Index: src/ResolvExpr/CandidateFinder.cpp
===================================================================
--- src/ResolvExpr/CandidateFinder.cpp	(revision 251ce804bfa9c794ec6fdf5d077a6553e6d749a4)
+++ src/ResolvExpr/CandidateFinder.cpp	(revision 3a7aa94e697cfe6575ffbfec969567cc0eecc91f)
@@ -25,16 +25,17 @@
 #include "AdjustExprType.hpp"
 #include "Candidate.hpp"
-#include "CastCost.hpp"           // for castCost
 #include "CompilationState.h"
-#include "ConversionCost.h"       // for conversionCast
 #include "Cost.h"
+#include "CastCost.hpp"
+#include "PolyCost.hpp"
+#include "SpecCost.hpp"
+#include "ConversionCost.h"
 #include "ExplodedArg.hpp"
-#include "PolyCost.hpp"
 #include "RenameVars.h"           // for renameTyVars
 #include "Resolver.h"
 #include "ResolveTypeof.h"
+#include "WidenMode.h"
 #include "SatisfyAssertions.hpp"
-#include "SpecCost.hpp"
-#include "typeops.h"              // for combos
+#include "typeops.h"              // for adjustExprType, conversionCost, polyCost, specCost
 #include "Unify.h"
 #include "AST/Expr.hpp"
@@ -55,1882 +56,4 @@
 namespace ResolvExpr {
 
-/// Unique identifier for matching expression resolutions to their requesting expression
-UniqueId globalResnSlot = 0;
-
-namespace {
-	/// First index is which argument, second is which alternative, third is which exploded element
-	using ExplodedArgs_new = std::deque< std::vector< ExplodedArg > >;
-
-	/// Returns a list of alternatives with the minimum cost in the given list
-	CandidateList findMinCost( const CandidateList & candidates ) {
-		CandidateList out;
-		Cost minCost = Cost::infinity;
-		for ( const CandidateRef & r : candidates ) {
-			if ( r->cost < minCost ) {
-				minCost = r->cost;
-				out.clear();
-				out.emplace_back( r );
-			} else if ( r->cost == minCost ) {
-				out.emplace_back( r );
-			}
-		}
-		return out;
-	}
-
-	/// Computes conversion cost for a given expression to a given type
-	const ast::Expr * computeExpressionConversionCost(
-		const ast::Expr * arg, const ast::Type * paramType, const ast::SymbolTable & symtab, const ast::TypeEnvironment & env, Cost & outCost
-	) {
-		Cost convCost = computeConversionCost(
-				arg->result, paramType, arg->get_lvalue(), symtab, env );
-		outCost += convCost;
-
-		// If there is a non-zero conversion cost, ignoring poly cost, then the expression requires
-		// conversion. Ignore poly cost for now, since this requires resolution of the cast to
-		// infer parameters and this does not currently work for the reason stated below
-		Cost tmpCost = convCost;
-		tmpCost.incPoly( -tmpCost.get_polyCost() );
-		if ( tmpCost != Cost::zero ) {
-			ast::ptr< ast::Type > newType = paramType;
-			env.apply( newType );
-			return new ast::CastExpr{ arg, newType };
-
-			// xxx - *should* be able to resolve this cast, but at the moment pointers are not
-			// castable to zero_t, but are implicitly convertible. This is clearly inconsistent,
-			// once this is fixed it should be possible to resolve the cast.
-			// xxx - this isn't working, it appears because type1 (parameter) is seen as widenable,
-			// but it shouldn't be because this makes the conversion from DT* to DT* since
-			// commontype(zero_t, DT*) is DT*, rather than nothing
-
-			// CandidateFinder finder{ symtab, env };
-			// finder.find( arg, ResolvMode::withAdjustment() );
-			// assertf( finder.candidates.size() > 0,
-			// 	"Somehow castable expression failed to find alternatives." );
-			// assertf( finder.candidates.size() == 1,
-			// 	"Somehow got multiple alternatives for known cast expression." );
-			// return finder.candidates.front()->expr;
-		}
-
-		return arg;
-	}
-
-	/// Computes conversion cost for a given candidate
-	Cost computeApplicationConversionCost(
-		CandidateRef cand, const ast::SymbolTable & symtab
-	) {
-		auto appExpr = cand->expr.strict_as< ast::ApplicationExpr >();
-		auto pointer = appExpr->func->result.strict_as< ast::PointerType >();
-		auto function = pointer->base.strict_as< ast::FunctionType >();
-
-		Cost convCost = Cost::zero;
-		const auto & params = function->params;
-		auto param = params.begin();
-		auto & args = appExpr->args;
-
-		for ( unsigned i = 0; i < args.size(); ++i ) {
-			const ast::Type * argType = args[i]->result;
-			PRINT(
-				std::cerr << "arg expression:" << std::endl;
-				ast::print( std::cerr, args[i], 2 );
-				std::cerr << "--- results are" << std::endl;
-				ast::print( std::cerr, argType, 2 );
-			)
-
-			if ( param == params.end() ) {
-				if ( function->isVarArgs ) {
-					convCost.incUnsafe();
-					PRINT( std::cerr << "end of params with varargs function: inc unsafe: "
-						<< convCost << std::endl; ; )
-					// convert reference-typed expressions into value-typed expressions
-					cand->expr = ast::mutate_field_index(
-						appExpr, &ast::ApplicationExpr::args, i,
-						referenceToRvalueConversion( args[i], convCost ) );
-					continue;
-				} else return Cost::infinity;
-			}
-
-			if ( auto def = args[i].as< ast::DefaultArgExpr >() ) {
-				// Default arguments should be free - don't include conversion cost.
-				// Unwrap them here because they are not relevant to the rest of the system
-				cand->expr = ast::mutate_field_index(
-					appExpr, &ast::ApplicationExpr::args, i, def->expr );
-				++param;
-				continue;
-			}
-
-			// mark conversion cost and also specialization cost of param type
-			// const ast::Type * paramType = (*param)->get_type();
-			cand->expr = ast::mutate_field_index(
-				appExpr, &ast::ApplicationExpr::args, i,
-				computeExpressionConversionCost(
-					args[i], *param, symtab, cand->env, convCost ) );
-			convCost.decSpec( specCost( *param ) );
-			++param;  // can't be in for-loop update because of the continue
-		}
-
-		if ( param != params.end() ) return Cost::infinity;
-
-		// specialization cost of return types can't be accounted for directly, it disables
-		// otherwise-identical calls, like this example based on auto-newline in the I/O lib:
-		//
-		//   forall(otype OS) {
-		//     void ?|?(OS&, int);  // with newline
-		//     OS&  ?|?(OS&, int);  // no newline, always chosen due to more specialization
-		//   }
-
-		// mark type variable and specialization cost of forall clause
-		convCost.incVar( function->forall.size() );
-		convCost.decSpec( function->assertions.size() );
-
-		return convCost;
-	}
-
-	void makeUnifiableVars(
-		const ast::FunctionType * type, ast::OpenVarSet & unifiableVars,
-		ast::AssertionSet & need
-	) {
-		for ( auto & tyvar : type->forall ) {
-			unifiableVars[ *tyvar ] = ast::TypeData{ tyvar->base };
-		}
-		for ( auto & assn : type->assertions ) {
-			need[ assn ].isUsed = true;
-		}
-	}
-
-	/// Gets a default value from an initializer, nullptr if not present
-	const ast::ConstantExpr * getDefaultValue( const ast::Init * init ) {
-		if ( auto si = dynamic_cast< const ast::SingleInit * >( init ) ) {
-			if ( auto ce = si->value.as< ast::CastExpr >() ) {
-				return ce->arg.as< ast::ConstantExpr >();
-			} else {
-				return si->value.as< ast::ConstantExpr >();
-			}
-		}
-		return nullptr;
-	}
-
-	/// State to iteratively build a match of parameter expressions to arguments
-	struct ArgPack {
-		std::size_t parent;          ///< Index of parent pack
-		ast::ptr< ast::Expr > expr;  ///< The argument stored here
-		Cost cost;                   ///< The cost of this argument
-		ast::TypeEnvironment env;    ///< Environment for this pack
-		ast::AssertionSet need;      ///< Assertions outstanding for this pack
-		ast::AssertionSet have;      ///< Assertions found for this pack
-		ast::OpenVarSet open;        ///< Open variables for this pack
-		unsigned nextArg;            ///< Index of next argument in arguments list
-		unsigned tupleStart;         ///< Number of tuples that start at this index
-		unsigned nextExpl;           ///< Index of next exploded element
-		unsigned explAlt;            ///< Index of alternative for nextExpl > 0
-
-		ArgPack()
-		: parent( 0 ), expr(), cost( Cost::zero ), env(), need(), have(), open(), nextArg( 0 ),
-		  tupleStart( 0 ), nextExpl( 0 ), explAlt( 0 ) {}
-
-		ArgPack(
-			const ast::TypeEnvironment & env, const ast::AssertionSet & need,
