Changeset 153d3440

Timestamp:

Apr 12, 2023, 10:42:12 AM (20 months ago)

Author:

Andrew Beach <ajbeach@…>

Branches:

ADT, ast-experimental, master

Children:

eb8d791

Parents:

94c98f0e

Message:

Reorganize CandidateFinder? to lower indentation. I did not flatten the anonymous namespace because there is a lot both inside and outside it.

File:

• src/ResolvExpr/CandidateFinder.cpp (modified) (3 diffs)

src/ResolvExpr/CandidateFinder.cpp

@@ -55 +55 @@
 namespace ResolvExpr {
 
+/// Unique identifier for matching expression resolutions to their requesting expression
+UniqueId globalResnSlot = 0;
+
+namespace {
+        /// First index is which argument, second is which alternative, third is which exploded element
+        using ExplodedArgs_new = std::deque< std::vector< ExplodedArg > >;
+
+        /// Returns a list of alternatives with the minimum cost in the given list
+        CandidateList findMinCost( const CandidateList & candidates ) {
+                CandidateList out;
+                Cost minCost = Cost::infinity;
+                for ( const CandidateRef & r : candidates ) {
+                        if ( r->cost < minCost ) {
+                                minCost = r->cost;
+                                out.clear();
+                                out.emplace_back( r );
+                        } else if ( r->cost == minCost ) {
+                                out.emplace_back( r );
+                        }
+                }
+                return out;
+        }
+
+        /// Computes conversion cost for a given expression to a given type
+        const ast::Expr * computeExpressionConversionCost(
+                const ast::Expr * arg, const ast::Type * paramType, const ast::SymbolTable & symtab, const ast::TypeEnvironment & env, Cost & outCost
+        ) {
+                Cost convCost = computeConversionCost(
+                                arg->result, paramType, arg->get_lvalue(), symtab, env );
+                outCost += convCost;
+
+                // If there is a non-zero conversion cost, ignoring poly cost, then the expression requires
+                // conversion. Ignore poly cost for now, since this requires resolution of the cast to
+                // infer parameters and this does not currently work for the reason stated below
+                Cost tmpCost = convCost;
+                tmpCost.incPoly( -tmpCost.get_polyCost() );
+                if ( tmpCost != Cost::zero ) {
+                        ast::ptr< ast::Type > newType = paramType;
+                        env.apply( newType );
+                        return new ast::CastExpr{ arg, newType };
+
+                        // xxx - *should* be able to resolve this cast, but at the moment pointers are not
+                        // castable to zero_t, but are implicitly convertible. This is clearly inconsistent,
+                        // once this is fixed it should be possible to resolve the cast.
+                        // xxx - this isn't working, it appears because type1 (parameter) is seen as widenable,
+                        // but it shouldn't be because this makes the conversion from DT* to DT* since
+                        // commontype(zero_t, DT*) is DT*, rather than nothing
+
+                        // CandidateFinder finder{ symtab, env };
+                        // finder.find( arg, ResolvMode::withAdjustment() );
+                        // assertf( finder.candidates.size() > 0,
+                        //      "Somehow castable expression failed to find alternatives." );
+                        // assertf( finder.candidates.size() == 1,
+                        //      "Somehow got multiple alternatives for known cast expression." );
+                        // return finder.candidates.front()->expr;
+                }
+
+                return arg;
+        }
+
+        /// Computes conversion cost for a given candidate
+        Cost computeApplicationConversionCost(
+                CandidateRef cand, const ast::SymbolTable & symtab
+        ) {
+                auto appExpr = cand->expr.strict_as< ast::ApplicationExpr >();
+                auto pointer = appExpr->func->result.strict_as< ast::PointerType >();
+                auto function = pointer->base.strict_as< ast::FunctionType >();
+
+                Cost convCost = Cost::zero;
+                const auto & params = function->params;
+                auto param = params.begin();
+                auto & args = appExpr->args;
+
+                for ( unsigned i = 0; i < args.size(); ++i ) {
+                        const ast::Type * argType = args[i]->result;
+                        PRINT(
+                                std::cerr << "arg expression:" << std::endl;
+                                ast::print( std::cerr, args[i], 2 );
+                                std::cerr << "--- results are" << std::endl;
+                                ast::print( std::cerr, argType, 2 );
+                        )
+
+                        if ( param == params.end() ) {
+                                if ( function->isVarArgs ) {
+                                        convCost.incUnsafe();
+                                        PRINT( std::cerr << "end of params with varargs function: inc unsafe: "
+                                                << convCost << std::endl; ; )
+                                        // convert reference-typed expressions into value-typed expressions
+                                        cand->expr = ast::mutate_field_index(
+                                                appExpr, &ast::ApplicationExpr::args, i,
+                                                referenceToRvalueConversion( args[i], convCost ) );
+                                        continue;
+                                } else return Cost::infinity;
+                        }
+
+                        if ( auto def = args[i].as< ast::DefaultArgExpr >() ) {
+                                // Default arguments should be free - don't include conversion cost.
+                                // Unwrap them here because they are not relevant to the rest of the system
+                                cand->expr = ast::mutate_field_index(
+                                        appExpr, &ast::ApplicationExpr::args, i, def->expr );
+                                ++param;
+                                continue;
+                        }
+
+                        // mark conversion cost and also specialization cost of param type
+                        // const ast::Type * paramType = (*param)->get_type();
+                        cand->expr = ast::mutate_field_index(
+                                appExpr, &ast::ApplicationExpr::args, i,
+                                computeExpressionConversionCost(
+                                        args[i], *param, symtab, cand->env, convCost ) );
+                        convCost.decSpec( specCost( *param ) );
+                        ++param;  // can't be in for-loop update because of the continue
+                }
+
+                if ( param != params.end() ) return Cost::infinity;
+
+                // specialization cost of return types can't be accounted for directly, it disables
+                // otherwise-identical calls, like this example based on auto-newline in the I/O lib:
+                //
+                //   forall(otype OS) {
+                //     void ?|?(OS&, int);  // with newline
+                //     OS&  ?|?(OS&, int);  // no newline, always chosen due to more specialization
+                //   }
+
+                // mark type variable and specialization cost of forall clause
+                convCost.incVar( function->forall.size() );
+                convCost.decSpec( function->assertions.size() );
+
+                return convCost;
+        }
+
+        void makeUnifiableVars(
+                const ast::FunctionType * type, ast::OpenVarSet & unifiableVars,
+                ast::AssertionSet & need
+        ) {
+                for ( auto & tyvar : type->forall ) {
+                        unifiableVars[ *tyvar ] = ast::TypeData{ tyvar->base };
+                }
+                for ( auto & assn : type->assertions ) {
+                        need[ assn ].isUsed = true;
+                }
+        }
+
+        /// Gets a default value from an initializer, nullptr if not present
+        const ast::ConstantExpr * getDefaultValue( const ast::Init * init ) {
+                if ( auto si = dynamic_cast< const ast::SingleInit * >( init ) ) {
+                        if ( auto ce = si->value.as< ast::CastExpr >() ) {
+                                return ce->arg.as< ast::ConstantExpr >();
+                        } else {
+                                return si->value.as< ast::ConstantExpr >();
+                        }
+                }
+                return nullptr;
+        }
+
+        /// State to iteratively build a match of parameter expressions to arguments
+        struct ArgPack {
+                std::size_t parent;          ///< Index of parent pack
+                ast::ptr< ast::Expr > expr;  ///< The argument stored here
+                Cost cost;                   ///< The cost of this argument
+                ast::TypeEnvironment env;    ///< Environment for this pack
+                ast::AssertionSet need;      ///< Assertions outstanding for this pack
+                ast::AssertionSet have;      ///< Assertions found for this pack
+                ast::OpenVarSet open;        ///< Open variables for this pack
+                unsigned nextArg;            ///< Index of next argument in arguments list
+                unsigned tupleStart;         ///< Number of tuples that start at this index
+                unsigned nextExpl;           ///< Index of next exploded element
+                unsigned explAlt;            ///< Index of alternative for nextExpl > 0
+
+                ArgPack()
+                : parent( 0 ), expr(), cost( Cost::zero ), env(), need(), have(), open(), nextArg( 0 ),
+                  tupleStart( 0 ), nextExpl( 0 ), explAlt( 0 ) {}
+
+                ArgPack(
+                        const ast::TypeEnvironment & env, const ast::AssertionSet & need,
+                        const ast::AssertionSet & have, const ast::OpenVarSet & open )
+                : parent( 0 ), expr(), cost( Cost::zero ), env( env ), need( need ), have( have ),
+                  open( open ), nextArg( 0 ), tupleStart( 0 ), nextExpl( 0 ), explAlt( 0 ) {}
+
+                ArgPack(
+                        std::size_t parent, const ast::Expr * expr, ast::TypeEnvironment && env,
+                        ast::AssertionSet && need, ast::AssertionSet && have, ast::OpenVarSet && open,
+                        unsigned nextArg, unsigned tupleStart = 0, Cost cost = Cost::zero,
+                        unsigned nextExpl = 0, unsigned explAlt = 0 )
+                : parent(parent), expr( expr ), cost( cost ), env( std::move( env ) ), need( std::move( need ) ),
+                  have( std::move( have ) ), open( std::move( open ) ), nextArg( nextArg ), tupleStart( tupleStart ),
+                  nextExpl( nextExpl ), explAlt( explAlt ) {}
+
+                ArgPack(
+                        const ArgPack & o, ast::TypeEnvironment && env, ast::AssertionSet && need,
+                        ast::AssertionSet && have, ast::OpenVarSet && open, unsigned nextArg, Cost added )
+                : parent( o.parent ), expr( o.expr ), cost( o.cost + added ), env( std::move( env ) ),
+                  need( std::move( need ) ), have( std::move( have ) ), open( std::move( open ) ), nextArg( nextArg ),
+                  tupleStart( o.tupleStart ), nextExpl( 0 ), explAlt( 0 ) {}
+
+                /// true if this pack is in the middle of an exploded argument
+                bool hasExpl() const { return nextExpl > 0; }
+
+                /// Gets the list of exploded candidates for this pack
+                const ExplodedArg & getExpl( const ExplodedArgs_new & args ) const {
+                        return args[ nextArg-1 ][ explAlt ];
+                }
+
+                /// Ends a tuple expression, consolidating the appropriate args
+                void endTuple( const std::vector< ArgPack > & packs ) {
+                        // add all expressions in tuple to list, summing cost
+                        std::deque< const ast::Expr * > exprs;
+                        const ArgPack * pack = this;
+                        if ( expr ) { exprs.emplace_front( expr ); }
+                        while ( pack->tupleStart == 0 ) {
+                                pack = &packs[pack->parent];
+                                exprs.emplace_front( pack->expr );
+                                cost += pack->cost;
+                        }
+                        // reset pack to appropriate tuple
+                        std::vector< ast::ptr< ast::Expr > > exprv( exprs.begin(), exprs.end() );
+                        expr = new ast::TupleExpr{ expr->location, std::move( exprv ) };
+                        tupleStart = pack->tupleStart - 1;
+                        parent = pack->parent;
+                }
+        };
+
+        /// Instantiates an argument to match a parameter, returns false if no matching results left
+        bool instantiateArgument(
+                const ast::Type * paramType, const ast::Init * init, const ExplodedArgs_new & args,
+                std::vector< ArgPack > & results, std::size_t & genStart, const ast::SymbolTable & symtab,
+                unsigned nTuples = 0
+        ) {
+                if ( auto tupleType = dynamic_cast< const ast::TupleType * >( paramType ) ) {
+                        // paramType is a TupleType -- group args into a TupleExpr
+                        ++nTuples;
+                        for ( const ast::Type * type : *tupleType ) {
+                                // xxx - dropping initializer changes behaviour from previous, but seems correct
+                                // ^^^ need to handle the case where a tuple has a default argument
+                                if ( ! instantiateArgument(
+                                        type, nullptr, args, results, genStart, symtab, nTuples ) ) return false;
+                                nTuples = 0;
+                        }
+                        // re-constitute tuples for final generation
+                        for ( auto i = genStart; i < results.size(); ++i ) {
+                                results[i].endTuple( results );
+                        }
+                        return true;
+                } else if ( const ast::TypeInstType * ttype = Tuples::isTtype( paramType ) ) {
+                        // paramType is a ttype, consumes all remaining arguments
+
+                        // completed tuples; will be spliced to end of results to finish
+                        std::vector< ArgPack > finalResults{};
+
+                        // iterate until all results completed
+                        std::size_t genEnd;
+                        ++nTuples;
+                        do {
+                                genEnd = results.size();
+
+                                // add another argument to results
+                                for ( std::size_t i = genStart; i < genEnd; ++i ) {
+                                        unsigned nextArg = results[i].nextArg;
+
+                                        // use next element of exploded tuple if present
+                                        if ( results[i].hasExpl() ) {
+                                                const ExplodedArg & expl = results[i].getExpl( args );
+
+                                                unsigned nextExpl = results[i].nextExpl + 1;
+                                                if ( nextExpl == expl.exprs.size() ) { nextExpl = 0; }
+
+                                                results.emplace_back(
+                                                        i, expl.exprs[ results[i].nextExpl ], copy( results[i].env ),
+                                                        copy( results[i].need ), copy( results[i].have ),
+                                                        copy( results[i].open ), nextArg, nTuples, Cost::zero, nextExpl,
+                                                        results[i].explAlt );
+
+                                                continue;
+                                        }
+
+                                        // finish result when out of arguments
+                                        if ( nextArg >= args.size() ) {
+                                                ArgPack newResult{
+                                                        results[i].env, results[i].need, results[i].have, results[i].open };
+                                                newResult.nextArg = nextArg;
+                                                const ast::Type * argType = nullptr;
+
+                                                if ( nTuples > 0 || ! results[i].expr ) {
+                                                        // first iteration or no expression to clone,
+                                                        // push empty tuple expression
+                                                        newResult.parent = i;
+                                                        newResult.expr = new ast::TupleExpr{ CodeLocation{}, {} };
+                                                        argType = newResult.expr->result;
+                                                } else {
+                                                        // clone result to collect tuple
+                                                        newResult.parent = results[i].parent;
+                                                        newResult.cost = results[i].cost;
+                                                        newResult.tupleStart = results[i].tupleStart;
+                                                        newResult.expr = results[i].expr;
+                                                        argType = newResult.expr->result;
+
+                                                        if ( results[i].tupleStart > 0 && Tuples::isTtype( argType ) ) {
+                                                                // the case where a ttype value is passed directly is special,
+                                                                // e.g. for argument forwarding purposes
+                                                                // xxx - what if passing multiple arguments, last of which is
+                                                                //       ttype?
+                                                                // xxx - what would happen if unify was changed so that unifying
+                                                                //       tuple
+                                                                // types flattened both before unifying lists? then pass in
+                                                                // TupleType (ttype) below.
+                                                                --newResult.tupleStart;
+                                                        } else {
+                                                                // collapse leftover arguments into tuple
+                                                                newResult.endTuple( results );
+                                                                argType = newResult.expr->result;
+                                                        }
+                                                }
+
+                                                // check unification for ttype before adding to final
+                                                if (
+                                                        unify(
+                                                                ttype, argType, newResult.env, newResult.need, newResult.have,
+                                                                newResult.open, symtab )
+                                                ) {
+                                                        finalResults.emplace_back( std::move( newResult ) );
+                                                }
+
+                                                continue;
+                                        }
+
+                                        // add each possible next argument
+                                        for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
+                                                const ExplodedArg & expl = args[nextArg][j];
+
+                                                // fresh copies of parent parameters for this iteration
+                                                ast::TypeEnvironment env = results[i].env;
+                                                ast::OpenVarSet open = results[i].open;
+
+                                                env.addActual( expl.env, open );
+
+                                                // skip empty tuple arguments by (nearly) cloning parent into next gen
+                                                if ( expl.exprs.empty() ) {
+                                                        results.emplace_back(
+                                                                results[i], std::move( env ), copy( results[i].need ),
+                                                                copy( results[i].have ), std::move( open ), nextArg + 1, expl.cost );
+
+                                                        continue;
+                                                }
+
+                                                // add new result
+                                                results.emplace_back(
+                                                        i, expl.exprs.front(), std::move( env ), copy( results[i].need ),
+                                                        copy( results[i].have ), std::move( open ), nextArg + 1, nTuples,
+                                                        expl.cost, expl.exprs.size() == 1 ? 0 : 1, j );
+                                        }
+                                }
+
+                                // reset for next round
+                                genStart = genEnd;
+                                nTuples = 0;
+                        } while ( genEnd != results.size() );
+
+                        // splice final results onto results
+                        for ( std::size_t i = 0; i < finalResults.size(); ++i ) {
+                                results.emplace_back( std::move( finalResults[i] ) );
+                        }
+                        return ! finalResults.empty();
+                }
+
+                // iterate each current subresult
+                std::size_t genEnd = results.size();
+                for ( std::size_t i = genStart; i < genEnd; ++i ) {
+                        unsigned nextArg = results[i].nextArg;
+
+                        // use remainder of exploded tuple if present
+                        if ( results[i].hasExpl() ) {
+                                const ExplodedArg & expl = results[i].getExpl( args );
+                                const ast::Expr * expr = expl.exprs[ results[i].nextExpl ];
+
+                                ast::TypeEnvironment env = results[i].env;
+                                ast::AssertionSet need = results[i].need, have = results[i].have;
+                                ast::OpenVarSet open = results[i].open;
+
+                                const ast::Type * argType = expr->result;
+
+                                PRINT(
+                                        std::cerr << "param type is ";
+                                        ast::print( std::cerr, paramType );
+                                        std::cerr << std::endl << "arg type is ";
+                                        ast::print( std::cerr, argType );
+                                        std::cerr << std::endl;
+                                )
+
+                                if ( unify( paramType, argType, env, need, have, open, symtab ) ) {
+                                        unsigned nextExpl = results[i].nextExpl + 1;
+                                        if ( nextExpl == expl.exprs.size() ) { nextExpl = 0; }
+
+                                        results.emplace_back(
+                                                i, expr, std::move( env ), std::move( need ), std::move( have ), std::move( open ), nextArg,
+                                                nTuples, Cost::zero, nextExpl, results[i].explAlt );
+                                }
+
+                                continue;
+                        }
+
+                        // use default initializers if out of arguments
+                        if ( nextArg >= args.size() ) {
+                                if ( const ast::ConstantExpr * cnst = getDefaultValue( init ) ) {
+                                        ast::TypeEnvironment env = results[i].env;
+                                        ast::AssertionSet need = results[i].need, have = results[i].have;
+                                        ast::OpenVarSet open = results[i].open;
+
+                                        if ( unify( paramType, cnst->result, env, need, have, open, symtab ) ) {
+                                                results.emplace_back(
+                                                        i, new ast::DefaultArgExpr{ cnst->location, cnst }, std::move( env ),
+                                                        std::move( need ), std::move( have ), std::move( open ), nextArg, nTuples );
+                                        }
+                                }
+
+                                continue;
+                        }
+
+                        // Check each possible next argument
+                        for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
+                                const ExplodedArg & expl = args[nextArg][j];
+
+                                // fresh copies of parent parameters for this iteration
+                                ast::TypeEnvironment env = results[i].env;
+                                ast::AssertionSet need = results[i].need, have = results[i].have;
+                                ast::OpenVarSet open = results[i].open;
+
+                                env.addActual( expl.env, open );
+
+                                // skip empty tuple arguments by (nearly) cloning parent into next gen
+                                if ( expl.exprs.empty() ) {
+                                        results.emplace_back(
+                                                results[i], std::move( env ), std::move( need ), std::move( have ), std::move( open ),
+                                                nextArg + 1, expl.cost );
+
