source: src/ResolvExpr/CandidateFinder.cpp @ 0bd3faf

//
// Cforall Version 1.0.0 Copyright (C) 2015 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// CandidateFinder.cpp --
//
// Author           : Aaron B. Moss
// Created On       : Wed Jun 5 14:30:00 2019
// Last Modified By : Andrew Beach
// Last Modified On : Wed Mar 16 11:58:00 2022
// Update Count     : 3
//

#include "CandidateFinder.hpp"

#include <deque>
#include <iterator>               // for back_inserter
#include <sstream>
#include <string>
#include <unordered_map>
#include <vector>

#include "AdjustExprType.hpp"
#include "Candidate.hpp"
#include "CastCost.hpp"           // for castCost
#include "CompilationState.h"
#include "ConversionCost.h"       // for conversionCast
#include "Cost.h"
#include "ExplodedArg.hpp"
#include "PolyCost.hpp"
#include "RenameVars.h"           // for renameTyVars
#include "Resolver.h"
#include "ResolveTypeof.h"
#include "SatisfyAssertions.hpp"
#include "SpecCost.hpp"
#include "typeops.h"              // for combos
#include "Unify.h"
#include "WidenMode.h"
#include "AST/Expr.hpp"
#include "AST/Node.hpp"
#include "AST/Pass.hpp"
#include "AST/Print.hpp"
#include "AST/SymbolTable.hpp"
#include "AST/Type.hpp"
#include "Common/utility.h"       // for move, copy
#include "SymTab/Mangler.h"
#include "Tuples/Tuples.h"        // for handleTupleAssignment
#include "InitTweak/InitTweak.h"  // for getPointerBase

#include "Common/Stats/Counter.h"

#define PRINT( text ) if ( resolvep ) { text }

namespace ResolvExpr {

/// Unique identifier for matching expression resolutions to their requesting expression
ast::UniqueId globalResnSlot = 0;

namespace {
        /// First index is which argument, second is which alternative, third is which exploded element
        using ExplodedArgs = 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 & 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 CodeLocation & location,
                const ast::Type * paramType, const ast::Init * init, const ExplodedArgs & 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( location,
                                        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( location, {} );
                                                        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 )
                                                ) {
                                                        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 ) ) {
                                        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 ) ) {
                                                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 ) ) {
                                        // 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 CodeLocation & location,
                        const CandidateRef & func, const ast::FunctionType * funcType,
                        const ExplodedArgs & 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
                ast::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 CodeLocation & location,
                const CandidateRef & func, const ast::FunctionType * funcType,
                const ExplodedArgs & 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 ( selfFinder.strictMode ) {
                                if ( ! unifyExact(
                                        returnType, targetType, funcEnv, funcNeed, funcHave, funcOpen, noWiden() ) // xxx - is no widening correct?
                                ) {
                                        // unification failed, do not pursue this candidate
                                        return;
                                }
                        }
                        else {
                                if ( ! unify(
                                        returnType, targetType, funcEnv, funcNeed, funcHave, funcOpen )
                                ) {
                                        // 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( location,
                                                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( location,
                                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::unsafe, "" );
                } else if ( auto unionInst = aggrExpr->result.as< ast::UnionInstType >() ) {
                        addAggMembers( unionInst, aggrExpr, *cand, Cost::unsafe, "" );
                }
        }

        /// 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 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 >() ) {
                                                // if (!selfFinder.allowVoid && function->returns.empty()) continue;
                                                CandidateRef newFunc{ new Candidate{ *func } };
                                                newFunc->expr =
                                                        referenceToRvalueConversion( newFunc->expr, newFunc->cost );
                                                makeFunctionCandidates( untypedExpr->location,
                                                        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( untypedExpr->location,
                                                                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( untypedExpr->location,
                                                                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; }

                // only keep the best matching intrinsic result to match C semantics (no unexpected narrowing/widening)
                // TODO: keep one for each set of argument candidates?
                Cost intrinsicCost = Cost::infinity;
                CandidateList intrinsicResult;

                // 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;
                                withFunc->cost += cvtCost;
                                auto func = withFunc->expr.strict_as<ast::ApplicationExpr>()->func.as<ast::VariableExpr>();
                                if (func && func->var->linkage == ast::Linkage::Intrinsic) {
                                        if (withFunc->cost < intrinsicCost) {
                                                intrinsicResult.clear();
                                                intrinsicCost = withFunc->cost;
                                        }
                                        if (withFunc->cost == intrinsicCost) {
                                                intrinsicResult.emplace_back(std::move(withFunc));
                                        }
                                }
                                else {
                                        candidates.emplace_back( std::move( withFunc ) );
                                }
                        }
                }
                spliceBegin( candidates, intrinsicResult );
                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 : found ) {
                        addAnonConversions( c );
                }
                // would this be too slow when we don't check cost anymore?
                spliceBegin( candidates, found );

                if ( candidates.empty() && targetType && ! targetType->isVoid() && !selfFinder.strictMode ) {
                        // 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 );
                if (toType->isVoid()) {
                        finder.allowVoid = true;
                }
                if ( castExpr->kind == ast::CastExpr::Return ) {
                        finder.strictMode = true;
                        finder.find( castExpr->arg, ResolvMode::withAdjustment() );