-			const ast::AssertionSet & have, const ast::OpenVarSet & open )
-		: parent( 0 ), expr(), cost( Cost::zero ), env( env ), need( need ), have( have ),
-		  open( open ), nextArg( 0 ), tupleStart( 0 ), nextExpl( 0 ), explAlt( 0 ) {}
-
-		ArgPack(
-			std::size_t parent, const ast::Expr * expr, ast::TypeEnvironment && env,
-			ast::AssertionSet && need, ast::AssertionSet && have, ast::OpenVarSet && open,
-			unsigned nextArg, unsigned tupleStart = 0, Cost cost = Cost::zero,
-			unsigned nextExpl = 0, unsigned explAlt = 0 )
-		: parent(parent), expr( expr ), cost( cost ), env( std::move( env ) ), need( std::move( need ) ),
-		  have( std::move( have ) ), open( std::move( open ) ), nextArg( nextArg ), tupleStart( tupleStart ),
-		  nextExpl( nextExpl ), explAlt( explAlt ) {}
-
-		ArgPack(
-			const ArgPack & o, ast::TypeEnvironment && env, ast::AssertionSet && need,
-			ast::AssertionSet && have, ast::OpenVarSet && open, unsigned nextArg, Cost added )
-		: parent( o.parent ), expr( o.expr ), cost( o.cost + added ), env( std::move( env ) ),
-		  need( std::move( need ) ), have( std::move( have ) ), open( std::move( open ) ), nextArg( nextArg ),
-		  tupleStart( o.tupleStart ), nextExpl( 0 ), explAlt( 0 ) {}
-
-		/// true if this pack is in the middle of an exploded argument
-		bool hasExpl() const { return nextExpl > 0; }
-
-		/// Gets the list of exploded candidates for this pack
-		const ExplodedArg & getExpl( const ExplodedArgs_new & args ) const {
-			return args[ nextArg-1 ][ explAlt ];
-		}
-
-		/// Ends a tuple expression, consolidating the appropriate args
-		void endTuple( const std::vector< ArgPack > & packs ) {
-			// add all expressions in tuple to list, summing cost
-			std::deque< const ast::Expr * > exprs;
-			const ArgPack * pack = this;
-			if ( expr ) { exprs.emplace_front( expr ); }
-			while ( pack->tupleStart == 0 ) {
-				pack = &packs[pack->parent];
-				exprs.emplace_front( pack->expr );
-				cost += pack->cost;
-			}
-			// reset pack to appropriate tuple
-			std::vector< ast::ptr< ast::Expr > > exprv( exprs.begin(), exprs.end() );
-			expr = new ast::TupleExpr{ expr->location, std::move( exprv ) };
-			tupleStart = pack->tupleStart - 1;
-			parent = pack->parent;
-		}
-	};
-
-	/// Instantiates an argument to match a parameter, returns false if no matching results left
-	bool instantiateArgument(
-		const CodeLocation & location,
-		const ast::Type * paramType, const ast::Init * init, const ExplodedArgs_new & args,
-		std::vector< ArgPack > & results, std::size_t & genStart, const ast::SymbolTable & symtab,
-		unsigned nTuples = 0
-	) {
-		if ( auto tupleType = dynamic_cast< const ast::TupleType * >( paramType ) ) {
-			// paramType is a TupleType -- group args into a TupleExpr
-			++nTuples;
-			for ( const ast::Type * type : *tupleType ) {
-				// xxx - dropping initializer changes behaviour from previous, but seems correct
-				// ^^^ need to handle the case where a tuple has a default argument
-				if ( ! instantiateArgument( 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_new & args, CandidateList & out );
-
-		/// Adds implicit struct-conversions to the alternative list
-		void addAnonConversions( const CandidateRef & cand );
-
-		/// Adds aggregate member interpretations
-		void addAggMembers(
-			const ast::BaseInstType * aggrInst, const ast::Expr * expr,
-			const Candidate & cand, const Cost & addedCost, const std::string & name
-		);
-
-		/// Adds tuple member interpretations
-		void addTupleMembers(
-			const ast::TupleType * tupleType, const ast::Expr * expr, const Candidate & cand,
-			const Cost & addedCost, const ast::Expr * member
-		);
-
-		/// true if expression is an lvalue
-		static bool isLvalue( const ast::Expr * x ) {
-			return x->result && ( x->get_lvalue() || x->result.as< ast::ReferenceType >() );
-		}
-
-		void postvisit( const ast::UntypedExpr * untypedExpr );
-		void postvisit( const ast::VariableExpr * variableExpr );
-		void postvisit( const ast::ConstantExpr * constantExpr );
-		void postvisit( const ast::SizeofExpr * sizeofExpr );
-		void postvisit( const ast::AlignofExpr * alignofExpr );
-		void postvisit( const ast::AddressExpr * addressExpr );
-		void postvisit( const ast::LabelAddressExpr * labelExpr );
-		void postvisit( const ast::CastExpr * castExpr );
-		void postvisit( const ast::VirtualCastExpr * castExpr );
-		void postvisit( const ast::KeywordCastExpr * castExpr );
-		void postvisit( const ast::UntypedMemberExpr * memberExpr );
-		void postvisit( const ast::MemberExpr * memberExpr );
-		void postvisit( const ast::NameExpr * nameExpr );
-		void postvisit( const ast::UntypedOffsetofExpr * offsetofExpr );
-		void postvisit( const ast::OffsetofExpr * offsetofExpr );
-		void postvisit( const ast::OffsetPackExpr * offsetPackExpr );
-		void postvisit( const ast::LogicalExpr * logicalExpr );
-		void postvisit( const ast::ConditionalExpr * conditionalExpr );
-		void postvisit( const ast::CommaExpr * commaExpr );
-		void postvisit( const ast::ImplicitCopyCtorExpr * ctorExpr );
-		void postvisit( const ast::ConstructorExpr * ctorExpr );
-		void postvisit( const ast::RangeExpr * rangeExpr );
-		void postvisit( const ast::UntypedTupleExpr * tupleExpr );
-		void postvisit( const ast::TupleExpr * tupleExpr );
-		void postvisit( const ast::TupleIndexExpr * tupleExpr );
-		void postvisit( const ast::TupleAssignExpr * tupleExpr );
-		void postvisit( const ast::UniqueExpr * unqExpr );
-		void postvisit( const ast::StmtExpr * stmtExpr );
-		void postvisit( const ast::UntypedInitExpr * initExpr );
-
-		void postvisit( const ast::InitExpr * ) {
-			assertf( false, "CandidateFinder should never see a resolved InitExpr." );
-		}
-
-		void postvisit( const ast::DeletedExpr * ) {
-			assertf( false, "CandidateFinder should never see a DeletedExpr." );
-		}
-
-		void postvisit( const ast::GenericExpr * ) {
-			assertf( false, "_Generic is not yet supported." );
-		}
-	};
-
-	/// Set up candidate assertions for inference
-	void Finder::inferParameters( CandidateRef & newCand, CandidateList & out ) {
-		// Set need bindings for any unbound assertions
-		UniqueId crntResnSlot = 0; // matching ID for this expression's assertions
-		for ( auto & assn : newCand->need ) {
-			// skip already-matched assertions
-			if ( assn.second.resnSlot != 0 ) continue;
-			// assign slot for expression if needed
-			if ( crntResnSlot == 0 ) { crntResnSlot = ++globalResnSlot; }
-			// fix slot to assertion
-			assn.second.resnSlot = crntResnSlot;
-		}
-		// pair slot to expression
-		if ( crntResnSlot != 0 ) {
-			newCand->expr.get_and_mutate()->inferred.resnSlots().emplace_back( crntResnSlot );
-		}
-
-		// add to output list; assertion satisfaction will occur later
-		out.emplace_back( newCand );
-	}
-
-	/// Completes a function candidate with arguments located
-	void Finder::validateFunctionCandidate(
-		const CandidateRef & func, ArgPack & result, const std::vector< ArgPack > & results,
-		CandidateList & out
-	) {
-		ast::ApplicationExpr * appExpr =
-			new ast::ApplicationExpr{ func->expr->location, func->expr };
-		// sum cost and accumulate arguments
-		std::deque< const ast::Expr * > args;
-		Cost cost = func->cost;
-		const ArgPack * pack = &result;
-		while ( pack->expr ) {
-			args.emplace_front( pack->expr );
-			cost += pack->cost;
-			pack = &results[pack->parent];
-		}
-		std::vector< ast::ptr< ast::Expr > > vargs( args.begin(), args.end() );
-		appExpr->args = std::move( vargs );
-		// build and validate new candidate
-		auto newCand =
-			std::make_shared<Candidate>( appExpr, result.env, result.open, result.need, cost );
-		PRINT(
-			std::cerr << "instantiate function success: " << appExpr << std::endl;
-			std::cerr << "need assertions:" << std::endl;
-			ast::print( std::cerr, result.need, 2 );
-		)
-		inferParameters( newCand, out );
-	}
-
-	/// Builds a list of candidates for a function, storing them in out
-	void Finder::makeFunctionCandidates(
-		const CodeLocation & location,
-		const CandidateRef & func, const ast::FunctionType * funcType,
-		const ExplodedArgs_new & args, CandidateList & out
-	) {
-		ast::OpenVarSet funcOpen;
-		ast::AssertionSet funcNeed, funcHave;
-		ast::TypeEnvironment funcEnv{ func->env };