+                                        continue;
+                                }
+
+                                // consider only first exploded arg
+                                const ast::Expr * expr = expl.exprs.front();
+                                const ast::Type * argType = expr->result;
+
+                                PRINT(
+                                        std::cerr << "param type is ";
+                                        ast::print( std::cerr, paramType );
+                                        std::cerr << std::endl << "arg type is ";
+                                        ast::print( std::cerr, argType );
+                                        std::cerr << std::endl;
+                                )
+
+                                // attempt to unify types
+                                if ( unify( paramType, argType, env, need, have, open, symtab ) ) {
+                                        // add new result
+                                        results.emplace_back(
+                                                i, expr, std::move( env ), std::move( need ), std::move( have ), std::move( open ),
+                                                nextArg + 1, nTuples, expl.cost, expl.exprs.size() == 1 ? 0 : 1, j );
+                                }
+                        }
+                }
+
+                // reset for next parameter
+                genStart = genEnd;
+
+                return genEnd != results.size();  // were any new results added?
+        }
+
+        /// Generate a cast expression from `arg` to `toType`
+        const ast::Expr * restructureCast(
+                ast::ptr< ast::Expr > & arg, const ast::Type * toType, ast::GeneratedFlag isGenerated = ast::GeneratedCast
+        ) {
+                if (
+                        arg->result->size() > 1
+                        && ! toType->isVoid()
+                        && ! dynamic_cast< const ast::ReferenceType * >( toType )
+                ) {
+                        // Argument is a tuple and the target type is neither void nor a reference. Cast each
+                        // member of the tuple to its corresponding target type, producing the tuple of those
+                        // cast expressions. If there are more components of the tuple than components in the
+                        // target type, then excess components do not come out in the result expression (but
+                        // UniqueExpr ensures that the side effects will still be produced)
+                        if ( Tuples::maybeImpureIgnoreUnique( arg ) ) {
+                                // expressions which may contain side effects require a single unique instance of
+                                // the expression
+                                arg = new ast::UniqueExpr{ arg->location, arg };
+                        }
+                        std::vector< ast::ptr< ast::Expr > > components;
+                        for ( unsigned i = 0; i < toType->size(); ++i ) {
+                                // cast each component
+                                ast::ptr< ast::Expr > idx = new ast::TupleIndexExpr{ arg->location, arg, i };
+                                components.emplace_back(
+                                        restructureCast( idx, toType->getComponent( i ), isGenerated ) );
+                        }
+                        return new ast::TupleExpr{ arg->location, std::move( components ) };
+                } else {
+                        // handle normally
+                        return new ast::CastExpr{ arg->location, arg, toType, isGenerated };
+                }
+        }
+
+        /// Gets the name from an untyped member expression (must be NameExpr)
+        const std::string & getMemberName( const ast::UntypedMemberExpr * memberExpr ) {
+                if ( memberExpr->member.as< ast::ConstantExpr >() ) {
+                        SemanticError( memberExpr, "Indexed access to struct fields unsupported: " );
+                }
+
+                return memberExpr->member.strict_as< ast::NameExpr >()->name;
+        }
+
+        /// Actually visits expressions to find their candidate interpretations
+        class Finder final : public ast::WithShortCircuiting {
+                const ResolveContext & context;
+                const ast::SymbolTable & symtab;
+        public:
+                // static size_t traceId;
+                CandidateFinder & selfFinder;
+                CandidateList & candidates;
+                const ast::TypeEnvironment & tenv;
+                ast::ptr< ast::Type > & targetType;
+
+                enum Errors {
+                        NotFound,
+                        NoMatch,
+                        ArgsToFew,
+                        ArgsToMany,
+                        RetsToFew,
+                        RetsToMany,
+                        NoReason
+                };
+
+                struct {
+                        Errors code = NotFound;
+                } reason;
+
+                Finder( CandidateFinder & f )
+                : context( f.context ), symtab( context.symtab ), selfFinder( f ),
+                  candidates( f.candidates ), tenv( f.env ), targetType( f.targetType ) {}
+
+                void previsit( const ast::Node * ) { visit_children = false; }
+
+                /// Convenience to add candidate to list
+                template<typename... Args>
+                void addCandidate( Args &&... args ) {
+                        candidates.emplace_back( new Candidate{ std::forward<Args>( args )... } );
+                        reason.code = NoReason;
+                }
+
+                void postvisit( const ast::ApplicationExpr * applicationExpr ) {
+                        addCandidate( applicationExpr, tenv );
+                }
+
+                /// Set up candidate assertions for inference
+                void inferParameters( CandidateRef & newCand, CandidateList & out );
+
+                /// Completes a function candidate with arguments located
+                void validateFunctionCandidate(
+                        const CandidateRef & func, ArgPack & result, const std::vector< ArgPack > & results,
+                        CandidateList & out );
+
+                /// Builds a list of candidates for a function, storing them in out
+                void makeFunctionCandidates(
+                        const CandidateRef & func, const ast::FunctionType * funcType,
+                        const ExplodedArgs_new & args, CandidateList & out );
+
+                /// Adds implicit struct-conversions to the alternative list
+                void addAnonConversions( const CandidateRef & cand );
+
+                /// Adds aggregate member interpretations
+                void addAggMembers(
+                        const ast::BaseInstType * aggrInst, const ast::Expr * expr,
+                        const Candidate & cand, const Cost & addedCost, const std::string & name
+                );
+
+                /// Adds tuple member interpretations
+                void addTupleMembers(
+                        const ast::TupleType * tupleType, const ast::Expr * expr, const Candidate & cand,
+                        const Cost & addedCost, const ast::Expr * member
+                );
+
+                /// true if expression is an lvalue
+                static bool isLvalue( const ast::Expr * x ) {
+                        return x->result && ( x->get_lvalue() || x->result.as< ast::ReferenceType >() );
+                }
+
+                void postvisit( const ast::UntypedExpr * untypedExpr );
+                void postvisit( const ast::VariableExpr * variableExpr );
+                void postvisit( const ast::ConstantExpr * constantExpr );
+                void postvisit( const ast::SizeofExpr * sizeofExpr );
+                void postvisit( const ast::AlignofExpr * alignofExpr );
+                void postvisit( const ast::AddressExpr * addressExpr );
+                void postvisit( const ast::LabelAddressExpr * labelExpr );
+                void postvisit( const ast::CastExpr * castExpr );
+                void postvisit( const ast::VirtualCastExpr * castExpr );
+                void postvisit( const ast::KeywordCastExpr * castExpr );
+                void postvisit( const ast::UntypedMemberExpr * memberExpr );
+                void postvisit( const ast::MemberExpr * memberExpr );
+                void postvisit( const ast::NameExpr * nameExpr );
+                void postvisit( const ast::UntypedOffsetofExpr * offsetofExpr );
+                void postvisit( const ast::OffsetofExpr * offsetofExpr );
+                void postvisit( const ast::OffsetPackExpr * offsetPackExpr );
+                void postvisit( const ast::LogicalExpr * logicalExpr );
+                void postvisit( const ast::ConditionalExpr * conditionalExpr );
+                void postvisit( const ast::CommaExpr * commaExpr );
+                void postvisit( const ast::ImplicitCopyCtorExpr * ctorExpr );
+                void postvisit( const ast::ConstructorExpr * ctorExpr );
+                void postvisit( const ast::RangeExpr * rangeExpr );
+                void postvisit( const ast::UntypedTupleExpr * tupleExpr );
+                void postvisit( const ast::TupleExpr * tupleExpr );
+                void postvisit( const ast::TupleIndexExpr * tupleExpr );
+                void postvisit( const ast::TupleAssignExpr * tupleExpr );
+                void postvisit( const ast::UniqueExpr * unqExpr );
+                void postvisit( const ast::StmtExpr * stmtExpr );
+                void postvisit( const ast::UntypedInitExpr * initExpr );
+
+                void postvisit( const ast::InitExpr * ) {
+                        assertf( false, "CandidateFinder should never see a resolved InitExpr." );
+                }
+
+                void postvisit( const ast::DeletedExpr * ) {
+                        assertf( false, "CandidateFinder should never see a DeletedExpr." );
+                }
+
+                void postvisit( const ast::GenericExpr * ) {
+                        assertf( false, "_Generic is not yet supported." );
+                }
+        };
+
+        /// Set up candidate assertions for inference
+        void Finder::inferParameters( CandidateRef & newCand, CandidateList & out ) {
+                // Set need bindings for any unbound assertions
+                UniqueId crntResnSlot = 0; // matching ID for this expression's assertions
+                for ( auto & assn : newCand->need ) {
+                        // skip already-matched assertions
+                        if ( assn.second.resnSlot != 0 ) continue;
+                        // assign slot for expression if needed
+                        if ( crntResnSlot == 0 ) { crntResnSlot = ++globalResnSlot; }
+                        // fix slot to assertion
+                        assn.second.resnSlot = crntResnSlot;
+                }
+                // pair slot to expression
+                if ( crntResnSlot != 0 ) {
+                        newCand->expr.get_and_mutate()->inferred.resnSlots().emplace_back( crntResnSlot );
+                }
+
+                // add to output list; assertion satisfaction will occur later
+                out.emplace_back( newCand );
+        }
+
+        /// Completes a function candidate with arguments located
+        void Finder::validateFunctionCandidate(
+                const CandidateRef & func, ArgPack & result, const std::vector< ArgPack > & results,
+                CandidateList & out
+        ) {
+                ast::ApplicationExpr * appExpr =
+                        new ast::ApplicationExpr{ func->expr->location, func->expr };
+                // sum cost and accumulate arguments
+                std::deque< const ast::Expr * > args;
+                Cost cost = func->cost;
+                const ArgPack * pack = &result;
+                while ( pack->expr ) {
+                        args.emplace_front( pack->expr );
+                        cost += pack->cost;
+                        pack = &results[pack->parent];
+                }
+                std::vector< ast::ptr< ast::Expr > > vargs( args.begin(), args.end() );
+                appExpr->args = std::move( vargs );
+                // build and validate new candidate
+                auto newCand =
+                        std::make_shared<Candidate>( appExpr, result.env, result.open, result.need, cost );
+                PRINT(
+                        std::cerr << "instantiate function success: " << appExpr << std::endl;
+                        std::cerr << "need assertions:" << std::endl;
+                        ast::print( std::cerr, result.need, 2 );
+                )
+                inferParameters( newCand, out );
+        }
+
+        /// Builds a list of candidates for a function, storing them in out
+        void Finder::makeFunctionCandidates(
+                const CandidateRef & func, const ast::FunctionType * funcType,
+                const ExplodedArgs_new & args, CandidateList & out
+        ) {
+                ast::OpenVarSet funcOpen;
+                ast::AssertionSet funcNeed, funcHave;
+                ast::TypeEnvironment funcEnv{ func->env };
+                makeUnifiableVars( funcType, funcOpen, funcNeed );
+                // add all type variables as open variables now so that those not used in the
+                // parameter list are still considered open
+                funcEnv.add( funcType->forall );
+
+                if ( targetType && ! targetType->isVoid() && ! funcType->returns.empty() ) {
+                        // attempt to narrow based on expected target type
+                        const ast::Type * returnType = funcType->returns.front();
+                        if ( ! unify(
+                                returnType, targetType, funcEnv, funcNeed, funcHave, funcOpen, symtab )
+                        ) {
+                                // unification failed, do not pursue this candidate
+                                return;
+                        }
+                }
+
+                // iteratively build matches, one parameter at a time
+                std::vector< ArgPack > results;
+                results.emplace_back( funcEnv, funcNeed, funcHave, funcOpen );
+                std::size_t genStart = 0;
+
+                // xxx - how to handle default arg after change to ftype representation?
+                if (const ast::VariableExpr * varExpr = func->expr.as<ast::VariableExpr>()) {
+                        if (const ast::FunctionDecl * funcDecl = varExpr->var.as<ast::FunctionDecl>()) {
+                                // function may have default args only if directly calling by name
+                                // must use types on candidate however, due to RenameVars substitution
+                                auto nParams = funcType->params.size();
+
+                                for (size_t i=0; i<nParams; ++i) {
+                                        auto obj = funcDecl->params[i].strict_as<ast::ObjectDecl>();
+                                        if (!instantiateArgument(
+                                                funcType->params[i], obj->init, args, results, genStart, symtab)) return;
+                                }
+                                goto endMatch;
+                        }
+                }
+                for ( const auto & param : funcType->params ) {
+                        // Try adding the arguments corresponding to the current parameter to the existing
+                        // matches
+                        // no default args for indirect calls
+                        if ( ! instantiateArgument(
+                                param, nullptr, args, results, genStart, symtab ) ) return;
+                }
+
+                endMatch:
+                if ( funcType->isVarArgs ) {
+                        // append any unused arguments to vararg pack
+                        std::size_t genEnd;
+                        do {
+                                genEnd = results.size();
+
+                                // iterate results
+                                for ( std::size_t i = genStart; i < genEnd; ++i ) {
+                                        unsigned nextArg = results[i].nextArg;
+
+                                        // use remainder of exploded tuple if present
+                                        if ( results[i].hasExpl() ) {
+                                                const ExplodedArg & expl = results[i].getExpl( args );
+
+                                                unsigned nextExpl = results[i].nextExpl + 1;
+                                                if ( nextExpl == expl.exprs.size() ) { nextExpl = 0; }
+
+                                                results.emplace_back(
+                                                        i, expl.exprs[ results[i].nextExpl ], copy( results[i].env ),
+                                                        copy( results[i].need ), copy( results[i].have ),
+                                                        copy( results[i].open ), nextArg, 0, Cost::zero, nextExpl,
+                                                        results[i].explAlt );
+
+                                                continue;
+                                        }
+
+                                        // finish result when out of arguments
+                                        if ( nextArg >= args.size() ) {
+                                                validateFunctionCandidate( func, results[i], results, out );
+
+                                                continue;
+                                        }
+
+                                        // add each possible next argument
+                                        for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
+                                                const ExplodedArg & expl = args[nextArg][j];
+
+                                                // fresh copies of parent parameters for this iteration
+                                                ast::TypeEnvironment env = results[i].env;
+                                                ast::OpenVarSet open = results[i].open;
+
+                                                env.addActual( expl.env, open );
+
+                                                // skip empty tuple arguments by (nearly) cloning parent into next gen
+                                                if ( expl.exprs.empty() ) {
+                                                        results.emplace_back(
+                                                                results[i], std::move( env ), copy( results[i].need ),
+                                                                copy( results[i].have ), std::move( open ), nextArg + 1,
+                                                                expl.cost );
+
+                                                        continue;
+                                                }
+
+                                                // add new result
+                                                results.emplace_back(
+                                                        i, expl.exprs.front(), std::move( env ), copy( results[i].need ),
+                                                        copy( results[i].have ), std::move( open ), nextArg + 1, 0, expl.cost,
+                                                        expl.exprs.size() == 1 ? 0 : 1, j );
+                                        }
+                                }
+
+                                genStart = genEnd;
+                        } while( genEnd != results.size() );
+                } else {
+                        // filter out the results that don't use all the arguments
+                        for ( std::size_t i = genStart; i < results.size(); ++i ) {
+                                ArgPack & result = results[i];
+                                if ( ! result.hasExpl() && result.nextArg >= args.size() ) {
+                                        validateFunctionCandidate( func, result, results, out );
+                                }
+                        }
+                }
+        }
+
+        /// Adds implicit struct-conversions to the alternative list
+        void Finder::addAnonConversions( const CandidateRef & cand ) {
+                // adds anonymous member interpretations whenever an aggregate value type is seen.
+                // it's okay for the aggregate expression to have reference type -- cast it to the
+                // base type to treat the aggregate as the referenced value
+                ast::ptr< ast::Expr > aggrExpr( cand->expr );
+                ast::ptr< ast::Type > & aggrType = aggrExpr.get_and_mutate()->result;
+                cand->env.apply( aggrType );
+
+                if ( aggrType.as< ast::ReferenceType >() ) {
+                        aggrExpr = new ast::CastExpr{ aggrExpr, aggrType->stripReferences() };
+                }
+
+                if ( auto structInst = aggrExpr->result.as< ast::StructInstType >() ) {
+                        addAggMembers( structInst, aggrExpr, *cand, Cost::safe, "" );
+                } else if ( auto unionInst = aggrExpr->result.as< ast::UnionInstType >() ) {
+                        addAggMembers( unionInst, aggrExpr, *cand, Cost::safe, "" );
+                }
+        }
+
+        /// Adds aggregate member interpretations
+        void Finder::addAggMembers(
+                const ast::BaseInstType * aggrInst, const ast::Expr * expr,
+                const Candidate & cand, const Cost & addedCost, const std::string & name
+        ) {
+                for ( const ast::Decl * decl : aggrInst->lookup( name ) ) {
+                        auto dwt = strict_dynamic_cast< const ast::DeclWithType * >( decl );
+                        CandidateRef newCand = std::make_shared<Candidate>(
+                                cand, new ast::MemberExpr{ expr->location, dwt, expr }, addedCost );
+                        // add anonymous member interpretations whenever an aggregate value type is seen
+                        // as a member expression
+                        addAnonConversions( newCand );
+                        candidates.emplace_back( std::move( newCand ) );
+                }
+        }
+
+        /// Adds tuple member interpretations
+        void Finder::addTupleMembers(
+                const ast::TupleType * tupleType, const ast::Expr * expr, const Candidate & cand,
+                const Cost & addedCost, const ast::Expr * member
+        ) {
+                if ( auto constantExpr = dynamic_cast< const ast::ConstantExpr * >( member ) ) {
+                        // get the value of the constant expression as an int, must be between 0 and the
+                        // length of the tuple to have meaning
+                        long long val = constantExpr->intValue();
+                        if ( val >= 0 && (unsigned long long)val < tupleType->size() ) {
+                                addCandidate(
+                                        cand, new ast::TupleIndexExpr{ expr->location, expr, (unsigned)val },
+                                        addedCost );
+                        }
+                }
+        }
+
+        void Finder::postvisit( const ast::UntypedExpr * untypedExpr ) {
+                std::vector< CandidateFinder > argCandidates =
+                        selfFinder.findSubExprs( untypedExpr->args );
+
+                // take care of possible tuple assignments
+                // if not tuple assignment, handled as normal function call
+                Tuples::handleTupleAssignment( selfFinder, untypedExpr, argCandidates );
+
+                CandidateFinder funcFinder( context, tenv );
+                if (auto nameExpr = untypedExpr->func.as<ast::NameExpr>()) {
+                        auto kind = ast::SymbolTable::getSpecialFunctionKind(nameExpr->name);
+                        if (kind != ast::SymbolTable::SpecialFunctionKind::NUMBER_OF_KINDS) {
+                                assertf(!argCandidates.empty(), "special function call without argument");
+                                for (auto & firstArgCand: argCandidates[0]) {
+                                        ast::ptr<ast::Type> argType = firstArgCand->expr->result;
+                                        firstArgCand->env.apply(argType);
+                                        // strip references
+                                        // xxx - is this correct?
+                                        while (argType.as<ast::ReferenceType>()) argType = argType.as<ast::ReferenceType>()->base;
+
+                                        // convert 1-tuple to plain type
+                                        if (auto tuple = argType.as<ast::TupleType>()) {
+                                                if (tuple->size() == 1) {
+                                                        argType = tuple->types[0];
+                                                }
+                                        }
+
+                                        // if argType is an unbound type parameter, all special functions need to be searched.
+                                        if (isUnboundType(argType)) {
+                                                funcFinder.otypeKeys.clear();
+                                                break;
+                                        }
+
+                                        if (argType.as<ast::PointerType>()) funcFinder.otypeKeys.insert(Mangle::Encoding::pointer);                                             
+                                        // else if (const ast::EnumInstType * enumInst = argType.as<ast::EnumInstType>()) {
+                                        //      const ast::EnumDecl * enumDecl = enumInst->base; // Here
+                                        //      if ( const ast::Type* enumType = enumDecl->base ) {
+                                        //              // instance of enum (T) is a instance of type (T)
+                                        //              funcFinder.otypeKeys.insert(Mangle::mangle(enumType, Mangle::NoGenericParams | Mangle::Type));
+                                        //      } else {
+                                        //              // instance of an untyped enum is techically int
+                                        //              funcFinder.otypeKeys.insert(Mangle::mangle(enumDecl, Mangle::NoGenericParams | Mangle::Type));
+                                        //      }
+                                        // }
+                                        else funcFinder.otypeKeys.insert(Mangle::mangle(argType, Mangle::NoGenericParams | Mangle::Type));
+                                }
+                        }
+                }
+                // if candidates are already produced, do not fail
+                // xxx - is it possible that handleTupleAssignment and main finder both produce candidates?