                        // return casts are eliminated (merely selecting an overload, no actual operation)
                        candidates = std::move(finder.candidates);
                }
                finder.find( castExpr->arg, ResolvMode::withAdjustment() );

                if ( !finder.candidates.empty() ) reason.code = NoMatch;

                CandidateList matches;
                Cost minExprCost = Cost::infinity;
                Cost minCastCost = Cost::infinity;
                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 );
                        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 );
                                // select first on argument cost, then conversion cost
                                if ( cand->cost < minExprCost || ( cand->cost == minExprCost && thisCost < minCastCost ) ) {
                                        minExprCost = cand->cost;
                                        minCastCost = thisCost;
                                        matches.clear();


                                }
                                // ambiguous case, still output candidates to print in error message
                                if ( cand->cost == minExprCost && thisCost == minCastCost ) {
                                        CandidateRef newCand = std::make_shared<Candidate>(
                                                restructureCast( cand->expr, toType, castExpr->isGenerated ),
                                                copy( cand->env ), std::move( open ), std::move( need ), cand->cost + thisCost);
                                        // currently assertions are always resolved immediately so this should have no effect. 
                                        // if this somehow changes in the future (e.g. delayed by indeterminate return type)
                                        // we may need to revisit the logic.
                                        inferParameters( newCand, matches );
                                }
                                // else skip, better alternatives found

                        }
                }
                candidates = std::move(matches);

                //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.allowVoid = true;
                finder2.find( conditionalExpr->arg2, ResolvMode::withAdjustment() );
                if ( finder2.candidates.empty() ) return;

                // candidates for false result
                CandidateFinder finder3( context, tenv );
                finder3.allowVoid = true;
                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,
                                                        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.allowVoid = true;
                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,
                                                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() );

                        Cost minExprCost = Cost::infinity;
                        Cost minCastCost = Cost::infinity;
                        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 );
                                (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 );
                                        if ( cand->cost < minExprCost || ( cand->cost == minExprCost && thisCost < minCastCost ) ) {
                                                minExprCost = cand->cost;
                                                minCastCost = thisCost;
                                                matches.clear();
                                        }
                                        // ambiguous case, still output candidates to print in error message
                                        if ( cand->cost == minExprCost && thisCost == minCastCost ) {
                                                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 );
                                                // currently assertions are always resolved immediately so this should have no effect.
                                                // if this somehow changes in the future (e.g. delayed by indeterminate return type)
                                                // we may need to revisit the logic.
                                                inferParameters( newCand, matches );
                                        }
                                }
                        }
                }

                // select first on argument cost, then conversion cost
                // CandidateList minArgCost = findMinCost( matches );
                // promoteCvtCost( minArgCost );
                // candidates = findMinCost( minArgCost );
                candidates = std::move(matches);
        }

        // 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() ) {
                                // tiebreaking by picking the lower cost on CURRENT expression
                                // NOTE: this behavior is different from C semantics.
                                // Specific remediations are performed for C operators at postvisit(UntypedExpr).
                                // Further investigations may take place.
                                if ( newCand->cost < found->second.candidate->cost
                                        || (newCand->cost == found->second.candidate->cost && newCand->cvtCost < found->second.candidate->cvtCost) ) {
                                        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 && newCand->cvtCost == found->second.candidate->cvtCost ) {
                                        // 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 );
        }
        */

        // optimization: don't prune for NameExpr since it never has cost
        if ( mode.prune && !dynamic_cast<const ast::NameExpr *>(expr) ) {
                // 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 >() ) {
                // cast away reference from expr
                cost.incReference();
                return new ast::CastExpr{ expr, expr->result->stripReferences() };
        }

        return expr;
}

Cost computeConversionCost(
        const ast::Type * argType, const ast::Type * paramType, bool argIsLvalue,
        const ast::SymbolTable & symtab, const ast::TypeEnvironment & env
) {
        PRINT(
                std::cerr << std::endl << "converting ";
                ast::print( std::cerr, argType, 2 );
                std::cerr << std::endl << " to ";
                ast::print( std::cerr, paramType, 2 );
                std::cerr << std::endl << "environment is: ";
                ast::print( std::cerr, env, 2 );
                std::cerr << std::endl;
        )
        Cost convCost = conversionCost( argType, paramType, argIsLvalue, symtab, env );
        PRINT(
                std::cerr << std::endl << "cost is " << convCost << std::endl;
        )
        if ( convCost == Cost::infinity ) return convCost;
        convCost.incPoly( polyCost( paramType, symtab, env ) + polyCost( argType, symtab, env ) );
        PRINT(
                std::cerr << "cost with polycost is " << convCost << std::endl;
        )
        return convCost;
}

} // namespace ResolvExpr

// Local Variables: //
// tab-width: 4 //
// mode: c++ //
// compile-command: "make install" //
// End: //