-		makeUnifiableVars( funcType, funcOpen, funcNeed );
-		// add all type variables as open variables now so that those not used in the
-		// parameter list are still considered open
-		funcEnv.add( funcType->forall );
-
-		if ( targetType && ! targetType->isVoid() && ! funcType->returns.empty() ) {
-			// attempt to narrow based on expected target type
-			const ast::Type * returnType = funcType->returns.front();
-			if ( ! unify(
-				returnType, targetType, funcEnv, funcNeed, funcHave, funcOpen )
-			) {
-				// 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::safe, "" );
-		} else if ( auto unionInst = aggrExpr->result.as< ast::UnionInstType >() ) {
-			addAggMembers( unionInst, aggrExpr, *cand, Cost::safe, "" );
-		}
-	}
-
-	/// Adds aggregate member interpretations
-	void Finder::addAggMembers(
-		const ast::BaseInstType * aggrInst, const ast::Expr * expr,
-		const Candidate & cand, const Cost & addedCost, const std::string & name
-	) {
-		for ( const ast::Decl * decl : aggrInst->lookup( name ) ) {
-			auto dwt = strict_dynamic_cast< const ast::DeclWithType * >( decl );
-			CandidateRef newCand = std::make_shared<Candidate>(
-				cand, new ast::MemberExpr{ expr->location, dwt, expr }, addedCost );
-			// add anonymous member interpretations whenever an aggregate value type is seen
-			// as a member expression
-			addAnonConversions( newCand );
-			candidates.emplace_back( std::move( newCand ) );
-		}
-	}
-
-	/// Adds tuple member interpretations
-	void Finder::addTupleMembers(
-		const ast::TupleType * tupleType, const ast::Expr * expr, const Candidate & cand,
-		const Cost & addedCost, const ast::Expr * member
-	) {
-		if ( auto constantExpr = dynamic_cast< const ast::ConstantExpr * >( member ) ) {
-			// get the value of the constant expression as an int, must be between 0 and the
-			// length of the tuple to have meaning
-			long long val = constantExpr->intValue();
-			if ( val >= 0 && (unsigned long long)val < tupleType->size() ) {
-				addCandidate(
-					cand, new ast::TupleIndexExpr{ expr->location, expr, (unsigned)val },
-					addedCost );
-			}
-		}
-	}
-
-	void Finder::postvisit( const ast::UntypedExpr * untypedExpr ) {
-		std::vector< CandidateFinder > argCandidates =
-			selfFinder.findSubExprs( untypedExpr->args );
-
-		// take care of possible tuple assignments
-		// if not tuple assignment, handled as normal function call
-		Tuples::handleTupleAssignment( selfFinder, untypedExpr, argCandidates );
-
-		CandidateFinder funcFinder( context, tenv );
-		if (auto nameExpr = untypedExpr->func.as<ast::NameExpr>()) {
-			auto kind = ast::SymbolTable::getSpecialFunctionKind(nameExpr->name);
-			if (kind != ast::SymbolTable::SpecialFunctionKind::NUMBER_OF_KINDS) {
-				assertf(!argCandidates.empty(), "special function call without argument");
-				for (auto & firstArgCand: argCandidates[0]) {
-					ast::ptr<ast::Type> argType = firstArgCand->expr->result;
-					firstArgCand->env.apply(argType);
-					// strip references
-					// xxx - is this correct?
-					while (argType.as<ast::ReferenceType>()) argType = argType.as<ast::ReferenceType>()->base;
-
-					// convert 1-tuple to plain type
-					if (auto tuple = argType.as<ast::TupleType>()) {
-						if (tuple->size() == 1) {
-							argType = tuple->types[0];
-						}
-					}
-
-					// if argType is an unbound type parameter, all special functions need to be searched.
-					if (isUnboundType(argType)) {
-						funcFinder.otypeKeys.clear();
-						break;
-					}
-
-					if (argType.as<ast::PointerType>()) funcFinder.otypeKeys.insert(Mangle::Encoding::pointer);						
-					// else if (const ast::EnumInstType * enumInst = argType.as<ast::EnumInstType>()) {
-					// 	const ast::EnumDecl * enumDecl = enumInst->base; // Here
-					// 	if ( const ast::Type* enumType = enumDecl->base ) {
-					// 		// instance of enum (T) is a instance of type (T)
-					// 		funcFinder.otypeKeys.insert(Mangle::mangle(enumType, Mangle::NoGenericParams | Mangle::Type));
-					// 	} else {
-					// 		// instance of an untyped enum is techically int
-					// 		funcFinder.otypeKeys.insert(Mangle::mangle(enumDecl, Mangle::NoGenericParams | Mangle::Type));
-					// 	}
-					// }
-					else funcFinder.otypeKeys.insert(Mangle::mangle(argType, Mangle::NoGenericParams | Mangle::Type));
-				}
-			}
-		}
-		// if candidates are already produced, do not fail
-		// xxx - is it possible that handleTupleAssignment and main finder both produce candidates?
-		// this means there exists ctor/assign functions with a tuple as first parameter.
-		ResolvMode mode = {
-			true, // adjust
-			!untypedExpr->func.as<ast::NameExpr>(), // prune if not calling by name
-			selfFinder.candidates.empty() // failfast if other options are not found
-		};
-		funcFinder.find( untypedExpr->func, mode );
-		// short-circuit if no candidates
-		// if ( funcFinder.candidates.empty() ) return;
-
-		reason.code = NoMatch;
-
-		// find function operators
-		ast::ptr< ast::Expr > opExpr = new ast::NameExpr{ untypedExpr->location, "?()" }; // ??? why not ?{}
-		CandidateFinder opFinder( context, tenv );
-		// okay if there aren't any function operations
-		opFinder.find( opExpr, ResolvMode::withoutFailFast() );
-		PRINT(
-			std::cerr << "known function ops:" << std::endl;
-			print( std::cerr, opFinder.candidates, 1 );
-		)
-
-		// pre-explode arguments
-		ExplodedArgs_new argExpansions;
-		for ( const CandidateFinder & args : argCandidates ) {
-			argExpansions.emplace_back();
-			auto & argE = argExpansions.back();
-			for ( const CandidateRef & arg : args ) { argE.emplace_back( *arg, symtab ); }
-		}
-
-		// Find function matches
-		CandidateList found;
-		SemanticErrorException errors;
-		for ( CandidateRef & func : funcFinder ) {
-			try {
-				PRINT(
-					std::cerr << "working on alternative:" << std::endl;
-					print( std::cerr, *func, 2 );
-				)
-
-				// check if the type is a pointer to function
-				const ast::Type * funcResult = func->expr->result->stripReferences();
-				if ( auto pointer = dynamic_cast< const ast::PointerType * >( funcResult ) ) {
-					if ( auto function = pointer->base.as< ast::FunctionType >() ) {
-						CandidateRef newFunc{ new Candidate{ *func } };
-						newFunc->expr =
-							referenceToRvalueConversion( newFunc->expr, newFunc->cost );
-						makeFunctionCandidates( 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; }
-
-		// Compute conversion costs
-		for ( CandidateRef & withFunc : found ) {
-			Cost cvtCost = computeApplicationConversionCost( withFunc, symtab );
-
-			PRINT(
-				auto appExpr = withFunc->expr.strict_as< ast::ApplicationExpr >();
-				auto pointer = appExpr->func->result.strict_as< ast::PointerType >();
-				auto function = pointer->base.strict_as< ast::FunctionType >();
-
-				std::cerr << "Case +++++++++++++ " << appExpr->func << std::endl;
-				std::cerr << "parameters are:" << std::endl;
-				ast::printAll( std::cerr, function->params, 2 );
-				std::cerr << "arguments are:" << std::endl;
-				ast::printAll( std::cerr, appExpr->args, 2 );
-				std::cerr << "bindings are:" << std::endl;
-				ast::print( std::cerr, withFunc->env, 2 );
-				std::cerr << "cost is: " << withFunc->cost << std::endl;
-				std::cerr << "cost of conversion is:" << cvtCost << std::endl;
-			)
-
-			if ( cvtCost != Cost::infinity ) {
-				withFunc->cvtCost = cvtCost;
-				candidates.emplace_back( std::move( withFunc ) );
-			}
-		}
-		found = std::move( candidates );
-
-		// use a new list so that candidates are not examined by addAnonConversions twice
-		CandidateList winners = findMinCost( found );
-		promoteCvtCost( winners );
-
-		// function may return a struct/union value, in which case we need to add candidates
-		// for implicit conversions to each of the anonymous members, which must happen after
-		// `findMinCost`, since anon conversions are never the cheapest
-		for ( const CandidateRef & c : winners ) {
-			addAnonConversions( c );
-		}
-		spliceBegin( candidates, winners );
-
-		if ( candidates.empty() && targetType && ! targetType->isVoid() ) {
-			// If resolution is unsuccessful with a target type, try again without, since it
-			// will sometimes succeed when it wouldn't with a target type binding.