+                // this means there exists ctor/assign functions with a tuple as first parameter.
+                ResolvMode mode = {
+                        true, // adjust
+                        !untypedExpr->func.as<ast::NameExpr>(), // prune if not calling by name
+                        selfFinder.candidates.empty() // failfast if other options are not found
+                };
+                funcFinder.find( untypedExpr->func, mode );
+                // short-circuit if no candidates
+                // if ( funcFinder.candidates.empty() ) return;
+
+                reason.code = NoMatch;
+
+                // find function operators
+                ast::ptr< ast::Expr > opExpr = new ast::NameExpr{ untypedExpr->location, "?()" }; // ??? why not ?{}
+                CandidateFinder opFinder( context, tenv );
+                // okay if there aren't any function operations
+                opFinder.find( opExpr, ResolvMode::withoutFailFast() );
+                PRINT(
+                        std::cerr << "known function ops:" << std::endl;
+                        print( std::cerr, opFinder.candidates, 1 );
+                )
+
+                // pre-explode arguments
+                ExplodedArgs_new argExpansions;
+                for ( const CandidateFinder & args : argCandidates ) {
+                        argExpansions.emplace_back();
+                        auto & argE = argExpansions.back();
+                        for ( const CandidateRef & arg : args ) { argE.emplace_back( *arg, symtab ); }
+                }
+
+                // Find function matches
+                CandidateList found;
+                SemanticErrorException errors;
+                for ( CandidateRef & func : funcFinder ) {
+                        try {
+                                PRINT(
+                                        std::cerr << "working on alternative:" << std::endl;
+                                        print( std::cerr, *func, 2 );
+                                )
+
+                                // check if the type is a pointer to function
+                                const ast::Type * funcResult = func->expr->result->stripReferences();
+                                if ( auto pointer = dynamic_cast< const ast::PointerType * >( funcResult ) ) {
+                                        if ( auto function = pointer->base.as< ast::FunctionType >() ) {
+                                                CandidateRef newFunc{ new Candidate{ *func } };
+                                                newFunc->expr =
+                                                        referenceToRvalueConversion( newFunc->expr, newFunc->cost );
+                                                makeFunctionCandidates( newFunc, function, argExpansions, found );
+                                        }
+                                } else if (
+                                        auto inst = dynamic_cast< const ast::TypeInstType * >( funcResult )
+                                ) {
+                                        if ( const ast::EqvClass * clz = func->env.lookup( *inst ) ) {
+                                                if ( auto function = clz->bound.as< ast::FunctionType >() ) {
+                                                        CandidateRef newFunc{ new Candidate{ *func } };
+                                                        newFunc->expr =
+                                                                referenceToRvalueConversion( newFunc->expr, newFunc->cost );
+                                                        makeFunctionCandidates( newFunc, function, argExpansions, found );
+                                                }
+                                        }
+                                }
+                        } catch ( SemanticErrorException & e ) { errors.append( e ); }
+                }
+
+                // Find matches on function operators `?()`
+                if ( ! opFinder.candidates.empty() ) {
+                        // add exploded function alternatives to front of argument list
+                        std::vector< ExplodedArg > funcE;
+                        funcE.reserve( funcFinder.candidates.size() );
+                        for ( const CandidateRef & func : funcFinder ) {
+                                funcE.emplace_back( *func, symtab );
+                        }
+                        argExpansions.emplace_front( std::move( funcE ) );
+
+                        for ( const CandidateRef & op : opFinder ) {
+                                try {
+                                        // check if type is pointer-to-function
+                                        const ast::Type * opResult = op->expr->result->stripReferences();
+                                        if ( auto pointer = dynamic_cast< const ast::PointerType * >( opResult ) ) {
+                                                if ( auto function = pointer->base.as< ast::FunctionType >() ) {
+                                                        CandidateRef newOp{ new Candidate{ *op} };
+                                                        newOp->expr =
+                                                                referenceToRvalueConversion( newOp->expr, newOp->cost );
+                                                        makeFunctionCandidates( newOp, function, argExpansions, found );
+                                                }
+                                        }
+                                } catch ( SemanticErrorException & e ) { errors.append( e ); }
+                        }
+                }
+
+                // Implement SFINAE; resolution errors are only errors if there aren't any non-error
+                // candidates
+                if ( found.empty() && ! errors.isEmpty() ) { throw errors; }
+
+                // Compute conversion costs
+                for ( CandidateRef & withFunc : found ) {
+                        Cost cvtCost = computeApplicationConversionCost( withFunc, symtab );
+
+                        PRINT(
+                                auto appExpr = withFunc->expr.strict_as< ast::ApplicationExpr >();
+                                auto pointer = appExpr->func->result.strict_as< ast::PointerType >();
+                                auto function = pointer->base.strict_as< ast::FunctionType >();
+
+                                std::cerr << "Case +++++++++++++ " << appExpr->func << std::endl;
+                                std::cerr << "parameters are:" << std::endl;
+                                ast::printAll( std::cerr, function->params, 2 );
+                                std::cerr << "arguments are:" << std::endl;
+                                ast::printAll( std::cerr, appExpr->args, 2 );
+                                std::cerr << "bindings are:" << std::endl;
+                                ast::print( std::cerr, withFunc->env, 2 );
+                                std::cerr << "cost is: " << withFunc->cost << std::endl;
+                                std::cerr << "cost of conversion is:" << cvtCost << std::endl;
+                        )
+
+                        if ( cvtCost != Cost::infinity ) {
+                                withFunc->cvtCost = cvtCost;
+                                candidates.emplace_back( std::move( withFunc ) );
+                        }
+                }
+                found = std::move( candidates );
+
+                // use a new list so that candidates are not examined by addAnonConversions twice
+                CandidateList winners = findMinCost( found );
+                promoteCvtCost( winners );
+
+                // function may return a struct/union value, in which case we need to add candidates
+                // for implicit conversions to each of the anonymous members, which must happen after
+                // `findMinCost`, since anon conversions are never the cheapest
+                for ( const CandidateRef & c : winners ) {
+                        addAnonConversions( c );
+                }
+                spliceBegin( candidates, winners );
+
+                if ( candidates.empty() && targetType && ! targetType->isVoid() ) {
+                        // If resolution is unsuccessful with a target type, try again without, since it
+                        // will sometimes succeed when it wouldn't with a target type binding.
+                        // For example:
+                        //   forall( otype T ) T & ?[]( T *, ptrdiff_t );
+                        //   const char * x = "hello world";
+                        //   unsigned char ch = x[0];
+                        // Fails with simple return type binding (xxx -- check this!) as follows:
+                        // * T is bound to unsigned char
+                        // * (x: const char *) is unified with unsigned char *, which fails
+                        // xxx -- fix this better
+                        targetType = nullptr;
+                        postvisit( untypedExpr );
+                }
+        }
+
+        void Finder::postvisit( const ast::AddressExpr * addressExpr ) {
+                CandidateFinder finder( context, tenv );
+                finder.find( addressExpr->arg );
+
+                if ( finder.candidates.empty() ) return;
+
+                reason.code = NoMatch;
+
+                for ( CandidateRef & r : finder.candidates ) {
+                        if ( ! isLvalue( r->expr ) ) continue;
+                        addCandidate( *r, new ast::AddressExpr{ addressExpr->location, r->expr } );
+                }
+        }
+
+        void Finder::postvisit( const ast::LabelAddressExpr * labelExpr ) {
+                addCandidate( labelExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::CastExpr * castExpr ) {
+                ast::ptr< ast::Type > toType = castExpr->result;
+                assert( toType );
+                toType = resolveTypeof( toType, context );
+                toType = adjustExprType( toType, tenv, symtab );
+
+                CandidateFinder finder( context, tenv, toType );
+                finder.find( castExpr->arg, ResolvMode::withAdjustment() );
+
+                if ( !finder.candidates.empty() ) reason.code = NoMatch;
+
+                CandidateList matches;
+                for ( CandidateRef & cand : finder.candidates ) {
+                        ast::AssertionSet need( cand->need.begin(), cand->need.end() ), have;
+                        ast::OpenVarSet open( cand->open );
+
+                        cand->env.extractOpenVars( open );
+
+                        // It is possible that a cast can throw away some values in a multiply-valued
+                        // expression, e.g. cast-to-void, one value to zero. Figure out the prefix of the
+                        // subexpression results that are cast directly. The candidate is invalid if it
+                        // has fewer results than there are types to cast to.
+                        int discardedValues = cand->expr->result->size() - toType->size();
+                        if ( discardedValues < 0 ) continue;
+
+                        // unification run for side-effects
+                        unify( toType, cand->expr->result, cand->env, need, have, open, symtab );
+                        Cost thisCost =
+                                (castExpr->isGenerated == ast::GeneratedFlag::GeneratedCast)
+                                        ? conversionCost( cand->expr->result, toType, cand->expr->get_lvalue(), symtab, cand->env )
+                                        : castCost( cand->expr->result, toType, cand->expr->get_lvalue(), symtab, cand->env );
+
+                        PRINT(
+                                std::cerr << "working on cast with result: " << toType << std::endl;
+                                std::cerr << "and expr type: " << cand->expr->result << std::endl;
+                                std::cerr << "env: " << cand->env << std::endl;
+                        )
+                        if ( thisCost != Cost::infinity ) {
+                                PRINT(
+                                        std::cerr << "has finite cost." << std::endl;
+                                )
+                                // count one safe conversion for each value that is thrown away
+                                thisCost.incSafe( discardedValues );
+                                CandidateRef newCand = std::make_shared<Candidate>(
+                                        restructureCast( cand->expr, toType, castExpr->isGenerated ),
+                                        copy( cand->env ), std::move( open ), std::move( need ), cand->cost,
+                                        cand->cost + thisCost );
+                                inferParameters( newCand, matches );
+                        }
+                }
+
+                // select first on argument cost, then conversion cost
+                CandidateList minArgCost = findMinCost( matches );
+                promoteCvtCost( minArgCost );
+                candidates = findMinCost( minArgCost );
+        }
+
+        void Finder::postvisit( const ast::VirtualCastExpr * castExpr ) {
+                assertf( castExpr->result, "Implicit virtual cast targets not yet supported." );
+                CandidateFinder finder( context, tenv );
+                // don't prune here, all alternatives guaranteed to have same type
+                finder.find( castExpr->arg, ResolvMode::withoutPrune() );
+                for ( CandidateRef & r : finder.candidates ) {
+                        addCandidate(
+                                *r,
+                                new ast::VirtualCastExpr{ castExpr->location, r->expr, castExpr->result } );
+                }
+        }
+
+        void Finder::postvisit( const ast::KeywordCastExpr * castExpr ) {
+                const auto & loc = castExpr->location;
+                assertf( castExpr->result, "Cast target should have been set in Validate." );
+                auto ref = castExpr->result.strict_as<ast::ReferenceType>();
+                auto inst = ref->base.strict_as<ast::StructInstType>();
+                auto target = inst->base.get();
+
+                CandidateFinder finder( context, tenv );
+
+                auto pick_alternatives = [target, this](CandidateList & found, bool expect_ref) {
+                        for (auto & cand : found) {
+                                const ast::Type * expr = cand->expr->result.get();
+                                if (expect_ref) {
+                                        auto res = dynamic_cast<const ast::ReferenceType*>(expr);
+                                        if (!res) { continue; }
+                                        expr = res->base.get();
+                                }
+
+                                if (auto insttype = dynamic_cast<const ast::TypeInstType*>(expr)) {
+                                        auto td = cand->env.lookup(*insttype);
+                                        if (!td) { continue; }
+                                        expr = td->bound.get();
+                                }
+
+                                if (auto base = dynamic_cast<const ast::StructInstType*>(expr)) {
+                                        if (base->base == target) {
+                                                candidates.push_back( std::move(cand) );
+                                                reason.code = NoReason;
+                                        }
+                                }
+                        }
+                };
+
+                try {
+                        // Attempt 1 : turn (thread&)X into (thread$&)X.__thrd
+                        // Clone is purely for memory management
+                        std::unique_ptr<const ast::Expr> tech1 { new ast::UntypedMemberExpr(loc, new ast::NameExpr(loc, castExpr->concrete_target.field), castExpr->arg) };
+
+                        // don't prune here, since it's guaranteed all alternatives will have the same type
+                        finder.find( tech1.get(), ResolvMode::withoutPrune() );
+                        pick_alternatives(finder.candidates, false);
+
+                        return;
+                } catch(SemanticErrorException & ) {}
+
+                // Fallback : turn (thread&)X into (thread$&)get_thread(X)
+                std::unique_ptr<const ast::Expr> fallback { ast::UntypedExpr::createDeref(loc,  new ast::UntypedExpr(loc, new ast::NameExpr(loc, castExpr->concrete_target.getter), { castExpr->arg })) };
+                // don't prune here, since it's guaranteed all alternatives will have the same type
+                finder.find( fallback.get(), ResolvMode::withoutPrune() );
+
+                pick_alternatives(finder.candidates, true);
+
+                // Whatever happens here, we have no more fallbacks
+        }
+
+        void Finder::postvisit( const ast::UntypedMemberExpr * memberExpr ) {
+                CandidateFinder aggFinder( context, tenv );
+                aggFinder.find( memberExpr->aggregate, ResolvMode::withAdjustment() );
+                for ( CandidateRef & agg : aggFinder.candidates ) {
+                        // it's okay for the aggregate expression to have reference type -- cast it to the
+                        // base type to treat the aggregate as the referenced value
+                        Cost addedCost = Cost::zero;
+                        agg->expr = referenceToRvalueConversion( agg->expr, addedCost );
+
+                        // find member of the given type
+                        if ( auto structInst = agg->expr->result.as< ast::StructInstType >() ) {
+                                addAggMembers(
+                                        structInst, agg->expr, *agg, addedCost, getMemberName( memberExpr ) );
+                        } else if ( auto unionInst = agg->expr->result.as< ast::UnionInstType >() ) {
+                                addAggMembers(
+                                        unionInst, agg->expr, *agg, addedCost, getMemberName( memberExpr ) );
+                        } else if ( auto tupleType = agg->expr->result.as< ast::TupleType >() ) {
+                                addTupleMembers( tupleType, agg->expr, *agg, addedCost, memberExpr->member );
+                        }
+                }
+        }
+
+        void Finder::postvisit( const ast::MemberExpr * memberExpr ) {
+                addCandidate( memberExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::NameExpr * nameExpr ) {
+                std::vector< ast::SymbolTable::IdData > declList;
+                if (!selfFinder.otypeKeys.empty()) {
+                        auto kind = ast::SymbolTable::getSpecialFunctionKind(nameExpr->name);
+                        assertf(kind != ast::SymbolTable::SpecialFunctionKind::NUMBER_OF_KINDS, "special lookup with non-special target: %s", nameExpr->name.c_str());
+
+                        for (auto & otypeKey: selfFinder.otypeKeys) {
+                                auto result = symtab.specialLookupId(kind, otypeKey);
+                                declList.insert(declList.end(), std::make_move_iterator(result.begin()), std::make_move_iterator(result.end()));
+                        }
+                } else {
+                        declList = symtab.lookupId( nameExpr->name );
+                }
+                PRINT( std::cerr << "nameExpr is " << nameExpr->name << std::endl; )
+
+                if ( declList.empty() ) return;
+
+                reason.code = NoMatch;
+
+                for ( auto & data : declList ) {
+                        Cost cost = Cost::zero;
+                        ast::Expr * newExpr = data.combine( nameExpr->location, cost );
+
+                        CandidateRef newCand = std::make_shared<Candidate>(
+                                newExpr, copy( tenv ), ast::OpenVarSet{}, ast::AssertionSet{}, Cost::zero,
+                                cost );
+
+                        if (newCand->expr->env) {
+                                newCand->env.add(*newCand->expr->env);
+                                auto mutExpr = newCand->expr.get_and_mutate();
+                                mutExpr->env  = nullptr;
+                                newCand->expr = mutExpr;
+                        }
+
+                        PRINT(
+                                std::cerr << "decl is ";
+                                ast::print( std::cerr, data.id );
+                                std::cerr << std::endl;
+                                std::cerr << "newExpr is ";
+                                ast::print( std::cerr, newExpr );
+                                std::cerr << std::endl;
+                        )
+                        newCand->expr = ast::mutate_field(
+                                newCand->expr.get(), &ast::Expr::result,
+                                renameTyVars( newCand->expr->result ) );
+                        // add anonymous member interpretations whenever an aggregate value type is seen
+                        // as a name expression
+                        addAnonConversions( newCand );
+                        candidates.emplace_back( std::move( newCand ) );
+                }
+        }
+
+        void Finder::postvisit( const ast::VariableExpr * variableExpr ) {
+                // not sufficient to just pass `variableExpr` here, type might have changed since
+                // creation
+                addCandidate(
+                        new ast::VariableExpr{ variableExpr->location, variableExpr->var }, tenv );
+        }
+
+        void Finder::postvisit( const ast::ConstantExpr * constantExpr ) {
+                addCandidate( constantExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::SizeofExpr * sizeofExpr ) {
+                if ( sizeofExpr->type ) {
+                        addCandidate(
+                                new ast::SizeofExpr{
+                                        sizeofExpr->location, resolveTypeof( sizeofExpr->type, context ) },
+                                tenv );
+                } else {
+                        // find all candidates for the argument to sizeof
+                        CandidateFinder finder( context, tenv );
+                        finder.find( sizeofExpr->expr );
+                        // find the lowest-cost candidate, otherwise ambiguous
+                        CandidateList winners = findMinCost( finder.candidates );
+                        if ( winners.size() != 1 ) {
+                                SemanticError(
+                                        sizeofExpr->expr.get(), "Ambiguous expression in sizeof operand: " );
+                        }
+                        // return the lowest-cost candidate
+                        CandidateRef & choice = winners.front();
+                        choice->expr = referenceToRvalueConversion( choice->expr, choice->cost );
+                        choice->cost = Cost::zero;
+                        addCandidate( *choice, new ast::SizeofExpr{ sizeofExpr->location, choice->expr } );
+                }
+        }
+
+        void Finder::postvisit( const ast::AlignofExpr * alignofExpr ) {
+                if ( alignofExpr->type ) {
+                        addCandidate(
+                                new ast::AlignofExpr{
+                                        alignofExpr->location, resolveTypeof( alignofExpr->type, context ) },
+                                tenv );
+                } else {
+                        // find all candidates for the argument to alignof
+                        CandidateFinder finder( context, tenv );
+                        finder.find( alignofExpr->expr );
+                        // find the lowest-cost candidate, otherwise ambiguous
+                        CandidateList winners = findMinCost( finder.candidates );
+                        if ( winners.size() != 1 ) {
+                                SemanticError(
+                                        alignofExpr->expr.get(), "Ambiguous expression in alignof operand: " );
+                        }
+                        // return the lowest-cost candidate
+                        CandidateRef & choice = winners.front();
+                        choice->expr = referenceToRvalueConversion( choice->expr, choice->cost );
+                        choice->cost = Cost::zero;
+                        addCandidate(
+                                *choice, new ast::AlignofExpr{ alignofExpr->location, choice->expr } );
+                }
+        }
+
+        void Finder::postvisit( const ast::UntypedOffsetofExpr * offsetofExpr ) {
+                const ast::BaseInstType * aggInst;
+                if (( aggInst = offsetofExpr->type.as< ast::StructInstType >() )) ;
+                else if (( aggInst = offsetofExpr->type.as< ast::UnionInstType >() )) ;
+                else return;
+
+                for ( const ast::Decl * member : aggInst->lookup( offsetofExpr->member ) ) {
+                        auto dwt = strict_dynamic_cast< const ast::DeclWithType * >( member );
+                        addCandidate(
+                                new ast::OffsetofExpr{ offsetofExpr->location, aggInst, dwt }, tenv );
+                }
+        }
+
+        void Finder::postvisit( const ast::OffsetofExpr * offsetofExpr ) {
+                addCandidate( offsetofExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::OffsetPackExpr * offsetPackExpr ) {
+                addCandidate( offsetPackExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::LogicalExpr * logicalExpr ) {
+                CandidateFinder finder1( context, tenv );
+                finder1.find( logicalExpr->arg1, ResolvMode::withAdjustment() );
+                if ( finder1.candidates.empty() ) return;
+
+                CandidateFinder finder2( context, tenv );
+                finder2.find( logicalExpr->arg2, ResolvMode::withAdjustment() );
+                if ( finder2.candidates.empty() ) return;
+
+                reason.code = NoMatch;
+
+                for ( const CandidateRef & r1 : finder1.candidates ) {
+                        for ( const CandidateRef & r2 : finder2.candidates ) {
+                                ast::TypeEnvironment env{ r1->env };
+                                env.simpleCombine( r2->env );
+                                ast::OpenVarSet open{ r1->open };
+                                mergeOpenVars( open, r2->open );
+                                ast::AssertionSet need;
+                                mergeAssertionSet( need, r1->need );
+                                mergeAssertionSet( need, r2->need );
+
+                                addCandidate(
+                                        new ast::LogicalExpr{
+                                                logicalExpr->location, r1->expr, r2->expr, logicalExpr->isAnd },