-			// For example:
-			//   forall( otype T ) T & ?[]( T *, ptrdiff_t );
-			//   const char * x = "hello world";
-			//   unsigned char ch = x[0];
-			// Fails with simple return type binding (xxx -- check this!) as follows:
-			// * T is bound to unsigned char
-			// * (x: const char *) is unified with unsigned char *, which fails
-			// xxx -- fix this better
-			targetType = nullptr;
-			postvisit( untypedExpr );
-		}
-	}
-
-	void Finder::postvisit( const ast::AddressExpr * addressExpr ) {
-		CandidateFinder finder( context, tenv );
-		finder.find( addressExpr->arg );
-
-		if ( finder.candidates.empty() ) return;
-
-		reason.code = NoMatch;
-
-		for ( CandidateRef & r : finder.candidates ) {
-			if ( ! isLvalue( r->expr ) ) continue;
-			addCandidate( *r, new ast::AddressExpr{ addressExpr->location, r->expr } );
-		}
-	}
-
-	void Finder::postvisit( const ast::LabelAddressExpr * labelExpr ) {
-		addCandidate( labelExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::CastExpr * castExpr ) {
-		ast::ptr< ast::Type > toType = castExpr->result;
-		assert( toType );
-		toType = resolveTypeof( toType, context );
-		toType = adjustExprType( toType, tenv, symtab );
-
-		CandidateFinder finder( context, tenv, toType );
-		finder.find( castExpr->arg, ResolvMode::withAdjustment() );
-
-		if ( !finder.candidates.empty() ) reason.code = NoMatch;
-
-		CandidateList matches;
-		for ( CandidateRef & cand : finder.candidates ) {
-			ast::AssertionSet need( cand->need.begin(), cand->need.end() ), have;
-			ast::OpenVarSet open( cand->open );
-
-			cand->env.extractOpenVars( open );
-
-			// It is possible that a cast can throw away some values in a multiply-valued
-			// expression, e.g. cast-to-void, one value to zero. Figure out the prefix of the
-			// subexpression results that are cast directly. The candidate is invalid if it
-			// has fewer results than there are types to cast to.
-			int discardedValues = cand->expr->result->size() - toType->size();
-			if ( discardedValues < 0 ) continue;
-
-			// unification run for side-effects
-			unify( toType, cand->expr->result, cand->env, need, have, open );
-			Cost thisCost =
-				(castExpr->isGenerated == ast::GeneratedFlag::GeneratedCast)
-					? conversionCost( cand->expr->result, toType, cand->expr->get_lvalue(), symtab, cand->env )
-					: castCost( cand->expr->result, toType, cand->expr->get_lvalue(), symtab, cand->env );
-
-			PRINT(
-				std::cerr << "working on cast with result: " << toType << std::endl;
-				std::cerr << "and expr type: " << cand->expr->result << std::endl;
-				std::cerr << "env: " << cand->env << std::endl;
-			)
-			if ( thisCost != Cost::infinity ) {
-				PRINT(
-					std::cerr << "has finite cost." << std::endl;
-				)
-				// count one safe conversion for each value that is thrown away
-				thisCost.incSafe( discardedValues );
-				CandidateRef newCand = std::make_shared<Candidate>(
-					restructureCast( cand->expr, toType, castExpr->isGenerated ),
-					copy( cand->env ), std::move( open ), std::move( need ), cand->cost,
-					cand->cost + thisCost );
-				inferParameters( newCand, matches );
-			}
-		}
-
-		// select first on argument cost, then conversion cost
-		CandidateList minArgCost = findMinCost( matches );
-		promoteCvtCost( minArgCost );
-		candidates = findMinCost( minArgCost );
-	}
-
-	void Finder::postvisit( const ast::VirtualCastExpr * castExpr ) {
-		assertf( castExpr->result, "Implicit virtual cast targets not yet supported." );
-		CandidateFinder finder( context, tenv );
-		// don't prune here, all alternatives guaranteed to have same type
-		finder.find( castExpr->arg, ResolvMode::withoutPrune() );
-		for ( CandidateRef & r : finder.candidates ) {
-			addCandidate(
-				*r,
-				new ast::VirtualCastExpr{ castExpr->location, r->expr, castExpr->result } );
-		}
-	}
-
-	void Finder::postvisit( const ast::KeywordCastExpr * castExpr ) {
-		const auto & loc = castExpr->location;
-		assertf( castExpr->result, "Cast target should have been set in Validate." );
-		auto ref = castExpr->result.strict_as<ast::ReferenceType>();
-		auto inst = ref->base.strict_as<ast::StructInstType>();
-		auto target = inst->base.get();
-
-		CandidateFinder finder( context, tenv );
-
-		auto pick_alternatives = [target, this](CandidateList & found, bool expect_ref) {
-			for (auto & cand : found) {
-				const ast::Type * expr = cand->expr->result.get();
-				if (expect_ref) {
-					auto res = dynamic_cast<const ast::ReferenceType*>(expr);
-					if (!res) { continue; }
-					expr = res->base.get();
-				}
-
-				if (auto insttype = dynamic_cast<const ast::TypeInstType*>(expr)) {
-					auto td = cand->env.lookup(*insttype);
-					if (!td) { continue; }
-					expr = td->bound.get();
-				}
-
-				if (auto base = dynamic_cast<const ast::StructInstType*>(expr)) {
-					if (base->base == target) {
-						candidates.push_back( std::move(cand) );
-						reason.code = NoReason;
-					}
-				}
-			}
-		};
-
-		try {
-			// Attempt 1 : turn (thread&)X into (thread$&)X.__thrd
-			// Clone is purely for memory management
-			std::unique_ptr<const ast::Expr> tech1 { new ast::UntypedMemberExpr(loc, new ast::NameExpr(loc, castExpr->concrete_target.field), castExpr->arg) };
-
-			// don't prune here, since it's guaranteed all alternatives will have the same type
-			finder.find( tech1.get(), ResolvMode::withoutPrune() );
-			pick_alternatives(finder.candidates, false);
-
-			return;
-		} catch(SemanticErrorException & ) {}
-
-		// Fallback : turn (thread&)X into (thread$&)get_thread(X)
-		std::unique_ptr<const ast::Expr> fallback { ast::UntypedExpr::createDeref(loc,  new ast::UntypedExpr(loc, new ast::NameExpr(loc, castExpr->concrete_target.getter), { castExpr->arg })) };
-		// don't prune here, since it's guaranteed all alternatives will have the same type
-		finder.find( fallback.get(), ResolvMode::withoutPrune() );
-
-		pick_alternatives(finder.candidates, true);
-
-		// Whatever happens here, we have no more fallbacks
-	}
-
-	void Finder::postvisit( const ast::UntypedMemberExpr * memberExpr ) {
-		CandidateFinder aggFinder( context, tenv );
-		aggFinder.find( memberExpr->aggregate, ResolvMode::withAdjustment() );
-		for ( CandidateRef & agg : aggFinder.candidates ) {
-			// it's okay for the aggregate expression to have reference type -- cast it to the
-			// base type to treat the aggregate as the referenced value
-			Cost addedCost = Cost::zero;
-			agg->expr = referenceToRvalueConversion( agg->expr, addedCost );
-
-			// find member of the given type
-			if ( auto structInst = agg->expr->result.as< ast::StructInstType >() ) {
-				addAggMembers(
-					structInst, agg->expr, *agg, addedCost, getMemberName( memberExpr ) );
-			} else if ( auto unionInst = agg->expr->result.as< ast::UnionInstType >() ) {
-				addAggMembers(
-					unionInst, agg->expr, *agg, addedCost, getMemberName( memberExpr ) );
-			} else if ( auto tupleType = agg->expr->result.as< ast::TupleType >() ) {
-				addTupleMembers( tupleType, agg->expr, *agg, addedCost, memberExpr->member );
-			}
-		}
-	}
-
-	void Finder::postvisit( const ast::MemberExpr * memberExpr ) {
-		addCandidate( memberExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::NameExpr * nameExpr ) {
-		std::vector< ast::SymbolTable::IdData > declList;
-		if (!selfFinder.otypeKeys.empty()) {
-			auto kind = ast::SymbolTable::getSpecialFunctionKind(nameExpr->name);
-			assertf(kind != ast::SymbolTable::SpecialFunctionKind::NUMBER_OF_KINDS, "special lookup with non-special target: %s", nameExpr->name.c_str());
-
-			for (auto & otypeKey: selfFinder.otypeKeys) {
-				auto result = symtab.specialLookupId(kind, otypeKey);
-				declList.insert(declList.end(), std::make_move_iterator(result.begin()), std::make_move_iterator(result.end()));
-			}
-		} else {
-			declList = symtab.lookupId( nameExpr->name );
-		}
-		PRINT( std::cerr << "nameExpr is " << nameExpr->name << std::endl; )
-
-		if ( declList.empty() ) return;
-
-		reason.code = NoMatch;
-
-		for ( auto & data : declList ) {
-			Cost cost = Cost::zero;
-			ast::Expr * newExpr = data.combine( nameExpr->location, cost );
-
-			CandidateRef newCand = std::make_shared<Candidate>(
-				newExpr, copy( tenv ), ast::OpenVarSet{}, ast::AssertionSet{}, Cost::zero,
-				cost );
-
-			if (newCand->expr->env) {
-				newCand->env.add(*newCand->expr->env);
-				auto mutExpr = newCand->expr.get_and_mutate();
-				mutExpr->env  = nullptr;
-				newCand->expr = mutExpr;
-			}
-
-			PRINT(
-				std::cerr << "decl is ";
-				ast::print( std::cerr, data.id );
-				std::cerr << std::endl;
-				std::cerr << "newExpr is ";
-				ast::print( std::cerr, newExpr );
-				std::cerr << std::endl;
-			)
-			newCand->expr = ast::mutate_field(
-				newCand->expr.get(), &ast::Expr::result,
-				renameTyVars( newCand->expr->result ) );
-			// add anonymous member interpretations whenever an aggregate value type is seen
-			// as a name expression
-			addAnonConversions( newCand );
-			candidates.emplace_back( std::move( newCand ) );
-		}
-	}
-
-	void Finder::postvisit( const ast::VariableExpr * variableExpr ) {
-		// not sufficient to just pass `variableExpr` here, type might have changed since
-		// creation
-		addCandidate(
-			new ast::VariableExpr{ variableExpr->location, variableExpr->var }, tenv );
-	}
-
-	void Finder::postvisit( const ast::ConstantExpr * constantExpr ) {
-		addCandidate( constantExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::SizeofExpr * sizeofExpr ) {
-		if ( sizeofExpr->type ) {
-			addCandidate(
-				new ast::SizeofExpr{
-					sizeofExpr->location, resolveTypeof( sizeofExpr->type, context ) },
-				tenv );
-		} else {
-			// find all candidates for the argument to sizeof
-			CandidateFinder finder( context, tenv );
-			finder.find( sizeofExpr->expr );
-			// find the lowest-cost candidate, otherwise ambiguous
-			CandidateList winners = findMinCost( finder.candidates );
-			if ( winners.size() != 1 ) {
-				SemanticError(
-					sizeofExpr->expr.get(), "Ambiguous expression in sizeof operand: " );
-			}
-			// return the lowest-cost candidate
-			CandidateRef & choice = winners.front();
-			choice->expr = referenceToRvalueConversion( choice->expr, choice->cost );
-			choice->cost = Cost::zero;
-			addCandidate( *choice, new ast::SizeofExpr{ sizeofExpr->location, choice->expr } );
-		}
-	}
-
-	void Finder::postvisit( const ast::AlignofExpr * alignofExpr ) {
-		if ( alignofExpr->type ) {
-			addCandidate(
-				new ast::AlignofExpr{
-					alignofExpr->location, resolveTypeof( alignofExpr->type, context ) },
-				tenv );
-		} else {
-			// find all candidates for the argument to alignof
-			CandidateFinder finder( context, tenv );
-			finder.find( alignofExpr->expr );
-			// find the lowest-cost candidate, otherwise ambiguous
-			CandidateList winners = findMinCost( finder.candidates );
-			if ( winners.size() != 1 ) {
-				SemanticError(
-					alignofExpr->expr.get(), "Ambiguous expression in alignof operand: " );
-			}
-			// return the lowest-cost candidate
-			CandidateRef & choice = winners.front();
-			choice->expr = referenceToRvalueConversion( choice->expr, choice->cost );
-			choice->cost = Cost::zero;
-			addCandidate(
-				*choice, new ast::AlignofExpr{ alignofExpr->location, choice->expr } );
-		}
-	}
-
-	void Finder::postvisit( const ast::UntypedOffsetofExpr * offsetofExpr ) {
-		const ast::BaseInstType * aggInst;
-		if (( aggInst = offsetofExpr->type.as< ast::StructInstType >() )) ;
-		else if (( aggInst = offsetofExpr->type.as< ast::UnionInstType >() )) ;
-		else return;
-
-		for ( const ast::Decl * member : aggInst->lookup( offsetofExpr->member ) ) {
-			auto dwt = strict_dynamic_cast< const ast::DeclWithType * >( member );
-			addCandidate(
-				new ast::OffsetofExpr{ offsetofExpr->location, aggInst, dwt }, tenv );
-		}
-	}
-
-	void Finder::postvisit( const ast::OffsetofExpr * offsetofExpr ) {
-		addCandidate( offsetofExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::OffsetPackExpr * offsetPackExpr ) {
-		addCandidate( offsetPackExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::LogicalExpr * logicalExpr ) {
-		CandidateFinder finder1( context, tenv );
-		finder1.find( logicalExpr->arg1, ResolvMode::withAdjustment() );
-		if ( finder1.candidates.empty() ) return;
-
-		CandidateFinder finder2( context, tenv );
-		finder2.find( logicalExpr->arg2, ResolvMode::withAdjustment() );
-		if ( finder2.candidates.empty() ) return;
-
-		reason.code = NoMatch;
-
-		for ( const CandidateRef & r1 : finder1.candidates ) {
-			for ( const CandidateRef & r2 : finder2.candidates ) {
-				ast::TypeEnvironment env{ r1->env };
-				env.simpleCombine( r2->env );
-				ast::OpenVarSet open{ r1->open };
-				mergeOpenVars( open, r2->open );
-				ast::AssertionSet need;
-				mergeAssertionSet( need, r1->need );
-				mergeAssertionSet( need, r2->need );
-
-				addCandidate(
-					new ast::LogicalExpr{
-						logicalExpr->location, r1->expr, r2->expr, logicalExpr->isAnd },
-					std::move( env ), std::move( open ), std::move( need ), r1->cost + r2->cost );
-			}
-		}
-	}
-
-	void Finder::postvisit( const ast::ConditionalExpr * conditionalExpr ) {
-		// candidates for condition
-		CandidateFinder finder1( context, tenv );
-		finder1.find( conditionalExpr->arg1, ResolvMode::withAdjustment() );
-		if ( finder1.candidates.empty() ) return;
-
-		// candidates for true result
-		CandidateFinder finder2( context, tenv );
-		finder2.find( conditionalExpr->arg2, ResolvMode::withAdjustment() );
-		if ( finder2.candidates.empty() ) return;
-
-		// candidates for false result
-		CandidateFinder finder3( context, tenv );
-		finder3.find( conditionalExpr->arg3, ResolvMode::withAdjustment() );
-		if ( finder3.candidates.empty() ) return;
-
-		reason.code = NoMatch;
-
-		for ( const CandidateRef & r1 : finder1.candidates ) {
-			for ( const CandidateRef & r2 : finder2.candidates ) {
-				for ( const CandidateRef & r3 : finder3.candidates ) {
-					ast::TypeEnvironment env{ r1->env };
-					env.simpleCombine( r2->env );
-					env.simpleCombine( r3->env );
-					ast::OpenVarSet open{ r1->open };
-					mergeOpenVars( open, r2->open );
-					mergeOpenVars( open, r3->open );
-					ast::AssertionSet need;
-					mergeAssertionSet( need, r1->need );
-					mergeAssertionSet( need, r2->need );
-					mergeAssertionSet( need, r3->need );
-					ast::AssertionSet have;
-
-					// unify true and false results, then infer parameters to produce new
-					// candidates
-					ast::ptr< ast::Type > common;
-					if (
-						unify(
-							r2->expr->result, r3->expr->result, env, need, have, open,
-							common )
-					) {
-						// generate typed expression
-						ast::ConditionalExpr * newExpr = new ast::ConditionalExpr{
-							conditionalExpr->location, r1->expr, r2->expr, r3->expr };
-						newExpr->result = common ? common : r2->expr->result;
-						// convert both options to result type
-						Cost cost = r1->cost + r2->cost + r3->cost;
-						newExpr->arg2 = computeExpressionConversionCost(
-							newExpr->arg2, newExpr->result, symtab, env, cost );
-						newExpr->arg3 = computeExpressionConversionCost(
-							newExpr->arg3, newExpr->result, symtab, env, cost );
-						// output candidate
-						CandidateRef newCand = std::make_shared<Candidate>(
-							newExpr, std::move( env ), std::move( open ), std::move( need ), cost );