+                                        std::move( env ), std::move( open ), std::move( need ), r1->cost + r2->cost );
+                        }
+                }
+        }
+
+        void Finder::postvisit( const ast::ConditionalExpr * conditionalExpr ) {
+                // candidates for condition
+                CandidateFinder finder1( context, tenv );
+                finder1.find( conditionalExpr->arg1, ResolvMode::withAdjustment() );
+                if ( finder1.candidates.empty() ) return;
+
+                // candidates for true result
+                CandidateFinder finder2( context, tenv );
+                finder2.find( conditionalExpr->arg2, ResolvMode::withAdjustment() );
+                if ( finder2.candidates.empty() ) return;
+
+                // candidates for false result
+                CandidateFinder finder3( context, tenv );
+                finder3.find( conditionalExpr->arg3, ResolvMode::withAdjustment() );
+                if ( finder3.candidates.empty() ) return;
+
+                reason.code = NoMatch;
+
+                for ( const CandidateRef & r1 : finder1.candidates ) {
+                        for ( const CandidateRef & r2 : finder2.candidates ) {
+                                for ( const CandidateRef & r3 : finder3.candidates ) {
+                                        ast::TypeEnvironment env{ r1->env };
+                                        env.simpleCombine( r2->env );
+                                        env.simpleCombine( r3->env );
+                                        ast::OpenVarSet open{ r1->open };
+                                        mergeOpenVars( open, r2->open );
+                                        mergeOpenVars( open, r3->open );
+                                        ast::AssertionSet need;
+                                        mergeAssertionSet( need, r1->need );
+                                        mergeAssertionSet( need, r2->need );
+                                        mergeAssertionSet( need, r3->need );
+                                        ast::AssertionSet have;
+
+                                        // unify true and false results, then infer parameters to produce new
+                                        // candidates
+                                        ast::ptr< ast::Type > common;
+                                        if (
+                                                unify(
+                                                        r2->expr->result, r3->expr->result, env, need, have, open, symtab,
+                                                        common )
+                                        ) {
+                                                // generate typed expression
+                                                ast::ConditionalExpr * newExpr = new ast::ConditionalExpr{
+                                                        conditionalExpr->location, r1->expr, r2->expr, r3->expr };
+                                                newExpr->result = common ? common : r2->expr->result;
+                                                // convert both options to result type
+                                                Cost cost = r1->cost + r2->cost + r3->cost;
+                                                newExpr->arg2 = computeExpressionConversionCost(
+                                                        newExpr->arg2, newExpr->result, symtab, env, cost );
+                                                newExpr->arg3 = computeExpressionConversionCost(
+                                                        newExpr->arg3, newExpr->result, symtab, env, cost );
+                                                // output candidate
+                                                CandidateRef newCand = std::make_shared<Candidate>(
+                                                        newExpr, std::move( env ), std::move( open ), std::move( need ), cost );
+                                                inferParameters( newCand, candidates );
+                                        }
+                                }
+                        }
+                }
+        }
+
+        void Finder::postvisit( const ast::CommaExpr * commaExpr ) {
+                ast::TypeEnvironment env{ tenv };
+                ast::ptr< ast::Expr > arg1 = resolveInVoidContext( commaExpr->arg1, context, env );
+
+                CandidateFinder finder2( context, env );
+                finder2.find( commaExpr->arg2, ResolvMode::withAdjustment() );
+
+                for ( const CandidateRef & r2 : finder2.candidates ) {
+                        addCandidate( *r2, new ast::CommaExpr{ commaExpr->location, arg1, r2->expr } );
+                }
+        }
+
+        void Finder::postvisit( const ast::ImplicitCopyCtorExpr * ctorExpr ) {
+                addCandidate( ctorExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::ConstructorExpr * ctorExpr ) {
+                CandidateFinder finder( context, tenv );
+                finder.find( ctorExpr->callExpr, ResolvMode::withoutPrune() );
+                for ( CandidateRef & r : finder.candidates ) {
+                        addCandidate( *r, new ast::ConstructorExpr{ ctorExpr->location, r->expr } );
+                }
+        }
+
+        void Finder::postvisit( const ast::RangeExpr * rangeExpr ) {
+                // resolve low and high, accept candidates where low and high types unify
+                CandidateFinder finder1( context, tenv );
+                finder1.find( rangeExpr->low, ResolvMode::withAdjustment() );
+                if ( finder1.candidates.empty() ) return;
+
+                CandidateFinder finder2( context, tenv );
+                finder2.find( rangeExpr->high, ResolvMode::withAdjustment() );
+                if ( finder2.candidates.empty() ) return;
+
+                reason.code = NoMatch;
+
+                for ( const CandidateRef & r1 : finder1.candidates ) {
+                        for ( const CandidateRef & r2 : finder2.candidates ) {
+                                ast::TypeEnvironment env{ r1->env };
+                                env.simpleCombine( r2->env );
+                                ast::OpenVarSet open{ r1->open };
+                                mergeOpenVars( open, r2->open );
+                                ast::AssertionSet need;
+                                mergeAssertionSet( need, r1->need );
+                                mergeAssertionSet( need, r2->need );
+                                ast::AssertionSet have;
+
+                                ast::ptr< ast::Type > common;
+                                if (
+                                        unify(
+                                                r1->expr->result, r2->expr->result, env, need, have, open, symtab,
+                                                common )
+                                ) {
+                                        // generate new expression
+                                        ast::RangeExpr * newExpr =
+                                                new ast::RangeExpr{ rangeExpr->location, r1->expr, r2->expr };
+                                        newExpr->result = common ? common : r1->expr->result;
+                                        // add candidate
+                                        CandidateRef newCand = std::make_shared<Candidate>(
+                                                newExpr, std::move( env ), std::move( open ), std::move( need ),
+                                                r1->cost + r2->cost );
+                                        inferParameters( newCand, candidates );
+                                }
+                        }
+                }
+        }
+
+        void Finder::postvisit( const ast::UntypedTupleExpr * tupleExpr ) {
+                std::vector< CandidateFinder > subCandidates =
+                        selfFinder.findSubExprs( tupleExpr->exprs );
+                std::vector< CandidateList > possibilities;
+                combos( subCandidates.begin(), subCandidates.end(), back_inserter( possibilities ) );
+
+                for ( const CandidateList & subs : possibilities ) {
+                        std::vector< ast::ptr< ast::Expr > > exprs;
+                        exprs.reserve( subs.size() );
+                        for ( const CandidateRef & sub : subs ) { exprs.emplace_back( sub->expr ); }
+
+                        ast::TypeEnvironment env;
+                        ast::OpenVarSet open;
+                        ast::AssertionSet need;
+                        for ( const CandidateRef & sub : subs ) {
+                                env.simpleCombine( sub->env );
+                                mergeOpenVars( open, sub->open );
+                                mergeAssertionSet( need, sub->need );
+                        }
+
+                        addCandidate(
+                                new ast::TupleExpr{ tupleExpr->location, std::move( exprs ) },
+                                std::move( env ), std::move( open ), std::move( need ), sumCost( subs ) );
+                }
+        }
+
+        void Finder::postvisit( const ast::TupleExpr * tupleExpr ) {
+                addCandidate( tupleExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::TupleIndexExpr * tupleExpr ) {
+                addCandidate( tupleExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::TupleAssignExpr * tupleExpr ) {
+                addCandidate( tupleExpr, tenv );
+        }
+
+        void Finder::postvisit( const ast::UniqueExpr * unqExpr ) {
+                CandidateFinder finder( context, tenv );
+                finder.find( unqExpr->expr, ResolvMode::withAdjustment() );
+                for ( CandidateRef & r : finder.candidates ) {
+                        // ensure that the the id is passed on so that the expressions are "linked"
+                        addCandidate( *r, new ast::UniqueExpr{ unqExpr->location, r->expr, unqExpr->id } );
+                }
+        }
+
+        void Finder::postvisit( const ast::StmtExpr * stmtExpr ) {
+                addCandidate( resolveStmtExpr( stmtExpr, context ), tenv );
+        }
+
+        void Finder::postvisit( const ast::UntypedInitExpr * initExpr ) {
+                // handle each option like a cast
+                CandidateList matches;
+                PRINT(
+                        std::cerr << "untyped init expr: " << initExpr << std::endl;
+                )
+                // O(n^2) checks of d-types with e-types
+                for ( const ast::InitAlternative & initAlt : initExpr->initAlts ) {
+                        // calculate target type
+                        const ast::Type * toType = resolveTypeof( initAlt.type, context );
+                        toType = adjustExprType( toType, tenv, symtab );
+                        // The call to find must occur inside this loop, otherwise polymorphic return
+                        // types are not bound to the initialization type, since return type variables are
+                        // only open for the duration of resolving the UntypedExpr.
+                        CandidateFinder finder( context, tenv, toType );
+                        finder.find( initExpr->expr, ResolvMode::withAdjustment() );
+                        for ( CandidateRef & cand : finder.candidates ) {
+                                if (reason.code == NotFound) reason.code = NoMatch;
+
+                                ast::TypeEnvironment env{ cand->env };
+                                ast::AssertionSet need( cand->need.begin(), cand->need.end() ), have;
+                                ast::OpenVarSet open{ cand->open };
+
+                                PRINT(
+                                        std::cerr << "  @ " << toType << " " << initAlt.designation << std::endl;
+                                )
+
+                                // It is possible that a cast can throw away some values in a multiply-valued
+                                // expression, e.g. cast-to-void, one value to zero. Figure out the prefix of
+                                // the subexpression results that are cast directly. The candidate is invalid
+                                // if it has fewer results than there are types to cast to.
+                                int discardedValues = cand->expr->result->size() - toType->size();
+                                if ( discardedValues < 0 ) continue;
+
+                                // unification run for side-effects
+                                bool canUnify = unify( toType, cand->expr->result, env, need, have, open, symtab );
+                                (void) canUnify;
+                                Cost thisCost = computeConversionCost( cand->expr->result, toType, cand->expr->get_lvalue(),
+                                        symtab, env );
+                                PRINT(
+                                        Cost legacyCost = castCost( cand->expr->result, toType, cand->expr->get_lvalue(),
+                                                symtab, env );
+                                        std::cerr << "Considering initialization:";
+                                        std::cerr << std::endl << "  FROM: " << cand->expr->result << std::endl;
+                                        std::cerr << std::endl << "  TO: "   << toType             << std::endl;
+                                        std::cerr << std::endl << "  Unification " << (canUnify ? "succeeded" : "failed");
+                                        std::cerr << std::endl << "  Legacy cost " << legacyCost;
+                                        std::cerr << std::endl << "  New cost " << thisCost;
+                                        std::cerr << std::endl;
+                                )
+                                if ( thisCost != Cost::infinity ) {
+                                        // count one safe conversion for each value that is thrown away
+                                        thisCost.incSafe( discardedValues );
+                                        CandidateRef newCand = std::make_shared<Candidate>(
+                                                new ast::InitExpr{
+                                                        initExpr->location, restructureCast( cand->expr, toType ),
+                                                        initAlt.designation },
+                                                std::move(env), std::move( open ), std::move( need ), cand->cost, thisCost );
+                                        inferParameters( newCand, matches );
+                                }
+                        }
+                }
+
+                // select first on argument cost, then conversion cost
+                CandidateList minArgCost = findMinCost( matches );
+                promoteCvtCost( minArgCost );
+                candidates = findMinCost( minArgCost );
+        }
+
+        // size_t Finder::traceId = Stats::Heap::new_stacktrace_id("Finder");
+        /// Prunes a list of candidates down to those that have the minimum conversion cost for a given
+        /// return type. Skips ambiguous candidates.
+
+} // anonymous namespace
+
+bool CandidateFinder::pruneCandidates( CandidateList & candidates, CandidateList & out, std::vector<std::string> & errors ) {
+        struct PruneStruct {
+                CandidateRef candidate;
+                bool ambiguous;
+
+                PruneStruct() = default;
+                PruneStruct( const CandidateRef & c ) : candidate( c ), ambiguous( false ) {}
+        };
+
+        // find lowest-cost candidate for each type
+        std::unordered_map< std::string, PruneStruct > selected;
+        // attempt to skip satisfyAssertions on more expensive alternatives if better options have been found
+        std::sort(candidates.begin(), candidates.end(), [](const CandidateRef & x, const CandidateRef & y){return x->cost < y->cost;});
+        for ( CandidateRef & candidate : candidates ) {
+                std::string mangleName;
+                {
+                        ast::ptr< ast::Type > newType = candidate->expr->result;
+                        assertf(candidate->expr->result, "Result of expression %p for candidate is null", candidate->expr.get());
+                        candidate->env.apply( newType );
+                        mangleName = Mangle::mangle( newType );
+                }
+
+                auto found = selected.find( mangleName );
+                if (found != selected.end() && found->second.candidate->cost < candidate->cost) {
+                        PRINT(
+                                std::cerr << "cost " << candidate->cost << " loses to "
+                                        << found->second.candidate->cost << std::endl;
+                        )
+                        continue;
+                }
+
+                // xxx - when do satisfyAssertions produce more than 1 result?
+                // this should only happen when initial result type contains
+                // unbound type parameters, then it should never be pruned by
+                // the previous step, since renameTyVars guarantees the mangled name
+                // is unique.
+                CandidateList satisfied;
+                bool needRecomputeKey = false;
+                if (candidate->need.empty()) {
+                        satisfied.emplace_back(candidate);
+                }
+                else {
+                        satisfyAssertions(candidate, context.symtab, satisfied, errors);
+                        needRecomputeKey = true;
+                }
+
+                for (auto & newCand : satisfied) {
+                        // recomputes type key, if satisfyAssertions changed it
+                        if (needRecomputeKey)
+                        {
+                                ast::ptr< ast::Type > newType = newCand->expr->result;
+                                assertf(newCand->expr->result, "Result of expression %p for candidate is null", newCand->expr.get());
+                                newCand->env.apply( newType );
+                                mangleName = Mangle::mangle( newType );
+                        }
+                        auto found = selected.find( mangleName );
+                        if ( found != selected.end() ) {
+                                if ( newCand->cost < found->second.candidate->cost ) {
+                                        PRINT(
+                                                std::cerr << "cost " << newCand->cost << " beats "
+                                                        << found->second.candidate->cost << std::endl;
+                                        )
+
+                                        found->second = PruneStruct{ newCand };
+                                } else if ( newCand->cost == found->second.candidate->cost ) {
+                                        // if one of the candidates contains a deleted identifier, can pick the other,
+                                        // since deleted expressions should not be ambiguous if there is another option
+                                        // that is at least as good
+                                        if ( findDeletedExpr( newCand->expr ) ) {
+                                                // do nothing
+                                                PRINT( std::cerr << "candidate is deleted" << std::endl; )
+                                        } else if ( findDeletedExpr( found->second.candidate->expr ) ) {
+                                                PRINT( std::cerr << "current is deleted" << std::endl; )
+                                                found->second = PruneStruct{ newCand };
+                                        } else {
+                                                PRINT( std::cerr << "marking ambiguous" << std::endl; )
+                                                found->second.ambiguous = true;
+                                        }
+                                } else {
+                                        // xxx - can satisfyAssertions increase the cost?
+                                        PRINT(
+                                                std::cerr << "cost " << newCand->cost << " loses to "
+                                                        << found->second.candidate->cost << std::endl;
+                                        )
+                                }
+                        } else {
+                                selected.emplace_hint( found, mangleName, newCand );
+                        }
+                }
+        }
+
+        // report unambiguous min-cost candidates
+        // CandidateList out;
+        for ( auto & target : selected ) {
+                if ( target.second.ambiguous ) continue;
+
+                CandidateRef cand = target.second.candidate;
+
+                ast::ptr< ast::Type > newResult = cand->expr->result;
+                cand->env.applyFree( newResult );
+                cand->expr = ast::mutate_field(
+                        cand->expr.get(), &ast::Expr::result, std::move( newResult ) );
+
+                out.emplace_back( cand );
+        }
+        // if everything is lost in satisfyAssertions, report the error
+        return !selected.empty();
+}
+
+void CandidateFinder::find( const ast::Expr * expr, ResolvMode mode ) {
+        // Find alternatives for expression
+        ast::Pass<Finder> finder{ *this };
+        expr->accept( finder );
+
+        if ( mode.failFast && candidates.empty() ) {
+                switch(finder.core.reason.code) {
+                case Finder::NotFound:
+                        { SemanticError( expr, "No alternatives for expression " ); break; }
+                case Finder::NoMatch:
+                        { SemanticError( expr, "Invalid application of existing declaration(s) in expression " ); break; }
+                case Finder::ArgsToFew:
+                case Finder::ArgsToMany:
+                case Finder::RetsToFew:
+                case Finder::RetsToMany:
+                case Finder::NoReason:
+                default:
+                        { SemanticError( expr->location, "No reasonable alternatives for expression : reasons unkown" ); }
+                }
+        }
+
+        /*
+        if ( mode.satisfyAssns || mode.prune ) {
+                // trim candidates to just those where the assertions are satisfiable
+                // - necessary pre-requisite to pruning
+                CandidateList satisfied;
+                std::vector< std::string > errors;
+                for ( CandidateRef & candidate : candidates ) {
+                        satisfyAssertions( candidate, localSyms, satisfied, errors );
+                }
+
+                // fail early if none such
+                if ( mode.failFast && satisfied.empty() ) {
+                        std::ostringstream stream;
+                        stream << "No alternatives with satisfiable assertions for " << expr << "\n";
+                        for ( const auto& err : errors ) {
+                                stream << err;
+                        }
+                        SemanticError( expr->location, stream.str() );
+                }
+
+                // reset candidates
+                candidates = move( satisfied );
+        }
+        */
+
+        if ( mode.prune ) {
+                // trim candidates to single best one
+                PRINT(
+                        std::cerr << "alternatives before prune:" << std::endl;
+                        print( std::cerr, candidates );
+                )
+
+                CandidateList pruned;
+                std::vector<std::string> errors;
+                bool found = pruneCandidates( candidates, pruned, errors );
+
+                if ( mode.failFast && pruned.empty() ) {
+                        std::ostringstream stream;
+                        if (found) {
+                                CandidateList winners = findMinCost( candidates );
+                                stream << "Cannot choose between " << winners.size() << " alternatives for "
+                                        "expression\n";
+                                ast::print( stream, expr );
+                                stream << " Alternatives are:\n";
+                                print( stream, winners, 1 );
+                                SemanticError( expr->location, stream.str() );
+                        }
+                        else {
+                                stream << "No alternatives with satisfiable assertions for " << expr << "\n";
+                                for ( const auto& err : errors ) {
+                                        stream << err;
+                                }
+                                SemanticError( expr->location, stream.str() );
+                        }
+                }
+
+                auto oldsize = candidates.size();
+                candidates = std::move( pruned );
+
+                PRINT(
+                        std::cerr << "there are " << oldsize << " alternatives before elimination" << std::endl;
+                )
+                PRINT(
+                        std::cerr << "there are " << candidates.size() << " alternatives after elimination"
+                                << std::endl;
+                )
+        }
+
+        // adjust types after pruning so that types substituted by pruneAlternatives are correctly
+        // adjusted
+        if ( mode.adjust ) {
+                for ( CandidateRef & r : candidates ) {
+                        r->expr = ast::mutate_field(
+                                r->expr.get(), &ast::Expr::result,
+                                adjustExprType( r->expr->result, r->env, context.symtab ) );
+                }
+        }
+
+        // Central location to handle gcc extension keyword, etc. for all expressions
+        for ( CandidateRef & r : candidates ) {
+                if ( r->expr->extension != expr->extension ) {
+                        r->expr.get_and_mutate()->extension = expr->extension;
+                }
+        }
+}
+
+std::vector< CandidateFinder > CandidateFinder::findSubExprs(
+        const std::vector< ast::ptr< ast::Expr > > & xs
+) {
+        std::vector< CandidateFinder > out;
+
+        for ( const auto & x : xs ) {
+                out.emplace_back( context, env );
+                out.back().find( x, ResolvMode::withAdjustment() );
+
+                PRINT(
+                        std::cerr << "findSubExprs" << std::endl;
+                        print( std::cerr, out.back().candidates );
+                )
+        }
+
+        return out;
+}
+
 const ast::Expr * referenceToRvalueConversion( const ast::Expr * expr, Cost & cost ) {
         if ( expr->result.as< ast::ReferenceType >() ) {
@@ -64 +1936 @@
         return expr;
 }
-
-/// Unique identifier for matching expression resolutions to their requesting expression
-UniqueId globalResnSlot = 0;
 