-						inferParameters( newCand, candidates );
-					}
-				}
-			}
-		}
-	}
-
-	void Finder::postvisit( const ast::CommaExpr * commaExpr ) {
-		ast::TypeEnvironment env{ tenv };
-		ast::ptr< ast::Expr > arg1 = resolveInVoidContext( commaExpr->arg1, context, env );
-
-		CandidateFinder finder2( context, env );
-		finder2.find( commaExpr->arg2, ResolvMode::withAdjustment() );
-
-		for ( const CandidateRef & r2 : finder2.candidates ) {
-			addCandidate( *r2, new ast::CommaExpr{ commaExpr->location, arg1, r2->expr } );
-		}
-	}
-
-	void Finder::postvisit( const ast::ImplicitCopyCtorExpr * ctorExpr ) {
-		addCandidate( ctorExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::ConstructorExpr * ctorExpr ) {
-		CandidateFinder finder( context, tenv );
-		finder.find( ctorExpr->callExpr, ResolvMode::withoutPrune() );
-		for ( CandidateRef & r : finder.candidates ) {
-			addCandidate( *r, new ast::ConstructorExpr{ ctorExpr->location, r->expr } );
-		}
-	}
-
-	void Finder::postvisit( const ast::RangeExpr * rangeExpr ) {
-		// resolve low and high, accept candidates where low and high types unify
-		CandidateFinder finder1( context, tenv );
-		finder1.find( rangeExpr->low, ResolvMode::withAdjustment() );
-		if ( finder1.candidates.empty() ) return;
-
-		CandidateFinder finder2( context, tenv );
-		finder2.find( rangeExpr->high, ResolvMode::withAdjustment() );
-		if ( finder2.candidates.empty() ) return;
-
-		reason.code = NoMatch;
-
-		for ( const CandidateRef & r1 : finder1.candidates ) {
-			for ( const CandidateRef & r2 : finder2.candidates ) {
-				ast::TypeEnvironment env{ r1->env };
-				env.simpleCombine( r2->env );
-				ast::OpenVarSet open{ r1->open };
-				mergeOpenVars( open, r2->open );
-				ast::AssertionSet need;
-				mergeAssertionSet( need, r1->need );
-				mergeAssertionSet( need, r2->need );
-				ast::AssertionSet have;
-
-				ast::ptr< ast::Type > common;
-				if (
-					unify(
-						r1->expr->result, r2->expr->result, env, need, have, open,
-						common )
-				) {
-					// generate new expression
-					ast::RangeExpr * newExpr =
-						new ast::RangeExpr{ rangeExpr->location, r1->expr, r2->expr };
-					newExpr->result = common ? common : r1->expr->result;
-					// add candidate
-					CandidateRef newCand = std::make_shared<Candidate>(
-						newExpr, std::move( env ), std::move( open ), std::move( need ),
-						r1->cost + r2->cost );
-					inferParameters( newCand, candidates );
-				}
-			}
-		}
-	}
-
-	void Finder::postvisit( const ast::UntypedTupleExpr * tupleExpr ) {
-		std::vector< CandidateFinder > subCandidates =
-			selfFinder.findSubExprs( tupleExpr->exprs );
-		std::vector< CandidateList > possibilities;
-		combos( subCandidates.begin(), subCandidates.end(), back_inserter( possibilities ) );
-
-		for ( const CandidateList & subs : possibilities ) {
-			std::vector< ast::ptr< ast::Expr > > exprs;
-			exprs.reserve( subs.size() );
-			for ( const CandidateRef & sub : subs ) { exprs.emplace_back( sub->expr ); }
-
-			ast::TypeEnvironment env;
-			ast::OpenVarSet open;
-			ast::AssertionSet need;
-			for ( const CandidateRef & sub : subs ) {
-				env.simpleCombine( sub->env );
-				mergeOpenVars( open, sub->open );
-				mergeAssertionSet( need, sub->need );
-			}
-
-			addCandidate(
-				new ast::TupleExpr{ tupleExpr->location, std::move( exprs ) },
-				std::move( env ), std::move( open ), std::move( need ), sumCost( subs ) );
-		}
-	}
-
-	void Finder::postvisit( const ast::TupleExpr * tupleExpr ) {
-		addCandidate( tupleExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::TupleIndexExpr * tupleExpr ) {
-		addCandidate( tupleExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::TupleAssignExpr * tupleExpr ) {
-		addCandidate( tupleExpr, tenv );
-	}
-
-	void Finder::postvisit( const ast::UniqueExpr * unqExpr ) {
-		CandidateFinder finder( context, tenv );
-		finder.find( unqExpr->expr, ResolvMode::withAdjustment() );
-		for ( CandidateRef & r : finder.candidates ) {
-			// ensure that the the id is passed on so that the expressions are "linked"
-			addCandidate( *r, new ast::UniqueExpr{ unqExpr->location, r->expr, unqExpr->id } );
-		}
-	}
-
-	void Finder::postvisit( const ast::StmtExpr * stmtExpr ) {
-		addCandidate( resolveStmtExpr( stmtExpr, context ), tenv );
-	}
-
-	void Finder::postvisit( const ast::UntypedInitExpr * initExpr ) {
-		// handle each option like a cast
-		CandidateList matches;
-		PRINT(
-			std::cerr << "untyped init expr: " << initExpr << std::endl;
-		)
-		// O(n^2) checks of d-types with e-types
-		for ( const ast::InitAlternative & initAlt : initExpr->initAlts ) {
-			// calculate target type
-			const ast::Type * toType = resolveTypeof( initAlt.type, context );
-			toType = adjustExprType( toType, tenv, symtab );
-			// The call to find must occur inside this loop, otherwise polymorphic return
-			// types are not bound to the initialization type, since return type variables are
-			// only open for the duration of resolving the UntypedExpr.
-			CandidateFinder finder( context, tenv, toType );
-			finder.find( initExpr->expr, ResolvMode::withAdjustment() );
-			for ( CandidateRef & cand : finder.candidates ) {
-				if (reason.code == NotFound) reason.code = NoMatch;
-
-				ast::TypeEnvironment env{ cand->env };
-				ast::AssertionSet need( cand->need.begin(), cand->need.end() ), have;
-				ast::OpenVarSet open{ cand->open };
-
-				PRINT(
-					std::cerr << "  @ " << toType << " " << initAlt.designation << std::endl;
-				)
-
-				// It is possible that a cast can throw away some values in a multiply-valued
-				// expression, e.g. cast-to-void, one value to zero. Figure out the prefix of
-				// the subexpression results that are cast directly. The candidate is invalid
-				// if it has fewer results than there are types to cast to.
-				int discardedValues = cand->expr->result->size() - toType->size();
-				if ( discardedValues < 0 ) continue;
-
-				// unification run for side-effects
-				bool canUnify = unify( toType, cand->expr->result, env, need, have, open );
-				(void) canUnify;
-				Cost thisCost = computeConversionCost( cand->expr->result, toType, cand->expr->get_lvalue(),
-					symtab, env );
-				PRINT(
-					Cost legacyCost = castCost( cand->expr->result, toType, cand->expr->get_lvalue(),
-						symtab, env );
-					std::cerr << "Considering initialization:";
-					std::cerr << std::endl << "  FROM: " << cand->expr->result << std::endl;
-					std::cerr << std::endl << "  TO: "   << toType             << std::endl;
-					std::cerr << std::endl << "  Unification " << (canUnify ? "succeeded" : "failed");
-					std::cerr << std::endl << "  Legacy cost " << legacyCost;
-					std::cerr << std::endl << "  New cost " << thisCost;
-					std::cerr << std::endl;
-				)
-				if ( thisCost != Cost::infinity ) {
-					// count one safe conversion for each value that is thrown away
-					thisCost.incSafe( discardedValues );
-					CandidateRef newCand = std::make_shared<Candidate>(
-						new ast::InitExpr{
-							initExpr->location, restructureCast( cand->expr, toType ),
-							initAlt.designation },
-						std::move(env), std::move( open ), std::move( need ), cand->cost, thisCost );
-					inferParameters( newCand, matches );
-				}
-			}
-		}
-
-		// select first on argument cost, then conversion cost
-		CandidateList minArgCost = findMinCost( matches );
-		promoteCvtCost( minArgCost );
-		candidates = findMinCost( minArgCost );
-	}
-
-	// size_t Finder::traceId = Stats::Heap::new_stacktrace_id("Finder");
-	/// Prunes a list of candidates down to those that have the minimum conversion cost for a given
-	/// return type. Skips ambiguous candidates.
-
-} // anonymous namespace
-
-bool CandidateFinder::pruneCandidates( CandidateList & candidates, CandidateList & out, std::vector<std::string> & errors ) {
-	struct PruneStruct {
-		CandidateRef candidate;
-		bool ambiguous;
-
-		PruneStruct() = default;
-		PruneStruct( const CandidateRef & c ) : candidate( c ), ambiguous( false ) {}
-	};
-
-	// find lowest-cost candidate for each type
-	std::unordered_map< std::string, PruneStruct > selected;
-	// attempt to skip satisfyAssertions on more expensive alternatives if better options have been found
-	std::sort(candidates.begin(), candidates.end(), [](const CandidateRef & x, const CandidateRef & y){return x->cost < y->cost;});
-	for ( CandidateRef & candidate : candidates ) {
-		std::string mangleName;
-		{
-			ast::ptr< ast::Type > newType = candidate->expr->result;
-			assertf(candidate->expr->result, "Result of expression %p for candidate is null", candidate->expr.get());
-			candidate->env.apply( newType );
-			mangleName = Mangle::mangle( newType );
-		}
-
-		auto found = selected.find( mangleName );
-		if (found != selected.end() && found->second.candidate->cost < candidate->cost) {
-			PRINT(
-				std::cerr << "cost " << candidate->cost << " loses to "
-					<< found->second.candidate->cost << std::endl;
-			)
-			continue;
-		}
-
-		// xxx - when do satisfyAssertions produce more than 1 result?
-		// this should only happen when initial result type contains
-		// unbound type parameters, then it should never be pruned by
-		// the previous step, since renameTyVars guarantees the mangled name
-		// is unique.
-		CandidateList satisfied;
-		bool needRecomputeKey = false;
-		if (candidate->need.empty()) {
-			satisfied.emplace_back(candidate);
-		}
-		else {
-			satisfyAssertions(candidate, context.symtab, satisfied, errors);
-			needRecomputeKey = true;
-		}
-
-		for (auto & newCand : satisfied) {
-			// recomputes type key, if satisfyAssertions changed it
-			if (needRecomputeKey)
-			{
-				ast::ptr< ast::Type > newType = newCand->expr->result;
-				assertf(newCand->expr->result, "Result of expression %p for candidate is null", newCand->expr.get());
-				newCand->env.apply( newType );
-				mangleName = Mangle::mangle( newType );
-			}
-			auto found = selected.find( mangleName );
-			if ( found != selected.end() ) {
-				if ( newCand->cost < found->second.candidate->cost ) {
-					PRINT(
-						std::cerr << "cost " << newCand->cost << " beats "
-							<< found->second.candidate->cost << std::endl;
-					)
-
-					found->second = PruneStruct{ newCand };
-				} else if ( newCand->cost == found->second.candidate->cost ) {
-					// if one of the candidates contains a deleted identifier, can pick the other,
-					// since deleted expressions should not be ambiguous if there is another option
-					// that is at least as good
-					if ( findDeletedExpr( newCand->expr ) ) {
-						// do nothing
-						PRINT( std::cerr << "candidate is deleted" << std::endl; )
-					} else if ( findDeletedExpr( found->second.candidate->expr ) ) {
-						PRINT( std::cerr << "current is deleted" << std::endl; )
-						found->second = PruneStruct{ newCand };
-					} else {
-						PRINT( std::cerr << "marking ambiguous" << std::endl; )
-						found->second.ambiguous = true;
-					}
-				} else {
-					// xxx - can satisfyAssertions increase the cost?
-					PRINT(
-						std::cerr << "cost " << newCand->cost << " loses to "
-							<< found->second.candidate->cost << std::endl;
-					)
-				}
-			} else {
-				selected.emplace_hint( found, mangleName, newCand );
-			}
-		}
-	}
-
-	// report unambiguous min-cost candidates
-	// CandidateList out;
-	for ( auto & target : selected ) {
-		if ( target.second.ambiguous ) continue;
-
-		CandidateRef cand = target.second.candidate;
-
-		ast::ptr< ast::Type > newResult = cand->expr->result;
-		cand->env.applyFree( newResult );
-		cand->expr = ast::mutate_field(
-			cand->expr.get(), &ast::Expr::result, std::move( newResult ) );
-
-		out.emplace_back( cand );
-	}
-	// if everything is lost in satisfyAssertions, report the error
-	return !selected.empty();
-}
-
-void CandidateFinder::find( const ast::Expr * expr, ResolvMode mode ) {
-	// Find alternatives for expression
-	ast::Pass<Finder> finder{ *this };
-	expr->accept( finder );
-
-	if ( mode.failFast && candidates.empty() ) {
-		switch(finder.core.reason.code) {
-		case Finder::NotFound:
-			{ SemanticError( expr, "No alternatives for expression " ); break; }
-		case Finder::NoMatch:
-			{ SemanticError( expr, "Invalid application of existing declaration(s) in expression " ); break; }
-		case Finder::ArgsToFew:
-		case Finder::ArgsToMany:
-		case Finder::RetsToFew:
-		case Finder::RetsToMany:
-		case Finder::NoReason:
-		default:
-			{ SemanticError( expr->location, "No reasonable alternatives for expression : reasons unkown" ); }
-		}
-	}
-
-	/*
-	if ( mode.satisfyAssns || mode.prune ) {
-		// trim candidates to just those where the assertions are satisfiable
-		// - necessary pre-requisite to pruning
-		CandidateList satisfied;
-		std::vector< std::string > errors;
-		for ( CandidateRef & candidate : candidates ) {
-			satisfyAssertions( candidate, localSyms, satisfied, errors );
-		}
-
-		// fail early if none such
-		if ( mode.failFast && satisfied.empty() ) {
-			std::ostringstream stream;
-			stream << "No alternatives with satisfiable assertions for " << expr << "\n";
-			for ( const auto& err : errors ) {
-				stream << err;
-			}
-			SemanticError( expr->location, stream.str() );
-		}
-
-		// reset candidates
-		candidates = move( satisfied );
-	}
-	*/
-
-	if ( mode.prune ) {
-		// trim candidates to single best one
-		PRINT(
-			std::cerr << "alternatives before prune:" << std::endl;
-			print( std::cerr, candidates );
-		)
-
-		CandidateList pruned;
-		std::vector<std::string> errors;
-		bool found = pruneCandidates( candidates, pruned, errors );
-
-		if ( mode.failFast && pruned.empty() ) {
-			std::ostringstream stream;
-			if (found) {
-				CandidateList winners = findMinCost( candidates );
-				stream << "Cannot choose between " << winners.size() << " alternatives for "
-					"expression\n";
-				ast::print( stream, expr );
-				stream << " Alternatives are:\n";
-				print( stream, winners, 1 );
-				SemanticError( expr->location, stream.str() );
-			}
-			else {
-				stream << "No alternatives with satisfiable assertions for " << expr << "\n";
-				for ( const auto& err : errors ) {
-					stream << err;
-				}
-				SemanticError( expr->location, stream.str() );
-			}
-		}
-
-		auto oldsize = candidates.size();
-		candidates = std::move( pruned );
-
-		PRINT(
-			std::cerr << "there are " << oldsize << " alternatives before elimination" << std::endl;
-		)
-		PRINT(
-			std::cerr << "there are " << candidates.size() << " alternatives after elimination"
-				<< std::endl;
-		)
-	}
-
-	// adjust types after pruning so that types substituted by pruneAlternatives are correctly
-	// adjusted
-	if ( mode.adjust ) {
-		for ( CandidateRef & r : candidates ) {
-			r->expr = ast::mutate_field(
-				r->expr.get(), &ast::Expr::result,
-				adjustExprType( r->expr->result, r->env, context.symtab ) );
-		}
-	}
-
-	// Central location to handle gcc extension keyword, etc. for all expressions
-	for ( CandidateRef & r : candidates ) {
-		if ( r->expr->extension != expr->extension ) {
-			r->expr.get_and_mutate()->extension = expr->extension;
-		}
-	}
-}
-
-std::vector< CandidateFinder > CandidateFinder::findSubExprs(
-	const std::vector< ast::ptr< ast::Expr > > & xs
-) {
-	std::vector< CandidateFinder > out;
-