 Cost computeConversionCost(
@@ -93 +1962 @@
 }
 
-namespace {
-        /// First index is which argument, second is which alternative, third is which exploded element
-        using ExplodedArgs_new = std::deque< std::vector< ExplodedArg > >;
-
-        /// Returns a list of alternatives with the minimum cost in the given list
-        CandidateList findMinCost( const CandidateList & candidates ) {
-                CandidateList out;
-                Cost minCost = Cost::infinity;
-                for ( const CandidateRef & r : candidates ) {
-                        if ( r->cost < minCost ) {
-                                minCost = r->cost;
-                                out.clear();
-                                out.emplace_back( r );
-                        } else if ( r->cost == minCost ) {
-                                out.emplace_back( r );
-                        }
-                }
-                return out;
-        }
-
-        /// Computes conversion cost for a given expression to a given type
-        const ast::Expr * computeExpressionConversionCost(
-                const ast::Expr * arg, const ast::Type * paramType, const ast::SymbolTable & symtab, const ast::TypeEnvironment & env, Cost & outCost
-        ) {
-                Cost convCost = computeConversionCost(
-                                arg->result, paramType, arg->get_lvalue(), symtab, env );
-                outCost += convCost;
-
-                // If there is a non-zero conversion cost, ignoring poly cost, then the expression requires
-                // conversion. Ignore poly cost for now, since this requires resolution of the cast to
-                // infer parameters and this does not currently work for the reason stated below
-                Cost tmpCost = convCost;
-                tmpCost.incPoly( -tmpCost.get_polyCost() );
-                if ( tmpCost != Cost::zero ) {
-                        ast::ptr< ast::Type > newType = paramType;
-                        env.apply( newType );
-                        return new ast::CastExpr{ arg, newType };
-
-                        // xxx - *should* be able to resolve this cast, but at the moment pointers are not
-                        // castable to zero_t, but are implicitly convertible. This is clearly inconsistent,
-                        // once this is fixed it should be possible to resolve the cast.
-                        // xxx - this isn't working, it appears because type1 (parameter) is seen as widenable,
-                        // but it shouldn't be because this makes the conversion from DT* to DT* since
-                        // commontype(zero_t, DT*) is DT*, rather than nothing
-
-                        // CandidateFinder finder{ symtab, env };
-                        // finder.find( arg, ResolvMode::withAdjustment() );
-                        // assertf( finder.candidates.size() > 0,
-                        //      "Somehow castable expression failed to find alternatives." );
-                        // assertf( finder.candidates.size() == 1,
-                        //      "Somehow got multiple alternatives for known cast expression." );
-                        // return finder.candidates.front()->expr;
-                }
-
-                return arg;
-        }
-
-        /// Computes conversion cost for a given candidate
-        Cost computeApplicationConversionCost(
-                CandidateRef cand, const ast::SymbolTable & symtab
-        ) {
-                auto appExpr = cand->expr.strict_as< ast::ApplicationExpr >();
-                auto pointer = appExpr->func->result.strict_as< ast::PointerType >();
-                auto function = pointer->base.strict_as< ast::FunctionType >();
-
-                Cost convCost = Cost::zero;
-                const auto & params = function->params;
-                auto param = params.begin();
-                auto & args = appExpr->args;
-
-                for ( unsigned i = 0; i < args.size(); ++i ) {
-                        const ast::Type * argType = args[i]->result;
-                        PRINT(
-                                std::cerr << "arg expression:" << std::endl;
-                                ast::print( std::cerr, args[i], 2 );
-                                std::cerr << "--- results are" << std::endl;
-                                ast::print( std::cerr, argType, 2 );
-                        )
-
-                        if ( param == params.end() ) {
-                                if ( function->isVarArgs ) {
-                                        convCost.incUnsafe();
-                                        PRINT( std::cerr << "end of params with varargs function: inc unsafe: "
-                                                << convCost << std::endl; ; )
-                                        // convert reference-typed expressions into value-typed expressions
-                                        cand->expr = ast::mutate_field_index(
-                                                appExpr, &ast::ApplicationExpr::args, i,
-                                                referenceToRvalueConversion( args[i], convCost ) );
-                                        continue;
-                                } else return Cost::infinity;
-                        }
-
-                        if ( auto def = args[i].as< ast::DefaultArgExpr >() ) {
-                                // Default arguments should be free - don't include conversion cost.
-                                // Unwrap them here because they are not relevant to the rest of the system
-                                cand->expr = ast::mutate_field_index(
-                                        appExpr, &ast::ApplicationExpr::args, i, def->expr );
-                                ++param;
-                                continue;
-                        }
-
-                        // mark conversion cost and also specialization cost of param type
-                        // const ast::Type * paramType = (*param)->get_type();
-                        cand->expr = ast::mutate_field_index(
-                                appExpr, &ast::ApplicationExpr::args, i,
-                                computeExpressionConversionCost(
-                                        args[i], *param, symtab, cand->env, convCost ) );
-                        convCost.decSpec( specCost( *param ) );
-                        ++param;  // can't be in for-loop update because of the continue
-                }
-
-                if ( param != params.end() ) return Cost::infinity;
-
-                // specialization cost of return types can't be accounted for directly, it disables
-                // otherwise-identical calls, like this example based on auto-newline in the I/O lib:
-                //
-                //   forall(otype OS) {
-                //     void ?|?(OS&, int);  // with newline
-                //     OS&  ?|?(OS&, int);  // no newline, always chosen due to more specialization
-                //   }
-
-                // mark type variable and specialization cost of forall clause
-                convCost.incVar( function->forall.size() );
-                convCost.decSpec( function->assertions.size() );
-
-                return convCost;
-        }
-
-        void makeUnifiableVars(
-                const ast::FunctionType * type, ast::OpenVarSet & unifiableVars,
-                ast::AssertionSet & need
-        ) {
-                for ( auto & tyvar : type->forall ) {
-                        unifiableVars[ *tyvar ] = ast::TypeData{ tyvar->base };
-                }
-                for ( auto & assn : type->assertions ) {
-                        need[ assn ].isUsed = true;
-                }
-        }
-
-        /// Gets a default value from an initializer, nullptr if not present
-        const ast::ConstantExpr * getDefaultValue( const ast::Init * init ) {
-                if ( auto si = dynamic_cast< const ast::SingleInit * >( init ) ) {
-                        if ( auto ce = si->value.as< ast::CastExpr >() ) {
-                                return ce->arg.as< ast::ConstantExpr >();
-                        } else {
-                                return si->value.as< ast::ConstantExpr >();
-                        }
-                }
-                return nullptr;
-        }
-
-        /// State to iteratively build a match of parameter expressions to arguments
-        struct ArgPack {
-                std::size_t parent;          ///< Index of parent pack
-                ast::ptr< ast::Expr > expr;  ///< The argument stored here
-                Cost cost;                   ///< The cost of this argument
-                ast::TypeEnvironment env;    ///< Environment for this pack
-                ast::AssertionSet need;      ///< Assertions outstanding for this pack
-                ast::AssertionSet have;      ///< Assertions found for this pack
-                ast::OpenVarSet open;        ///< Open variables for this pack
-                unsigned nextArg;            ///< Index of next argument in arguments list
-                unsigned tupleStart;         ///< Number of tuples that start at this index
-                unsigned nextExpl;           ///< Index of next exploded element
-                unsigned explAlt;            ///< Index of alternative for nextExpl > 0
-
-                ArgPack()
-                : parent( 0 ), expr(), cost( Cost::zero ), env(), need(), have(), open(), nextArg( 0 ),
-                  tupleStart( 0 ), nextExpl( 0 ), explAlt( 0 ) {}
-
-                ArgPack(
-                        const ast::TypeEnvironment & env, const ast::AssertionSet & need,
-                        const ast::AssertionSet & have, const ast::OpenVarSet & open )
-                : parent( 0 ), expr(), cost( Cost::zero ), env( env ), need( need ), have( have ),
-                  open( open ), nextArg( 0 ), tupleStart( 0 ), nextExpl( 0 ), explAlt( 0 ) {}
-
-                ArgPack(
-                        std::size_t parent, const ast::Expr * expr, ast::TypeEnvironment && env,
-                        ast::AssertionSet && need, ast::AssertionSet && have, ast::OpenVarSet && open,
-                        unsigned nextArg, unsigned tupleStart = 0, Cost cost = Cost::zero,
-                        unsigned nextExpl = 0, unsigned explAlt = 0 )
-                : parent(parent), expr( expr ), cost( cost ), env( std::move( env ) ), need( std::move( need ) ),
-                  have( std::move( have ) ), open( std::move( open ) ), nextArg( nextArg ), tupleStart( tupleStart ),
-                  nextExpl( nextExpl ), explAlt( explAlt ) {}
-
-                ArgPack(
-                        const ArgPack & o, ast::TypeEnvironment && env, ast::AssertionSet && need,
-                        ast::AssertionSet && have, ast::OpenVarSet && open, unsigned nextArg, Cost added )
-                : parent( o.parent ), expr( o.expr ), cost( o.cost + added ), env( std::move( env ) ),
-                  need( std::move( need ) ), have( std::move( have ) ), open( std::move( open ) ), nextArg( nextArg ),
-                  tupleStart( o.tupleStart ), nextExpl( 0 ), explAlt( 0 ) {}
-
-                /// true if this pack is in the middle of an exploded argument
-                bool hasExpl() const { return nextExpl > 0; }
-
-                /// Gets the list of exploded candidates for this pack
-                const ExplodedArg & getExpl( const ExplodedArgs_new & args ) const {
-                        return args[ nextArg-1 ][ explAlt ];
-                }
-
-                /// Ends a tuple expression, consolidating the appropriate args
-                void endTuple( const std::vector< ArgPack > & packs ) {
-                        // add all expressions in tuple to list, summing cost
-                        std::deque< const ast::Expr * > exprs;
-                        const ArgPack * pack = this;
-                        if ( expr ) { exprs.emplace_front( expr ); }
-                        while ( pack->tupleStart == 0 ) {
-                                pack = &packs[pack->parent];
-                                exprs.emplace_front( pack->expr );
-                                cost += pack->cost;
-                        }
-                        // reset pack to appropriate tuple
-                        std::vector< ast::ptr< ast::Expr > > exprv( exprs.begin(), exprs.end() );
-                        expr = new ast::TupleExpr{ expr->location, std::move( exprv ) };
-                        tupleStart = pack->tupleStart - 1;
-                        parent = pack->parent;
-                }
-        };
-
-        /// Instantiates an argument to match a parameter, returns false if no matching results left
-        bool instantiateArgument(
-                const ast::Type * paramType, const ast::Init * init, const ExplodedArgs_new & args,
-                std::vector< ArgPack > & results, std::size_t & genStart, const ast::SymbolTable & symtab,
-                unsigned nTuples = 0
-        ) {
-                if ( auto tupleType = dynamic_cast< const ast::TupleType * >( paramType ) ) {
-                        // paramType is a TupleType -- group args into a TupleExpr
-                        ++nTuples;
-                        for ( const ast::Type * type : *tupleType ) {
-                                // xxx - dropping initializer changes behaviour from previous, but seems correct
-                                // ^^^ need to handle the case where a tuple has a default argument
-                                if ( ! instantiateArgument(
-                                        type, nullptr, args, results, genStart, symtab, nTuples ) ) return false;
-                                nTuples = 0;
-                        }
-                        // re-constitute tuples for final generation
-                        for ( auto i = genStart; i < results.size(); ++i ) {
-                                results[i].endTuple( results );
-                        }
-                        return true;
-                } else if ( const ast::TypeInstType * ttype = Tuples::isTtype( paramType ) ) {
-                        // paramType is a ttype, consumes all remaining arguments
-
-                        // completed tuples; will be spliced to end of results to finish
-                        std::vector< ArgPack > finalResults{};
-
-                        // iterate until all results completed
-                        std::size_t genEnd;
-                        ++nTuples;
-                        do {
-                                genEnd = results.size();
-
-                                // add another argument to results
-                                for ( std::size_t i = genStart; i < genEnd; ++i ) {
-                                        unsigned nextArg = results[i].nextArg;
-
-                                        // use next element of exploded tuple if present
-                                        if ( results[i].hasExpl() ) {
-                                                const ExplodedArg & expl = results[i].getExpl( args );
-
-                                                unsigned nextExpl = results[i].nextExpl + 1;
-                                                if ( nextExpl == expl.exprs.size() ) { nextExpl = 0; }
-
-                                                results.emplace_back(
-                                                        i, expl.exprs[ results[i].nextExpl ], copy( results[i].env ),
-                                                        copy( results[i].need ), copy( results[i].have ),
-                                                        copy( results[i].open ), nextArg, nTuples, Cost::zero, nextExpl,
-                                                        results[i].explAlt );
-
-                                                continue;
-                                        }
-
-                                        // finish result when out of arguments
-                                        if ( nextArg >= args.size() ) {
-                                                ArgPack newResult{
-                                                        results[i].env, results[i].need, results[i].have, results[i].open };
-                                                newResult.nextArg = nextArg;
-                                                const ast::Type * argType = nullptr;
-
-                                                if ( nTuples > 0 || ! results[i].expr ) {
-                                                        // first iteration or no expression to clone,
-                                                        // push empty tuple expression
-                                                        newResult.parent = i;
-                                                        newResult.expr = new ast::TupleExpr{ CodeLocation{}, {} };
-                                                        argType = newResult.expr->result;
-                                                } else {
-                                                        // clone result to collect tuple
-                                                        newResult.parent = results[i].parent;
-                                                        newResult.cost = results[i].cost;
-                                                        newResult.tupleStart = results[i].tupleStart;
-                                                        newResult.expr = results[i].expr;
-                                                        argType = newResult.expr->result;
-
-                                                        if ( results[i].tupleStart > 0 && Tuples::isTtype( argType ) ) {
-                                                                // the case where a ttype value is passed directly is special,
-                                                                // e.g. for argument forwarding purposes
-                                                                // xxx - what if passing multiple arguments, last of which is
-                                                                //       ttype?
-                                                                // xxx - what would happen if unify was changed so that unifying
-                                                                //       tuple
-                                                                // types flattened both before unifying lists? then pass in
-                                                                // TupleType (ttype) below.
-                                                                --newResult.tupleStart;
-                                                        } else {
-                                                                // collapse leftover arguments into tuple
-                                                                newResult.endTuple( results );
-                                                                argType = newResult.expr->result;
-                                                        }
-                                                }
-
-                                                // check unification for ttype before adding to final
-                                                if (
-                                                        unify(
-                                                                ttype, argType, newResult.env, newResult.need, newResult.have,
-                                                                newResult.open, symtab )
-                                                ) {
-                                                        finalResults.emplace_back( std::move( newResult ) );
-                                                }
-
-                                                continue;
-                                        }
-
-                                        // add each possible next argument
-                                        for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
-                                                const ExplodedArg & expl = args[nextArg][j];
-
-                                                // fresh copies of parent parameters for this iteration
-                                                ast::TypeEnvironment env = results[i].env;
-                                                ast::OpenVarSet open = results[i].open;
-
-                                                env.addActual( expl.env, open );
-
-                                                // skip empty tuple arguments by (nearly) cloning parent into next gen
-                                                if ( expl.exprs.empty() ) {
-                                                        results.emplace_back(
-                                                                results[i], std::move( env ), copy( results[i].need ),
-                                                                copy( results[i].have ), std::move( open ), nextArg + 1, expl.cost );
-
-                                                        continue;
-                                                }
-
-                                                // add new result
-                                                results.emplace_back(
-                                                        i, expl.exprs.front(), std::move( env ), copy( results[i].need ),
-                                                        copy( results[i].have ), std::move( open ), nextArg + 1, nTuples,
-                                                        expl.cost, expl.exprs.size() == 1 ? 0 : 1, j );
-                                        }
-                                }
-
-                                // reset for next round
-                                genStart = genEnd;
-                                nTuples = 0;
-                        } while ( genEnd != results.size() );
-
-                        // splice final results onto results
-                        for ( std::size_t i = 0; i < finalResults.size(); ++i ) {
-                                results.emplace_back( std::move( finalResults[i] ) );
-                        }
-                        return ! finalResults.empty();
-                }
-
-                // iterate each current subresult
-                std::size_t genEnd = results.size();
-                for ( std::size_t i = genStart; i < genEnd; ++i ) {
-                        unsigned nextArg = results[i].nextArg;
-
-                        // use remainder of exploded tuple if present
-                        if ( results[i].hasExpl() ) {
-                                const ExplodedArg & expl = results[i].getExpl( args );
-                                const ast::Expr * expr = expl.exprs[ results[i].nextExpl ];
-
-                                ast::TypeEnvironment env = results[i].env;
-                                ast::AssertionSet need = results[i].need, have = results[i].have;
-                                ast::OpenVarSet open = results[i].open;
-
-                                const ast::Type * argType = expr->result;
-
-                                PRINT(
-                                        std::cerr << "param type is ";
-                                        ast::print( std::cerr, paramType );
-                                        std::cerr << std::endl << "arg type is ";
-                                        ast::print( std::cerr, argType );
-                                        std::cerr << std::endl;
-                                )
-
-                                if ( unify( paramType, argType, env, need, have, open, symtab ) ) {
-                                        unsigned nextExpl = results[i].nextExpl + 1;
-                                        if ( nextExpl == expl.exprs.size() ) { nextExpl = 0; }
-
-                                        results.emplace_back(
-                                                i, expr, std::move( env ), std::move( need ), std::move( have ), std::move( open ), nextArg,
-                                                nTuples, Cost::zero, nextExpl, results[i].explAlt );
-                                }
-
-                                continue;
-                        }
-
-                        // use default initializers if out of arguments
-                        if ( nextArg >= args.size() ) {
-                                if ( const ast::ConstantExpr * cnst = getDefaultValue( init ) ) {
-                                        ast::TypeEnvironment env = results[i].env;
-                                        ast::AssertionSet need = results[i].need, have = results[i].have;
-                                        ast::OpenVarSet open = results[i].open;
-
-                                        if ( unify( paramType, cnst->result, env, need, have, open, symtab ) ) {
-                                                results.emplace_back(
-                                                        i, new ast::DefaultArgExpr{ cnst->location, cnst }, std::move( env ),
-                                                        std::move( need ), std::move( have ), std::move( open ), nextArg, nTuples );
-                                        }
-                                }
-
-                                continue;
-                        }
-
-                        // Check each possible next argument
-                        for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
-                                const ExplodedArg & expl = args[nextArg][j];
-
-                                // fresh copies of parent parameters for this iteration
-                                ast::TypeEnvironment env = results[i].env;
-                                ast::AssertionSet need = results[i].need, have = results[i].have;
-                                ast::OpenVarSet open = results[i].open;
-
-                                env.addActual( expl.env, open );
-
-                                // skip empty tuple arguments by (nearly) cloning parent into next gen
-                                if ( expl.exprs.empty() ) {
-                                        results.emplace_back(
-                                                results[i], std::move( env ), std::move( need ), std::move( have ), std::move( open ),
-                                                nextArg + 1, expl.cost );
-
-                                        continue;
-                                }
-
-                                // consider only first exploded arg
-                                const ast::Expr * expr = expl.exprs.front();
-                                const ast::Type * argType = expr->result;
-
-                                PRINT(
-                                        std::cerr << "param type is ";
-                                        ast::print( std::cerr, paramType );
-                                        std::cerr << std::endl << "arg type is ";
-                                        ast::print( std::cerr, argType );
-                                        std::cerr << std::endl;
-                                )
-
-                                // attempt to unify types
-                                if ( unify( paramType, argType, env, need, have, open, symtab ) ) {
-                                        // add new result
-                                        results.emplace_back(
-                                                i, expr, std::move( env ), std::move( need ), std::move( have ), std::move( open ),
-                                                nextArg + 1, nTuples, expl.cost, expl.exprs.size() == 1 ? 0 : 1, j );
-                                }
-                        }
-                }
-
-                // reset for next parameter
-                genStart = genEnd;
-
-                return genEnd != results.size();  // were any new results added?
-        }
-
-        /// Generate a cast expression from `arg` to `toType`
-        const ast::Expr * restructureCast(
-                ast::ptr< ast::Expr > & arg, const ast::Type * toType, ast::GeneratedFlag isGenerated = ast::GeneratedCast
-        ) {
-                if (
-                        arg->result->size() > 1
-                        && ! toType->isVoid()
-                        && ! dynamic_cast< const ast::ReferenceType * >( toType )
-                ) {
-                        // Argument is a tuple and the target type is neither void nor a reference. Cast each
-                        // member of the tuple to its corresponding target type, producing the tuple of those
-                        // cast expressions. If there are more components of the tuple than components in the
-                        // target type, then excess components do not come out in the result expression (but
-                        // UniqueExpr ensures that the side effects will still be produced)
-                        if ( Tuples::maybeImpureIgnoreUnique( arg ) ) {
-                                // expressions which may contain side effects require a single unique instance of
-                                // the expression
-                                arg = new ast::UniqueExpr{ arg->location, arg };
-                        }
-                        std::vector< ast::ptr< ast::Expr > > components;
-                        for ( unsigned i = 0; i < toType->size(); ++i ) {
-                                // cast each component
-                                ast::ptr< ast::Expr > idx = new ast::TupleIndexExpr{ arg->location, arg, i };
-                                components.emplace_back(
-                                        restructureCast( idx, toType->getComponent( i ), isGenerated ) );
-                        }
-                        return new ast::TupleExpr{ arg->location, std::move( components ) };
-                } else {
-                        // handle normally
-                        return new ast::CastExpr{ arg->location, arg, toType, isGenerated };
-                }
-        }
-
-        /// Gets the name from an untyped member expression (must be NameExpr)
-        const std::string & getMemberName( const ast::UntypedMemberExpr * memberExpr ) {
-                if ( memberExpr->member.as< ast::ConstantExpr >() ) {
-                        SemanticError( memberExpr, "Indexed access to struct fields unsupported: " );
-                }
-
-                return memberExpr->member.strict_as< ast::NameExpr >()->name;
-        }
-
-        /// Actually visits expressions to find their candidate interpretations
-        class Finder final : public ast::WithShortCircuiting {
-                const ResolveContext & context;
-                const ast::SymbolTable & symtab;
-        public:
-                // static size_t traceId;
-                CandidateFinder & selfFinder;
-                CandidateList & candidates;
-                const ast::TypeEnvironment & tenv;
-                ast::ptr< ast::Type > & targetType;
-
-                enum Errors {
-                        NotFound,
-                        NoMatch,
-                        ArgsToFew,
-                        ArgsToMany,
-                        RetsToFew,
-                        RetsToMany,
-                        NoReason
-                };
-
-                struct {
-                        Errors code = NotFound;
-                } reason;
-
-                Finder( CandidateFinder & f )
-                : context( f.context ), symtab( context.symtab ), selfFinder( f ),
-                  candidates( f.candidates ), tenv( f.env ), targetType( f.targetType ) {}
-
-                void previsit( const ast::Node * ) { visit_children = false; }
-
-                /// Convenience to add candidate to list
-                template<typename... Args>
-                void addCandidate( Args &&... args ) {
-                        candidates.emplace_back( new Candidate{ std::forward<Args>( args )... } );
-                        reason.code = NoReason;
-                }
-
-                void postvisit( const ast::ApplicationExpr * applicationExpr ) {
-                        addCandidate( applicationExpr, tenv );
-                }
-
-                /// Set up candidate assertions for inference
-                void inferParameters( CandidateRef & newCand, CandidateList & out ) {
-                        // Set need bindings for any unbound assertions
-                        UniqueId crntResnSlot = 0; // matching ID for this expression's assertions
-                        for ( auto & assn : newCand->need ) {
-                                // skip already-matched assertions
-                                if ( assn.second.resnSlot != 0 ) continue;
-                                // assign slot for expression if needed
-                                if ( crntResnSlot == 0 ) { crntResnSlot = ++globalResnSlot; }
-                                // fix slot to assertion
-                                assn.second.resnSlot = crntResnSlot;
-                        }
-                        // pair slot to expression
-                        if ( crntResnSlot != 0 ) {
-                                newCand->expr.get_and_mutate()->inferred.resnSlots().emplace_back( crntResnSlot );
-                        }
-
-                        // add to output list; assertion satisfaction will occur later
-                        out.emplace_back( newCand );
-                }
-
-                /// Completes a function candidate with arguments located
-                void validateFunctionCandidate(
-                        const CandidateRef & func, ArgPack & result, const std::vector< ArgPack > & results,
-                        CandidateList & out
-                ) {
-                        ast::ApplicationExpr * appExpr =
-                                new ast::ApplicationExpr{ func->expr->location, func->expr };
-                        // sum cost and accumulate arguments
-                        std::deque< const ast::Expr * > args;
-                        Cost cost = func->cost;
-                        const ArgPack * pack = &result;
-                        while ( pack->expr ) {
-                                args.emplace_front( pack->expr );
-                                cost += pack->cost;
-                                pack = &results[pack->parent];
-                        }
-                        std::vector< ast::ptr< ast::Expr > > vargs( args.begin(), args.end() );
-                        appExpr->args = std::move( vargs );
-                        // build and validate new candidate
-                        auto newCand =
-                                std::make_shared<Candidate>( appExpr, result.env, result.open, result.need, cost );
-                        PRINT(
-                                std::cerr << "instantiate function success: " << appExpr << std::endl;
-                                std::cerr << "need assertions:" << std::endl;
-                                ast::print( std::cerr, result.need, 2 );
-                        )
-                        inferParameters( newCand, out );
-                }
-
-                /// Builds a list of candidates for a function, storing them in out
-                void makeFunctionCandidates(
-                        const CandidateRef & func, const ast::FunctionType * funcType,
-                        const ExplodedArgs_new & args, CandidateList & out
-                ) {
-                        ast::OpenVarSet funcOpen;
-                        ast::AssertionSet funcNeed, funcHave;
-                        ast::TypeEnvironment funcEnv{ func->env };
-                        makeUnifiableVars( funcType, funcOpen, funcNeed );
-                        // add all type variables as open variables now so that those not used in the
-                        // parameter list are still considered open
-                        funcEnv.add( funcType->forall );
-
-                        if ( targetType && ! targetType->isVoid() && ! funcType->returns.empty() ) {
-                                // attempt to narrow based on expected target type
-                                const ast::Type * returnType = funcType->returns.front();
-                                if ( ! unify(
-                                        returnType, targetType, funcEnv, funcNeed, funcHave, funcOpen, symtab )
-                                ) {
-                                        // unification failed, do not pursue this candidate
-                                        return;
-                                }
-                        }
-
-                        // iteratively build matches, one parameter at a time
-                        std::vector< ArgPack > results;
-                        results.emplace_back( funcEnv, funcNeed, funcHave, funcOpen );
-                        std::size_t genStart = 0;
-
-                        // xxx - how to handle default arg after change to ftype representation?
-                        if (const ast::VariableExpr * varExpr = func->expr.as<ast::VariableExpr>()) {
-                                if (const ast::FunctionDecl * funcDecl = varExpr->var.as<ast::FunctionDecl>()) {
-                                        // function may have default args only if directly calling by name
-                                        // must use types on candidate however, due to RenameVars substitution
-                                        auto nParams = funcType->params.size();
-
-                                        for (size_t i=0; i<nParams; ++i) {
-                                                auto obj = funcDecl->params[i].strict_as<ast::ObjectDecl>();
-                                                if (!instantiateArgument(
-                                                        funcType->params[i], obj->init, args, results, genStart, symtab)) return;
-                                        }
-                                        goto endMatch;
-                                }
-                        }
-                        for ( const auto & param : funcType->params ) {
-                                // Try adding the arguments corresponding to the current parameter to the existing
-                                // matches
-                                // no default args for indirect calls
-                                if ( ! instantiateArgument(
-                                        param, nullptr, args, results, genStart, symtab ) ) return;
-                        }
-
-                        endMatch:
-                        if ( funcType->isVarArgs ) {
-                                // append any unused arguments to vararg pack
-                                std::size_t genEnd;
-                                do {
-                                        genEnd = results.size();
-
-                                        // iterate results
-                                        for ( std::size_t i = genStart; i < genEnd; ++i ) {
-                                                unsigned nextArg = results[i].nextArg;
-
-                                                // use remainder of exploded tuple if present
-                                                if ( results[i].hasExpl() ) {
-                                                        const ExplodedArg & expl = results[i].getExpl( args );
-
-                                                        unsigned nextExpl = results[i].nextExpl + 1;
-                                                        if ( nextExpl == expl.exprs.size() ) { nextExpl = 0; }
-
-                                                        results.emplace_back(
-                                                                i, expl.exprs[ results[i].nextExpl ], copy( results[i].env ),
-                                                                copy( results[i].need ), copy( results[i].have ),
-                                                                copy( results[i].open ), nextArg, 0, Cost::zero, nextExpl,
-                                                                results[i].explAlt );
-
-                                                        continue;
-                                                }
-
-                                                // finish result when out of arguments
-                                                if ( nextArg >= args.size() ) {
-                                                        validateFunctionCandidate( func, results[i], results, out );
-
-                                                        continue;
-                                                }
-
-                                                // add each possible next argument
-                                                for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
-                                                        const ExplodedArg & expl = args[nextArg][j];
-
-                                                        // fresh copies of parent parameters for this iteration
-                                                        ast::TypeEnvironment env = results[i].env;
-                                                        ast::OpenVarSet open = results[i].open;
-
-                                                        env.addActual( expl.env, open );
-
-                                                        // skip empty tuple arguments by (nearly) cloning parent into next gen
-                                                        if ( expl.exprs.empty() ) {
-                                                                results.emplace_back(
-                                                                        results[i], std::move( env ), copy( results[i].need ),
-                                                                        copy( results[i].have ), std::move( open ), nextArg + 1,
-                                                                        expl.cost );
-
-                                                                continue;
-                                                        }
-
-                                                        // add new result
-                                                        results.emplace_back(
-                                                                i, expl.exprs.front(), std::move( env ), copy( results[i].need ),
-                                                                copy( results[i].have ), std::move( open ), nextArg + 1, 0, expl.cost,
-                                                                expl.exprs.size() == 1 ? 0 : 1, j );
-                                                }
-                                        }
-
-                                        genStart = genEnd;
-                                } while( genEnd != results.size() );
-                        } else {
-                                // filter out the results that don't use all the arguments
-                                for ( std::size_t i = genStart; i < results.size(); ++i ) {
-                                        ArgPack & result = results[i];
-                                        if ( ! result.hasExpl() && result.nextArg >= args.size() ) {
-                                                validateFunctionCandidate( func, result, results, out );
-                                        }
-                                }
-                        }
-                }
-
-                /// Adds implicit struct-conversions to the alternative list
-                void addAnonConversions( const CandidateRef & cand ) {
-                        // adds anonymous member interpretations whenever an aggregate value type is seen.
-                        // it's okay for the aggregate expression to have reference type -- cast it to the
-                        // base type to treat the aggregate as the referenced value
-                        ast::ptr< ast::Expr > aggrExpr( cand->expr );
-                        ast::ptr< ast::Type > & aggrType = aggrExpr.get_and_mutate()->result;
-                        cand->env.apply( aggrType );
-
-                        if ( aggrType.as< ast::ReferenceType >() ) {
-                                aggrExpr = new ast::CastExpr{ aggrExpr, aggrType->stripReferences() };
-                        }
-
-                        if ( auto structInst = aggrExpr->result.as< ast::StructInstType >() ) {
-                                addAggMembers( structInst, aggrExpr, *cand, Cost::safe, "" );
-                        } else if ( auto unionInst = aggrExpr->result.as< ast::UnionInstType >() ) {
-                                addAggMembers( unionInst, aggrExpr, *cand, Cost::safe, "" );
-                        }
-                }
-
-                /// Adds aggregate member interpretations
-                void addAggMembers(
-                        const ast::BaseInstType * aggrInst, const ast::Expr * expr,
-                        const Candidate & cand, const Cost & addedCost, const std::string & name
-                ) {
-                        for ( const ast::Decl * decl : aggrInst->lookup( name ) ) {
-                                auto dwt = strict_dynamic_cast< const ast::DeclWithType * >( decl );
-                                CandidateRef newCand = std::make_shared<Candidate>(
-                                        cand, new ast::MemberExpr{ expr->location, dwt, expr }, addedCost );
-                                // add anonymous member interpretations whenever an aggregate value type is seen
-                                // as a member expression
-                                addAnonConversions( newCand );
-                                candidates.emplace_back( std::move( newCand ) );
-                        }
-                }
-
-                /// Adds tuple member interpretations
-                void addTupleMembers(
-                        const ast::TupleType * tupleType, const ast::Expr * expr, const Candidate & cand,
-                        const Cost & addedCost, const ast::Expr * member
-                ) {
-                        if ( auto constantExpr = dynamic_cast< const ast::ConstantExpr * >( member ) ) {
-                                // get the value of the constant expression as an int, must be between 0 and the
-                                // length of the tuple to have meaning
-                                long long val = constantExpr->intValue();
-                                if ( val >= 0 && (unsigned long long)val < tupleType->size() ) {
-                                        addCandidate(
-                                                cand, new ast::TupleIndexExpr{ expr->location, expr, (unsigned)val },
-                                                addedCost );
-                                }
-                        }
-                }
-
-                void postvisit( const ast::UntypedExpr * untypedExpr ) {
-                        std::vector< CandidateFinder > argCandidates =
-                                selfFinder.findSubExprs( untypedExpr->args );
-
-                        // take care of possible tuple assignments
-                        // if not tuple assignment, handled as normal function call
-                        Tuples::handleTupleAssignment( selfFinder, untypedExpr, argCandidates );
-
-                        CandidateFinder funcFinder( context, tenv );
-                        if (auto nameExpr = untypedExpr->func.as<ast::NameExpr>()) {
-                                auto kind = ast::SymbolTable::getSpecialFunctionKind(nameExpr->name);
-                                if (kind != ast::SymbolTable::SpecialFunctionKind::NUMBER_OF_KINDS) {
-                                        assertf(!argCandidates.empty(), "special function call without argument");
-                                        for (auto & firstArgCand: argCandidates[0]) {
-                                                ast::ptr<ast::Type> argType = firstArgCand->expr->result;
-                                                firstArgCand->env.apply(argType);
-                                                // strip references
-                                                // xxx - is this correct?
-                                                while (argType.as<ast::ReferenceType>()) argType = argType.as<ast::ReferenceType>()->base;
-
-                                                // convert 1-tuple to plain type
-                                                if (auto tuple = argType.as<ast::TupleType>()) {
-                                                        if (tuple->size() == 1) {
-                                                                argType = tuple->types[0];
-                                                        }
-                                                }
-
-                                                // if argType is an unbound type parameter, all special functions need to be searched.
-                                                if (isUnboundType(argType)) {
-                                                        funcFinder.otypeKeys.clear();
-                                                        break;
-                                                }
-
-                                                if (argType.as<ast::PointerType>()) funcFinder.otypeKeys.insert(Mangle::Encoding::pointer);                                             
-                                                // else if (const ast::EnumInstType * enumInst = argType.as<ast::EnumInstType>()) {
-                                                //      const ast::EnumDecl * enumDecl = enumInst->base; // Here
-                                                //      if ( const ast::Type* enumType = enumDecl->base ) {
-                                                //              // instance of enum (T) is a instance of type (T) 
-                                                //              funcFinder.otypeKeys.insert(Mangle::mangle(enumType, Mangle::NoGenericParams | Mangle::Type));
-                                                //      } else {
-                                                //              // instance of an untyped enum is techically int
-                                                //              funcFinder.otypeKeys.insert(Mangle::mangle(enumDecl, Mangle::NoGenericParams | Mangle::Type));
-                                                //      }
-                                                // }
-                                                else funcFinder.otypeKeys.insert(Mangle::mangle(argType, Mangle::NoGenericParams | Mangle::Type));
-                                        }
-                                }
-                        }
-                        // if candidates are already produced, do not fail
-                        // xxx - is it possible that handleTupleAssignment and main finder both produce candidates?
-                        // this means there exists ctor/assign functions with a tuple as first parameter.
-                        ResolvMode mode = {
-                                true, // adjust
-                                !untypedExpr->func.as<ast::NameExpr>(), // prune if not calling by name
-                                selfFinder.candidates.empty() // failfast if other options are not found
-                        };
-                        funcFinder.find( untypedExpr->func, mode );
-                        // short-circuit if no candidates
-                        // if ( funcFinder.candidates.empty() ) return;
-
-                        reason.code = NoMatch;
-
-                        // find function operators
-                        ast::ptr< ast::Expr > opExpr = new ast::NameExpr{ untypedExpr->location, "?()" }; // ??? why not ?{}
-                        CandidateFinder opFinder( context, tenv );
-                        // okay if there aren't any function operations
-                        opFinder.find( opExpr, ResolvMode::withoutFailFast() );
-                        PRINT(
-                                std::cerr << "known function ops:" << std::endl;
-                                print( std::cerr, opFinder.candidates, 1 );
-                        )
-
-                        // pre-explode arguments
-                        ExplodedArgs_new argExpansions;
-                        for ( const CandidateFinder & args : argCandidates ) {
-                                argExpansions.emplace_back();
-                                auto & argE = argExpansions.back();
-                                for ( const CandidateRef & arg : args ) { argE.emplace_back( *arg, symtab ); }
-                        }
-
-                        // Find function matches
-                        CandidateList found;
-                        SemanticErrorException errors;
-                        for ( CandidateRef & func : funcFinder ) {
-                                try {
-                                        PRINT(
-                                                std::cerr << "working on alternative:" << std::endl;
-                                                print( std::cerr, *func, 2 );
-                                        )
-
-                                        // check if the type is a pointer to function
-                                        const ast::Type * funcResult = func->expr->result->stripReferences();
-                                        if ( auto pointer = dynamic_cast< const ast::PointerType * >( funcResult ) ) {
-                                                if ( auto function = pointer->base.as< ast::FunctionType >() ) {
-                                                        CandidateRef newFunc{ new Candidate{ *func } };
-                                                        newFunc->expr =
-                                                                referenceToRvalueConversion( newFunc->expr, newFunc->cost );
-                                                        makeFunctionCandidates( newFunc, function, argExpansions, found );
-                                                }
-                                        } else if (
-                                                auto inst = dynamic_cast< const ast::TypeInstType * >( funcResult )
-                                        ) {
-                                                if ( const ast::EqvClass * clz = func->env.lookup( *inst ) ) {
-                                                        if ( auto function = clz->bound.as< ast::FunctionType >() ) {
-                                                                CandidateRef newFunc{ new Candidate{ *func } };
-                                                                newFunc->expr =
-                                                                        referenceToRvalueConversion( newFunc->expr, newFunc->cost );
-                                                                makeFunctionCandidates( newFunc, function, argExpansions, found );
-                                                        }
-                                                }
-                                        }
-                                } catch ( SemanticErrorException & e ) { errors.append( e ); }
-                        }
-
-                        // Find matches on function operators `?()`
-                        if ( ! opFinder.candidates.empty() ) {
-                                // add exploded function alternatives to front of argument list
-                                std::vector< ExplodedArg > funcE;
-                                funcE.reserve( funcFinder.candidates.size() );
-                                for ( const CandidateRef & func : funcFinder ) {
-                                        funcE.emplace_back( *func, symtab );
-                                }
-                                argExpansions.emplace_front( std::move( funcE ) );
-
-                                for ( const CandidateRef & op : opFinder ) {
-                                        try {
-                                                // check if type is pointer-to-function
-                                                const ast::Type * opResult = op->expr->result->stripReferences();
-                                                if ( auto pointer = dynamic_cast< const ast::PointerType * >( opResult ) ) {
-                                                        if ( auto function = pointer->base.as< ast::FunctionType >() ) {
-                                                                CandidateRef newOp{ new Candidate{ *op} };
-                                                                newOp->expr =
-                                                                        referenceToRvalueConversion( newOp->expr, newOp->cost );
-                                                                makeFunctionCandidates( newOp, function, argExpansions, found );
-                                                        }
-                                                }
-                                        } catch ( SemanticErrorException & e ) { errors.append( e ); }
-                                }
-                        }
-
-                        // Implement SFINAE; resolution errors are only errors if there aren't any non-error
-                        // candidates
-                        if ( found.empty() && ! errors.isEmpty() ) { throw errors; }
-
-                        // Compute conversion costs
-                        for ( CandidateRef & withFunc : found ) {
-                                Cost cvtCost = computeApplicationConversionCost( withFunc, symtab );
-
-                                PRINT(
-                                        auto appExpr = withFunc->expr.strict_as< ast::ApplicationExpr >();
-                                        auto pointer = appExpr->func->result.strict_as< ast::PointerType >();
-                                        auto function = pointer->base.strict_as< ast::FunctionType >();
-
-                                        std::cerr << "Case +++++++++++++ " << appExpr->func << std::endl;
-                                        std::cerr << "parameters are:" << std::endl;
-                                        ast::printAll( std::cerr, function->params, 2 );
-                                        std::cerr << "arguments are:" << std::endl;
-                                        ast::printAll( std::cerr, appExpr->args, 2 );
-                                        std::cerr << "bindings are:" << std::endl;
-                                        ast::print( std::cerr, withFunc->env, 2 );
-                                        std::cerr << "cost is: " << withFunc->cost << std::endl;
-                                        std::cerr << "cost of conversion is:" << cvtCost << std::endl;
-                                )
-
-                                if ( cvtCost != Cost::infinity ) {
-                                        withFunc->cvtCost = cvtCost;
-                                        candidates.emplace_back( std::move( withFunc ) );
-                                }
-                        }
-                        found = std::move( candidates );
-
-                        // use a new list so that candidates are not examined by addAnonConversions twice
-                        CandidateList winners = findMinCost( found );
-                        promoteCvtCost( winners );
-
-                        // function may return a struct/union value, in which case we need to add candidates
-                        // for implicit conversions to each of the anonymous members, which must happen after
-                        // `findMinCost`, since anon conversions are never the cheapest
-                        for ( const CandidateRef & c : winners ) {
-                                addAnonConversions( c );
-                        }
-                        spliceBegin( candidates, winners );
-
-                        if ( candidates.empty() && targetType && ! targetType->isVoid() ) {
-                                // If resolution is unsuccessful with a target type, try again without, since it
-                                // will sometimes succeed when it wouldn't with a target type binding.
-                                // For example:
-                                //   forall( otype T ) T & ?[]( T *, ptrdiff_t );
-                                //   const char * x = "hello world";
-                                //   unsigned char ch = x[0];
-                                // Fails with simple return type binding (xxx -- check this!) as follows:
-                                // * T is bound to unsigned char
-                                // * (x: const char *) is unified with unsigned char *, which fails
-                                // xxx -- fix this better
-                                targetType = nullptr;
-                                postvisit( untypedExpr );
-                        }
-                }
-
-                /// true if expression is an lvalue
-                static bool isLvalue( const ast::Expr * x ) {
-                        return x->result && ( x->get_lvalue() || x->result.as< ast::ReferenceType >() );
-                }
-
-                void postvisit( const ast::AddressExpr * addressExpr ) {
-                        CandidateFinder finder( context, tenv );
-                        finder.find( addressExpr->arg );
-
-                        if( finder.candidates.empty() ) return;
-
-                        reason.code = NoMatch;
-
-                        for ( CandidateRef & r : finder.candidates ) {
-                                if ( ! isLvalue( r->expr ) ) continue;
-                                addCandidate( *r, new ast::AddressExpr{ addressExpr->location, r->expr } );
-                        }
-                }
-
-                void postvisit( const ast::LabelAddressExpr * labelExpr ) {
-                        addCandidate( labelExpr, tenv );
-                }
-
-                void postvisit( const ast::CastExpr * castExpr ) {
-                        ast::ptr< ast::Type > toType = castExpr->result;
-                        assert( toType );
-                        toType = resolveTypeof( toType, context );
-                        toType = adjustExprType( toType, tenv, symtab );
-
-                        CandidateFinder finder( context, tenv, toType );
-                        finder.find( castExpr->arg, ResolvMode::withAdjustment() );
-
-                        if( !finder.candidates.empty() ) reason.code = NoMatch;
-
-                        CandidateList matches;
-                        for ( CandidateRef & cand : finder.candidates ) {
-                                ast::AssertionSet need( cand->need.begin(), cand->need.end() ), have;
-                                ast::OpenVarSet open( cand->open );
-
-                                cand->env.extractOpenVars( open );
-
-                                // It is possible that a cast can throw away some values in a multiply-valued
-                                // expression, e.g. cast-to-void, one value to zero. Figure out the prefix of the
-                                // subexpression results that are cast directly. The candidate is invalid if it
-                                // has fewer results than there are types to cast to.
-                                int discardedValues = cand->expr->result->size() - toType->size();
-                                if ( discardedValues < 0 ) continue;
-
-                                // unification run for side-effects
-                                unify( toType, cand->expr->result, cand->env, need, have, open, symtab );
-                                Cost thisCost =
-                                        (castExpr->isGenerated == ast::GeneratedFlag::GeneratedCast)
-                            ? conversionCost( cand->expr->result, toType, cand->expr->get_lvalue(), symtab, cand->env )
-                            : castCost( cand->expr->result, toType, cand->expr->get_lvalue(), symtab, cand->env );
-
-                                PRINT(
-                                        std::cerr << "working on cast with result: " << toType << std::endl;
-                                        std::cerr << "and expr type: " << cand->expr->result << std::endl;
-                                        std::cerr << "env: " << cand->env << std::endl;
-                                )
-                                if ( thisCost != Cost::infinity ) {
-                                        PRINT(
-                                                std::cerr << "has finite cost." << std::endl;
-                                        )
-                                        // count one safe conversion for each value that is thrown away
-                                        thisCost.incSafe( discardedValues );
-                                        CandidateRef newCand = std::make_shared<Candidate>(
-                                                restructureCast( cand->expr, toType, castExpr->isGenerated ),
-                                                copy( cand->env ), std::move( open ), std::move( need ), cand->cost,
-                                                cand->cost + thisCost );
-                                        inferParameters( newCand, matches );
-                                }
-                        }
-
-                        // select first on argument cost, then conversion cost
-                        CandidateList minArgCost = findMinCost( matches );
-                        promoteCvtCost( minArgCost );
-                        candidates = findMinCost( minArgCost );
-                }
-
-                void postvisit( const ast::VirtualCastExpr * castExpr ) {
-                        assertf( castExpr->result, "Implicit virtual cast targets not yet supported." );
-                        CandidateFinder finder( context, tenv );
-                        // don't prune here, all alternatives guaranteed to have same type
-                        finder.find( castExpr->arg, ResolvMode::withoutPrune() );
-                        for ( CandidateRef & r : finder.candidates ) {
-                                addCandidate(
-                                        *r,
-                                        new ast::VirtualCastExpr{ castExpr->location, r->expr, castExpr->result } );
-                        }
-                }
-
-                void postvisit( const ast::KeywordCastExpr * castExpr ) {
-                        const auto & loc = castExpr->location;
-                        assertf( castExpr->result, "Cast target should have been set in Validate." );
-                        auto ref = castExpr->result.strict_as<ast::ReferenceType>();
-                        auto inst = ref->base.strict_as<ast::StructInstType>();
-                        auto target = inst->base.get();
-
-                        CandidateFinder finder( context, tenv );
-
-                        auto pick_alternatives = [target, this](CandidateList & found, bool expect_ref) {
-                                for(auto & cand : found) {
-                                        const ast::Type * expr = cand->expr->result.get();
-                                        if(expect_ref) {
-                                                auto res = dynamic_cast<const ast::ReferenceType*>(expr);
-                                                if(!res) { continue; }
-                                                expr = res->base.get();
-                                        }
-
-                                        if(auto insttype = dynamic_cast<const ast::TypeInstType*>(expr)) {
-                                                auto td = cand->env.lookup(*insttype);
-                                                if(!td) { continue; }
-                                                expr = td->bound.get();
-                                        }
-
-                                        if(auto base = dynamic_cast<const ast::StructInstType*>(expr)) {
-                                                if(base->base == target) {
-                                                        candidates.push_back( std::move(cand) );
-                                                        reason.code = NoReason;
-                                                }
-                                        }
-                                }
-                        };
-
-                        try {
-                                // Attempt 1 : turn (thread&)X into (thread$&)X.__thrd
-                                // Clone is purely for memory management
-                                std::unique_ptr<const ast::Expr> tech1 { new ast::UntypedMemberExpr(loc, new ast::NameExpr(loc, castExpr->concrete_target.field), castExpr->arg) };
-
-                                // don't prune here, since it's guaranteed all alternatives will have the same type
-                                finder.find( tech1.get(), ResolvMode::withoutPrune() );
-                                pick_alternatives(finder.candidates, false);
-
-                                return;
-                        } catch(SemanticErrorException & ) {}
-
-                        // Fallback : turn (thread&)X into (thread$&)get_thread(X)
-                        std::unique_ptr<const ast::Expr> fallback { ast::UntypedExpr::createDeref(loc,  new ast::UntypedExpr(loc, new ast::NameExpr(loc, castExpr->concrete_target.getter), { castExpr->arg })) };
-                        // don't prune here, since it's guaranteed all alternatives will have the same type
-                        finder.find( fallback.get(), ResolvMode::withoutPrune() );
-
-                        pick_alternatives(finder.candidates, true);
-
-                        // Whatever happens here, we have no more fallbacks
-                }
-
-                void postvisit( const ast::UntypedMemberExpr * memberExpr ) {
-                        CandidateFinder aggFinder( context, tenv );
-                        aggFinder.find( memberExpr->aggregate, ResolvMode::withAdjustment() );
-                        for ( CandidateRef & agg : aggFinder.candidates ) {
-                                // it's okay for the aggregate expression to have reference type -- cast it to the
-                                // base type to treat the aggregate as the referenced value
-                                Cost addedCost = Cost::zero;
-                                agg->expr = referenceToRvalueConversion( agg->expr, addedCost );
-
-                                // find member of the given type
-                                if ( auto structInst = agg->expr->result.as< ast::StructInstType >() ) {
-                                        addAggMembers(
-                                                structInst, agg->expr, *agg, addedCost, getMemberName( memberExpr ) );
-                                } else if ( auto unionInst = agg->expr->result.as< ast::UnionInstType >() ) {
-                                        addAggMembers(
-                                                unionInst, agg->expr, *agg, addedCost, getMemberName( memberExpr ) );
-                                } else if ( auto tupleType = agg->expr->result.as< ast::TupleType >() ) {
-                                        addTupleMembers( tupleType, agg->expr, *agg, addedCost, memberExpr->member );
-                                }
-                        }
-                }
-
-                void postvisit( const ast::MemberExpr * memberExpr ) {
-                        addCandidate( memberExpr, tenv );
-                }
-
-                void postvisit( const ast::NameExpr * nameExpr ) {
-                        std::vector< ast::SymbolTable::IdData > declList;
-                        if (!selfFinder.otypeKeys.empty()) {
-                                auto kind = ast::SymbolTable::getSpecialFunctionKind(nameExpr->name);
-                                assertf(kind != ast::SymbolTable::SpecialFunctionKind::NUMBER_OF_KINDS, "special lookup with non-special target: %s", nameExpr->name.c_str());
-
-                                for (auto & otypeKey: selfFinder.otypeKeys) {
-                                        auto result = symtab.specialLookupId(kind, otypeKey);
-                                        declList.insert(declList.end(), std::make_move_iterator(result.begin()), std::make_move_iterator(result.end()));
-                                }
-                        }
-                        else {
-                                declList = symtab.lookupId( nameExpr->name );
-                        }
-                        PRINT( std::cerr << "nameExpr is " << nameExpr->name << std::endl; )
-
-                        if( declList.empty() ) return;
-
-                        reason.code = NoMatch;
-
-                        for ( auto & data : declList ) {
-                                Cost cost = Cost::zero;
-                                ast::Expr * newExpr = data.combine( nameExpr->location, cost );
-
-                                CandidateRef newCand = std::make_shared<Candidate>(
-                                        newExpr, copy( tenv ), ast::OpenVarSet{}, ast::AssertionSet{}, Cost::zero,
-                                        cost );
-
-                                if (newCand->expr->env) {
-                                        newCand->env.add(*newCand->expr->env);
-                                        auto mutExpr = newCand->expr.get_and_mutate();
-                                        mutExpr->env  = nullptr;
-                                        newCand->expr = mutExpr;
-                                }
-
-                                PRINT(
-                                        std::cerr << "decl is ";
-                                        ast::print( std::cerr, data.id );
-                                        std::cerr << std::endl;
-                                        std::cerr << "newExpr is ";
-                                        ast::print( std::cerr, newExpr );
-                                        std::cerr << std::endl;
-                                )
-                                newCand->expr = ast::mutate_field(
-                                        newCand->expr.get(), &ast::Expr::result,
-                                        renameTyVars( newCand->expr->result ) );
-                                // add anonymous member interpretations whenever an aggregate value type is seen
-                                // as a name expression
-                                addAnonConversions( newCand );
-                                candidates.emplace_back( std::move( newCand ) );
-                        }
-                }
-
-                void postvisit( const ast::VariableExpr * variableExpr ) {
-                        // not sufficient to just pass `variableExpr` here, type might have changed since
-                        // creation
-                        addCandidate(
-                                new ast::VariableExpr{ variableExpr->location, variableExpr->var }, tenv );
-                }
-
-                void postvisit( const ast::ConstantExpr * constantExpr ) {
-                        addCandidate( constantExpr, tenv );
-                }
-
-                void postvisit( const ast::SizeofExpr * sizeofExpr ) {
-                        if ( sizeofExpr->type ) {
-                                addCandidate(
-                                        new ast::SizeofExpr{
-                                                sizeofExpr->location, resolveTypeof( sizeofExpr->type, context ) },
-                                        tenv );
-                        } else {
-                                // find all candidates for the argument to sizeof
-                                CandidateFinder finder( context, tenv );
-                                finder.find( sizeofExpr->expr );
-                                // find the lowest-cost candidate, otherwise ambiguous
-                                CandidateList winners = findMinCost( finder.candidates );
-                                if ( winners.size() != 1 ) {
-                                        SemanticError(
-                                                sizeofExpr->expr.get(), "Ambiguous expression in sizeof operand: " );
-                                }
-                                // return the lowest-cost candidate
-                                CandidateRef & choice = winners.front();
-                                choice->expr = referenceToRvalueConversion( choice->expr, choice->cost );
-                                choice->cost = Cost::zero;
-                                addCandidate( *choice, new ast::SizeofExpr{ sizeofExpr->location, choice->expr } );
-                        }
-                }
-
-                void postvisit( const ast::AlignofExpr * alignofExpr ) {
-                        if ( alignofExpr->type ) {
-                                addCandidate(
-                                        new ast::AlignofExpr{
-                                                alignofExpr->location, resolveTypeof( alignofExpr->type, context ) },
-                                        tenv );
-                        } else {
-                                // find all candidates for the argument to alignof
-                                CandidateFinder finder( context, tenv );
-                                finder.find( alignofExpr->expr );
-                                // find the lowest-cost candidate, otherwise ambiguous
-                                CandidateList winners = findMinCost( finder.candidates );
-                                if ( winners.size() != 1 ) {
-                                        SemanticError(
-                                                alignofExpr->expr.get(), "Ambiguous expression in alignof operand: " );
-                                }
-                                // return the lowest-cost candidate
-                                CandidateRef & choice = winners.front();
-                                choice->expr = referenceToRvalueConversion( choice->expr, choice->cost );
-                                choice->cost = Cost::zero;
-                                addCandidate(
-                                        *choice, new ast::AlignofExpr{ alignofExpr->location, choice->expr } );
-                        }
-                }
-
-                void postvisit( const ast::UntypedOffsetofExpr * offsetofExpr ) {
-                        const ast::BaseInstType * aggInst;
-                        if (( aggInst = offsetofExpr->type.as< ast::StructInstType >() )) ;
-                        else if (( aggInst = offsetofExpr->type.as< ast::UnionInstType >() )) ;
-                        else return;
-
-                        for ( const ast::Decl * member : aggInst->lookup( offsetofExpr->member ) ) {
-                                auto dwt = strict_dynamic_cast< const ast::DeclWithType * >( member );
-                                addCandidate(
-                                        new ast::OffsetofExpr{ offsetofExpr->location, aggInst, dwt }, tenv );
-                        }
-                }
-
-                void postvisit( const ast::OffsetofExpr * offsetofExpr ) {
-                        addCandidate( offsetofExpr, tenv );
-                }
-
-                void postvisit( const ast::OffsetPackExpr * offsetPackExpr ) {
-                        addCandidate( offsetPackExpr, tenv );
-                }
-
-                void postvisit( const ast::LogicalExpr * logicalExpr ) {
-                        CandidateFinder finder1( context, tenv );
-                        finder1.find( logicalExpr->arg1, ResolvMode::withAdjustment() );
-                        if ( finder1.candidates.empty() ) return;
-
-                        CandidateFinder finder2( context, tenv );
-                        finder2.find( logicalExpr->arg2, ResolvMode::withAdjustment() );
-                        if ( finder2.candidates.empty() ) return;
-
-                        reason.code = NoMatch;
-
-                        for ( const CandidateRef & r1 : finder1.candidates ) {
-                                for ( const CandidateRef & r2 : finder2.candidates ) {
-                                        ast::TypeEnvironment env{ r1->env };
-                                        env.simpleCombine( r2->env );
-                                        ast::OpenVarSet open{ r1->open };
-                                        mergeOpenVars( open, r2->open );
-                                        ast::AssertionSet need;
-                                        mergeAssertionSet( need, r1->need );
-                                        mergeAssertionSet( need, r2->need );
-
-                                        addCandidate(
-                                                new ast::LogicalExpr{
-                                                        logicalExpr->location, r1->expr, r2->expr, logicalExpr->isAnd },
-                                                std::move( env ), std::move( open ), std::move( need ), r1->cost + r2->cost );
-                                }
-                        }
-                }
-
-                void postvisit( const ast::ConditionalExpr * conditionalExpr ) {
-                        // candidates for condition
-                        CandidateFinder finder1( context, tenv );
-                        finder1.find( conditionalExpr->arg1, ResolvMode::withAdjustment() );
-                        if ( finder1.candidates.empty() ) return;
-
-                        // candidates for true result
-                        CandidateFinder finder2( context, tenv );
-                        finder2.find( conditionalExpr->arg2, ResolvMode::withAdjustment() );
-                        if ( finder2.candidates.empty() ) return;
-
-                        // candidates for false result
-                        CandidateFinder finder3( context, tenv );
-                        finder3.find( conditionalExpr->arg3, ResolvMode::withAdjustment() );
-                        if ( finder3.candidates.empty() ) return;
-
-                        reason.code = NoMatch;
-
-                        for ( const CandidateRef & r1 : finder1.candidates ) {
-                                for ( const CandidateRef & r2 : finder2.candidates ) {
-                                        for ( const CandidateRef & r3 : finder3.candidates ) {
-                                                ast::TypeEnvironment env{ r1->env };
-                                                env.simpleCombine( r2->env );
-                                                env.simpleCombine( r3->env );
-                                                ast::OpenVarSet open{ r1->open };
-                                                mergeOpenVars( open, r2->open );
-                                                mergeOpenVars( open, r3->open );
-                                                ast::AssertionSet need;
-                                                mergeAssertionSet( need, r1->need );
-                                                mergeAssertionSet( need, r2->need );
-                                                mergeAssertionSet( need, r3->need );
-                                                ast::AssertionSet have;
-
-                                                // unify true and false results, then infer parameters to produce new
-                                                // candidates
-                                                ast::ptr< ast::Type > common;
-                                                if (
-                                                        unify(
-                                                                r2->expr->result, r3->expr->result, env, need, have, open, symtab,
-                                                                common )
-                                                ) {
-                                                        // generate typed expression
-                                                        ast::ConditionalExpr * newExpr = new ast::ConditionalExpr{
-                                                                conditionalExpr->location, r1->expr, r2->expr, r3->expr };
-                                                        newExpr->result = common ? common : r2->expr->result;
-                                                        // convert both options to result type
-                                                        Cost cost = r1->cost + r2->cost + r3->cost;
-                                                        newExpr->arg2 = computeExpressionConversionCost(
-                                                                newExpr->arg2, newExpr->result, symtab, env, cost );
-                                                        newExpr->arg3 = computeExpressionConversionCost(
-                                                                newExpr->arg3, newExpr->result, symtab, env, cost );
-                                                        // output candidate
-                                                        CandidateRef newCand = std::make_shared<Candidate>(
-                                                                newExpr, std::move( env ), std::move( open ), std::move( need ), cost );
-                                                        inferParameters( newCand, candidates );
-                                                }
-                                        }
-                                }
-                        }
-                }
-
-                void postvisit( const ast::CommaExpr * commaExpr ) {
-                        ast::TypeEnvironment env{ tenv };
-                        ast::ptr< ast::Expr > arg1 = resolveInVoidContext( commaExpr->arg1, context, env );
-
-                        CandidateFinder finder2( context, env );
-                        finder2.find( commaExpr->arg2, ResolvMode::withAdjustment() );
-
-                        for ( const CandidateRef & r2 : finder2.candidates ) {
-                                addCandidate( *r2, new ast::CommaExpr{ commaExpr->location, arg1, r2->expr } );
-                        }
-                }
-
-                void postvisit( const ast::ImplicitCopyCtorExpr * ctorExpr ) {
-                        addCandidate( ctorExpr, tenv );
-                }
-
-                void postvisit( const ast::ConstructorExpr * ctorExpr ) {
-                        CandidateFinder finder( context, tenv );
-                        finder.find( ctorExpr->callExpr, ResolvMode::withoutPrune() );
-                        for ( CandidateRef & r : finder.candidates ) {
-                                addCandidate( *r, new ast::ConstructorExpr{ ctorExpr->location, r->expr } );
-                        }
-                }
-
-                void postvisit( const ast::RangeExpr * rangeExpr ) {
-                        // resolve low and high, accept candidates where low and high types unify
-                        CandidateFinder finder1( context, tenv );
-                        finder1.find( rangeExpr->low, ResolvMode::withAdjustment() );
-                        if ( finder1.candidates.empty() ) return;
-
-                        CandidateFinder finder2( context, tenv );
-                        finder2.find( rangeExpr->high, ResolvMode::withAdjustment() );
-                        if ( finder2.candidates.empty() ) return;
-
-                        reason.code = NoMatch;
-
-                        for ( const CandidateRef & r1 : finder1.candidates ) {
-                                for ( const CandidateRef & r2 : finder2.candidates ) {
-                                        ast::TypeEnvironment env{ r1->env };
-                                        env.simpleCombine( r2->env );
-                                        ast::OpenVarSet open{ r1->open };
-                                        mergeOpenVars( open, r2->open );
-                                        ast::AssertionSet need;
-                                        mergeAssertionSet( need, r1->need );
-                                        mergeAssertionSet( need, r2->need );
-                                        ast::AssertionSet have;
-
-                                        ast::ptr< ast::Type > common;
-                                        if (
-                                                unify(
-                                                        r1->expr->result, r2->expr->result, env, need, have, open, symtab,
-                                                        common )
-                                        ) {
-                                                // generate new expression
-                                                ast::RangeExpr * newExpr =
-                                                        new ast::RangeExpr{ rangeExpr->location, r1->expr, r2->expr };
-                                                newExpr->result = common ? common : r1->expr->result;
-                                                // add candidate
-                                                CandidateRef newCand = std::make_shared<Candidate>(
-                                                        newExpr, std::move( env ), std::move( open ), std::move( need ),
-                                                        r1->cost + r2->cost );
-                                                inferParameters( newCand, candidates );
-                                        }
-                                }
-                        }
-                }
-
-                void postvisit( const ast::UntypedTupleExpr * tupleExpr ) {
-                        std::vector< CandidateFinder > subCandidates =
-                                selfFinder.findSubExprs( tupleExpr->exprs );
-                        std::vector< CandidateList > possibilities;
-                        combos( subCandidates.begin(), subCandidates.end(), back_inserter( possibilities ) );
-
-                        for ( const CandidateList & subs : possibilities ) {
-                                std::vector< ast::ptr< ast::Expr > > exprs;
-                                exprs.reserve( subs.size() );
-                                for ( const CandidateRef & sub : subs ) { exprs.emplace_back( sub->expr ); }
-
-                                ast::TypeEnvironment env;
-                                ast::OpenVarSet open;
-                                ast::AssertionSet need;
-                                for ( const CandidateRef & sub : subs ) {
-                                        env.simpleCombine( sub->env );
-                                        mergeOpenVars( open, sub->open );
-                                        mergeAssertionSet( need, sub->need );
-                                }
-
-                                addCandidate(
-                                        new ast::TupleExpr{ tupleExpr->location, std::move( exprs ) },
-                                        std::move( env ), std::move( open ), std::move( need ), sumCost( subs ) );
-                        }
-                }
-
-                void postvisit( const ast::TupleExpr * tupleExpr ) {
-                        addCandidate( tupleExpr, tenv );
-                }
-
-                void postvisit( const ast::TupleIndexExpr * tupleExpr ) {
-                        addCandidate( tupleExpr, tenv );
-                }
-
-                void postvisit( const ast::TupleAssignExpr * tupleExpr ) {
-                        addCandidate( tupleExpr, tenv );
-                }
-
-                void postvisit( const ast::UniqueExpr * unqExpr ) {
-                        CandidateFinder finder( context, tenv );
-                        finder.find( unqExpr->expr, ResolvMode::withAdjustment() );
-                        for ( CandidateRef & r : finder.candidates ) {
-                                // ensure that the the id is passed on so that the expressions are "linked"
-                                addCandidate( *r, new ast::UniqueExpr{ unqExpr->location, r->expr, unqExpr->id } );
-                        }
-                }
-
-                void postvisit( const ast::StmtExpr * stmtExpr ) {
-                        addCandidate( resolveStmtExpr( stmtExpr, context ), tenv );
-                }
-
-                void postvisit( const ast::UntypedInitExpr * initExpr ) {
-                        // handle each option like a cast
-                        CandidateList matches;
-                        PRINT(
-                                std::cerr << "untyped init expr: " << initExpr << std::endl;
-                        )
-                        // O(n^2) checks of d-types with e-types
-                        for ( const ast::InitAlternative & initAlt : initExpr->initAlts ) {
-                                // calculate target type
-                                const ast::Type * toType = resolveTypeof( initAlt.type, context );
-                                toType = adjustExprType( toType, tenv, symtab );
-                                // The call to find must occur inside this loop, otherwise polymorphic return
-                                // types are not bound to the initialization type, since return type variables are
-                                // only open for the duration of resolving the UntypedExpr.
-                                CandidateFinder finder( context, tenv, toType );
-                                finder.find( initExpr->expr, ResolvMode::withAdjustment() );
-                                for ( CandidateRef & cand : finder.candidates ) {
-                                        if(reason.code == NotFound) reason.code = NoMatch;
-
-                                        ast::TypeEnvironment env{ cand->env };
-                                        ast::AssertionSet need( cand->need.begin(), cand->need.end() ), have;
-                                        ast::OpenVarSet open{ cand->open };
-
-                                        PRINT(
-                                                std::cerr << "  @ " << toType << " " << initAlt.designation << std::endl;
-                                        )
-
-                                        // It is possible that a cast can throw away some values in a multiply-valued
-                                        // expression, e.g. cast-to-void, one value to zero. Figure out the prefix of
-                                        // the subexpression results that are cast directly. The candidate is invalid
-                                        // if it has fewer results than there are types to cast to.
-                                        int discardedValues = cand->expr->result->size() - toType->size();
-                                        if ( discardedValues < 0 ) continue;
-
-                                        // unification run for side-effects
-                                        bool canUnify = unify( toType, cand->expr->result, env, need, have, open, symtab );
-                                        (void) canUnify;
-                                        Cost thisCost = computeConversionCost( cand->expr->result, toType, cand->expr->get_lvalue(),
-                                                symtab, env );
-                                        PRINT(
-                                                Cost legacyCost = castCost( cand->expr->result, toType, cand->expr->get_lvalue(),
-                                                        symtab, env );
-                                                std::cerr << "Considering initialization:";
-                                                std::cerr << std::endl << "  FROM: " << cand->expr->result << std::endl;
-                                                std::cerr << std::endl << "  TO: "   << toType             << std::endl;
-                                                std::cerr << std::endl << "  Unification " << (canUnify ? "succeeded" : "failed");
-                                                std::cerr << std::endl << "  Legacy cost " << legacyCost;
-                                                std::cerr << std::endl << "  New cost " << thisCost;
-                                                std::cerr << std::endl;
-                                        )
-                                        if ( thisCost != Cost::infinity ) {
-                                                // count one safe conversion for each value that is thrown away
-                                                thisCost.incSafe( discardedValues );
-                                                CandidateRef newCand = std::make_shared<Candidate>(
-                                                        new ast::InitExpr{
-                                                                initExpr->location, restructureCast( cand->expr, toType ),
-                                                                initAlt.designation },
-                                                        std::move(env), std::move( open ), std::move( need ), cand->cost, thisCost );
-                                                inferParameters( newCand, matches );
-                                        }
-                                }
-
-                        }
-
-                        // select first on argument cost, then conversion cost
-                        CandidateList minArgCost = findMinCost( matches );
-                        promoteCvtCost( minArgCost );
-                        candidates = findMinCost( minArgCost );
-                }
-
-                void postvisit( const ast::InitExpr * ) {
-                        assertf( false, "CandidateFinder should never see a resolved InitExpr." );
-                }
-
-                void postvisit( const ast::DeletedExpr * ) {
-                        assertf( false, "CandidateFinder should never see a DeletedExpr." );
-                }
-
-                void postvisit( const ast::GenericExpr * ) {
-                        assertf( false, "_Generic is not yet supported." );
-                }
-        };
-
-        // size_t Finder::traceId = Stats::Heap::new_stacktrace_id("Finder");
-        /// Prunes a list of candidates down to those that have the minimum conversion cost for a given
-        /// return type. Skips ambiguous candidates.
-
-} // anonymous namespace
-
-bool CandidateFinder::pruneCandidates( CandidateList & candidates, CandidateList & out, std::vector<std::string> & errors ) {
-        struct PruneStruct {
-                CandidateRef candidate;
-                bool ambiguous;
-
-                PruneStruct() = default;
-                PruneStruct( const CandidateRef & c ) : candidate( c ), ambiguous( false ) {}
-        };
-
-        // find lowest-cost candidate for each type
-        std::unordered_map< std::string, PruneStruct > selected;
-        // attempt to skip satisfyAssertions on more expensive alternatives if better options have been found
-        std::sort(candidates.begin(), candidates.end(), [](const CandidateRef & x, const CandidateRef & y){return x->cost < y->cost;});
-        for ( CandidateRef & candidate : candidates ) {
-                std::string mangleName;
-                {
-                        ast::ptr< ast::Type > newType = candidate->expr->result;
-                        assertf(candidate->expr->result, "Result of expression %p for candidate is null", candidate->expr.get());
-                        candidate->env.apply( newType );
-                        mangleName = Mangle::mangle( newType );
-                }
-
-                auto found = selected.find( mangleName );
-                if (found != selected.end() && found->second.candidate->cost < candidate->cost) {
-                        PRINT(
-                                std::cerr << "cost " << candidate->cost << " loses to "
-                                        << found->second.candidate->cost << std::endl;
-                        )
-                        continue;
-                }
-
-                // xxx - when do satisfyAssertions produce more than 1 result?
-                // this should only happen when initial result type contains
-                // unbound type parameters, then it should never be pruned by
-                // the previous step, since renameTyVars guarantees the mangled name
-                // is unique.
-                CandidateList satisfied;
-                bool needRecomputeKey = false;
-                if (candidate->need.empty()) {
-                        satisfied.emplace_back(candidate);
-                }
-                else {
-                        satisfyAssertions(candidate, context.symtab, satisfied, errors);
-                        needRecomputeKey = true;
-                }
-
-                for (auto & newCand : satisfied) {
-                        // recomputes type key, if satisfyAssertions changed it
-                        if (needRecomputeKey)
-                        {
-                                ast::ptr< ast::Type > newType = newCand->expr->result;
-                                assertf(newCand->expr->result, "Result of expression %p for candidate is null", newCand->expr.get());
-                                newCand->env.apply( newType );
-                                mangleName = Mangle::mangle( newType );
-                        }
-                        auto found = selected.find( mangleName );
-                        if ( found != selected.end() ) {
-                                if ( newCand->cost < found->second.candidate->cost ) {
-                                        PRINT(
-                                                std::cerr << "cost " << newCand->cost << " beats "
-                                                        << found->second.candidate->cost << std::endl;
-                                        )
-
-                                        found->second = PruneStruct{ newCand };
-                                } else if ( newCand->cost == found->second.candidate->cost ) {
-                                        // if one of the candidates contains a deleted identifier, can pick the other,
-                                        // since deleted expressions should not be ambiguous if there is another option
-                                        // that is at least as good
-                                        if ( findDeletedExpr( newCand->expr ) ) {
-                                                // do nothing
-                                                PRINT( std::cerr << "candidate is deleted" << std::endl; )
-                                        } else if ( findDeletedExpr( found->second.candidate->expr ) ) {
-                                                PRINT( std::cerr << "current is deleted" << std::endl; )
-                                                found->second = PruneStruct{ newCand };
-                                        } else {
-                                                PRINT( std::cerr << "marking ambiguous" << std::endl; )
-                                                found->second.ambiguous = true;
-                                        }
-                                } else {
-                                        // xxx - can satisfyAssertions increase the cost?
-                                        PRINT(
-                                                std::cerr << "cost " << newCand->cost << " loses to "
-                                                        << found->second.candidate->cost << std::endl;
-                                        )
-                                }
-                        } else {
-                                selected.emplace_hint( found, mangleName, newCand );
-                        }
-                }
-        }
-
-        // report unambiguous min-cost candidates
-        // CandidateList out;
-        for ( auto & target : selected ) {
-                if ( target.second.ambiguous ) continue;
-
-                CandidateRef cand = target.second.candidate;
-
-                ast::ptr< ast::Type > newResult = cand->expr->result;
-                cand->env.applyFree( newResult );
-                cand->expr = ast::mutate_field(
-                        cand->expr.get(), &ast::Expr::result, std::move( newResult ) );
-
-                out.emplace_back( cand );
-        }
-        // if everything is lost in satisfyAssertions, report the error
-        return !selected.empty();
-}
-
-void CandidateFinder::find( const ast::Expr * expr, ResolvMode mode ) {
-        // Find alternatives for expression
-        ast::Pass<Finder> finder{ *this };
-        expr->accept( finder );
-
-        if ( mode.failFast && candidates.empty() ) {
-                switch(finder.core.reason.code) {
-                case Finder::NotFound:
-                        { SemanticError( expr, "No alternatives for expression " ); break; }
-                case Finder::NoMatch:
-                        { SemanticError( expr, "Invalid application of existing declaration(s) in expression " ); break; }
-                case Finder::ArgsToFew:
-                case Finder::ArgsToMany:
-                case Finder::RetsToFew:
-                case Finder::RetsToMany:
-                case Finder::NoReason:
-                default:
-                        { SemanticError( expr->location, "No reasonable alternatives for expression : reasons unkown" ); }
-                }
-        }
-
-        /*
-        if ( mode.satisfyAssns || mode.prune ) {
-                // trim candidates to just those where the assertions are satisfiable
-                // - necessary pre-requisite to pruning
-                CandidateList satisfied;
-                std::vector< std::string > errors;
-                for ( CandidateRef & candidate : candidates ) {
-                        satisfyAssertions( candidate, localSyms, satisfied, errors );
-                }
-
-                // fail early if none such
-                if ( mode.failFast && satisfied.empty() ) {
-                        std::ostringstream stream;
-                        stream << "No alternatives with satisfiable assertions for " << expr << "\n";
-                        for ( const auto& err : errors ) {
-                                stream << err;
-                        }
-                        SemanticError( expr->location, stream.str() );
-                }
-
-                // reset candidates
-                candidates = move( satisfied );
-        }
-        */
-
-        if ( mode.prune ) {
-                // trim candidates to single best one
-                PRINT(
-                        std::cerr << "alternatives before prune:" << std::endl;
-                        print( std::cerr, candidates );
-                )
-
-                CandidateList pruned;
-                std::vector<std::string> errors;
-                bool found = pruneCandidates( candidates, pruned, errors );
-
-                if ( mode.failFast && pruned.empty() ) {
-                        std::ostringstream stream;
-                        if (found) {
-                                CandidateList winners = findMinCost( candidates );
-                                stream << "Cannot choose between " << winners.size() << " alternatives for "
-                                        "expression\n";
-                                ast::print( stream, expr );
-                                stream << " Alternatives are:\n";
-                                print( stream, winners, 1 );
-                                SemanticError( expr->location, stream.str() );
-                        }
-                        else {
-                                stream << "No alternatives with satisfiable assertions for " << expr << "\n";
-                                for ( const auto& err : errors ) {
-                                        stream << err;
-                                }
-                                SemanticError( expr->location, stream.str() );
-                        }
-                }
-
-                auto oldsize = candidates.size();
-                candidates = std::move( pruned );
-
-                PRINT(
-                        std::cerr << "there are " << oldsize << " alternatives before elimination" << std::endl;
-                )
-                PRINT(
-                        std::cerr << "there are " << candidates.size() << " alternatives after elimination"
-                                << std::endl;
-                )
-        }
-
-        // adjust types after pruning so that types substituted by pruneAlternatives are correctly
-        // adjusted
-        if ( mode.adjust ) {
-                for ( CandidateRef & r : candidates ) {
-                        r->expr = ast::mutate_field(
-                                r->expr.get(), &ast::Expr::result,
-                                adjustExprType( r->expr->result, r->env, context.symtab ) );
-                }
-        }
-
-        // Central location to handle gcc extension keyword, etc. for all expressions
-        for ( CandidateRef & r : candidates ) {
-                if ( r->expr->extension != expr->extension ) {
-                        r->expr.get_and_mutate()->extension = expr->extension;
-                }
-        }
-}
-
-std::vector< CandidateFinder > CandidateFinder::findSubExprs(
-        const std::vector< ast::ptr< ast::Expr > > & xs
-) {
-        std::vector< CandidateFinder > out;
-
-        for ( const auto & x : xs ) {
-                out.emplace_back( context, env );
-                out.back().find( x, ResolvMode::withAdjustment() );
-
-                PRINT(
-                        std::cerr << "findSubExprs" << std::endl;
-                        print( std::cerr, out.back().candidates );
-                )
-        }
-
-        return out;
-}
-
 } // namespace ResolvExpr