-	for ( const auto & x : xs ) {
-		out.emplace_back( context, env );
-		out.back().find( x, ResolvMode::withAdjustment() );
-
-		PRINT(
-			std::cerr << "findSubExprs" << std::endl;
-			print( std::cerr, out.back().candidates );
-		)
-	}
-
-	return out;
-}
-
 const ast::Expr * referenceToRvalueConversion( const ast::Expr * expr, Cost & cost ) {
 	if ( expr->result.as< ast::ReferenceType >() ) {
@@ -1942,4 +65,7 @@
 	return expr;
 }
+
+/// Unique identifier for matching expression resolutions to their requesting expression
+UniqueId globalResnSlot = 0;
 
 Cost computeConversionCost(
@@ -1968,4 +94,1901 @@
 }
 
+namespace {
+	/// First index is which argument, second is which alternative, third is which exploded element
+	using ExplodedArgs_new = std::deque< std::vector< ExplodedArg > >;
+
+	/// Returns a list of alternatives with the minimum cost in the given list
+	CandidateList findMinCost( const CandidateList & candidates ) {
+		CandidateList out;
+		Cost minCost = Cost::infinity;
+		for ( const CandidateRef & r : candidates ) {
+			if ( r->cost < minCost ) {
+				minCost = r->cost;
+				out.clear();
+				out.emplace_back( r );
+			} else if ( r->cost == minCost ) {
+				out.emplace_back( r );
+			}
+		}
+		return out;
+	}
+
+	/// Computes conversion cost for a given expression to a given type
+	const ast::Expr * computeExpressionConversionCost(
+		const ast::Expr * arg, const ast::Type * paramType, const ast::SymbolTable & symtab, const ast::TypeEnvironment & env, Cost & outCost
+	) {
+		Cost convCost = computeConversionCost(
+				arg->result, paramType, arg->get_lvalue(), symtab, env );
+		outCost += convCost;
+
+		// If there is a non-zero conversion cost, ignoring poly cost, then the expression requires
+		// conversion. Ignore poly cost for now, since this requires resolution of the cast to
+		// infer parameters and this does not currently work for the reason stated below
+		Cost tmpCost = convCost;
+		tmpCost.incPoly( -tmpCost.get_polyCost() );
+		if ( tmpCost != Cost::zero ) {
+			ast::ptr< ast::Type > newType = paramType;
+			env.apply( newType );
+			return new ast::CastExpr{ arg, newType };
+
+			// xxx - *should* be able to resolve this cast, but at the moment pointers are not
+			// castable to zero_t, but are implicitly convertible. This is clearly inconsistent,
+			// once this is fixed it should be possible to resolve the cast.
+			// xxx - this isn't working, it appears because type1 (parameter) is seen as widenable,
+			// but it shouldn't be because this makes the conversion from DT* to DT* since
+			// commontype(zero_t, DT*) is DT*, rather than nothing
+
+			// CandidateFinder finder{ symtab, env };
+			// finder.find( arg, ResolvMode::withAdjustment() );
+			// assertf( finder.candidates.size() > 0,
+			// 	"Somehow castable expression failed to find alternatives." );
+			// assertf( finder.candidates.size() == 1,
+			// 	"Somehow got multiple alternatives for known cast expression." );
+			// return finder.candidates.front()->expr;
+		}
+
+		return arg;
+	}
+
+	/// Computes conversion cost for a given candidate
+	Cost computeApplicationConversionCost(
+		CandidateRef cand, const ast::SymbolTable & symtab
+	) {
+		auto appExpr = cand->expr.strict_as< ast::ApplicationExpr >();
+		auto pointer = appExpr->func->result.strict_as< ast::PointerType >();
+		auto function = pointer->base.strict_as< ast::FunctionType >();
+
+		Cost convCost = Cost::zero;
+		const auto & params = function->params;
+		auto param = params.begin();
+		auto & args = appExpr->args;
+
+		for ( unsigned i = 0; i < args.size(); ++i ) {
+			const ast::Type * argType = args[i]->result;
+			PRINT(
+				std::cerr << "arg expression:" << std::endl;
+				ast::print( std::cerr, args[i], 2 );
+				std::cerr << "--- results are" << std::endl;
+				ast::print( std::cerr, argType, 2 );
+			)
+
+			if ( param == params.end() ) {
+				if ( function->isVarArgs ) {
+					convCost.incUnsafe();
+					PRINT( std::cerr << "end of params with varargs function: inc unsafe: "
+						<< convCost << std::endl; ; )
+					// convert reference-typed expressions into value-typed expressions
+					cand->expr = ast::mutate_field_index(
+						appExpr, &ast::ApplicationExpr::args, i,
+						referenceToRvalueConversion( args[i], convCost ) );
+					continue;
+				} else return Cost::infinity;
+			}
+
+			if ( auto def = args[i].as< ast::DefaultArgExpr >() ) {
+				// Default arguments should be free - don't include conversion cost.
+				// Unwrap them here because they are not relevant to the rest of the system
+				cand->expr = ast::mutate_field_index(
+					appExpr, &ast::ApplicationExpr::args, i, def->expr );
+				++param;
+				continue;
+			}
+
+			// mark conversion cost and also specialization cost of param type
+			// const ast::Type * paramType = (*param)->get_type();
+			cand->expr = ast::mutate_field_index(
+				appExpr, &ast::ApplicationExpr::args, i,
+				computeExpressionConversionCost(
+					args[i], *param, symtab, cand->env, convCost ) );
+			convCost.decSpec( specCost( *param ) );
+			++param;  // can't be in for-loop update because of the continue
+		}
+
+		if ( param != params.end() ) return Cost::infinity;
+
+		// specialization cost of return types can't be accounted for directly, it disables
+		// otherwise-identical calls, like this example based on auto-newline in the I/O lib:
+		//
+		//   forall(otype OS) {
+		//     void ?|?(OS&, int);  // with newline
+		//     OS&  ?|?(OS&, int);  // no newline, always chosen due to more specialization
+		//   }
+
+		// mark type variable and specialization cost of forall clause
+		convCost.incVar( function->forall.size() );
+		convCost.decSpec( function->assertions.size() );
+
+		return convCost;
+	}
+
+	void makeUnifiableVars(
+		const ast::FunctionType * type, ast::OpenVarSet & unifiableVars,
+		ast::AssertionSet & need
+	) {
+		for ( auto & tyvar : type->forall ) {
+			unifiableVars[ *tyvar ] = ast::TypeData{ tyvar->base };
+		}
+		for ( auto & assn : type->assertions ) {
+			need[ assn ].isUsed = true;
+		}
+	}
+
+	/// Gets a default value from an initializer, nullptr if not present
+	const ast::ConstantExpr * getDefaultValue( const ast::Init * init ) {
+		if ( auto si = dynamic_cast< const ast::SingleInit * >( init ) ) {
+			if ( auto ce = si->value.as< ast::CastExpr >() ) {
+				return ce->arg.as< ast::ConstantExpr >();
+			} else {
+				return si->value.as< ast::ConstantExpr >();
+			}
+		}
+		return nullptr;
+	}
+
+	/// State to iteratively build a match of parameter expressions to arguments
+	struct ArgPack {
+		std::size_t parent;          ///< Index of parent pack
+		ast::ptr< ast::Expr > expr;  ///< The argument stored here
+		Cost cost;                   ///< The cost of this argument
+		ast::TypeEnvironment env;    ///< Environment for this pack
+		ast::AssertionSet need;      ///< Assertions outstanding for this pack
+		ast::AssertionSet have;      ///< Assertions found for this pack
+		ast::OpenVarSet open;        ///< Open variables for this pack
+		unsigned nextArg;            ///< Index of next argument in arguments list
+		unsigned tupleStart;         ///< Number of tuples that start at this index
+		unsigned nextExpl;           ///< Index of next exploded element
+		unsigned explAlt;            ///< Index of alternative for nextExpl > 0
+
+		ArgPack()
+		: parent( 0 ), expr(), cost( Cost::zero ), env(), need(), have(), open(), nextArg( 0 ),
+		  tupleStart( 0 ), nextExpl( 0 ), explAlt( 0 ) {}
+
+		ArgPack(
+			const ast::TypeEnvironment & env, const ast::AssertionSet & need,
+			const ast::AssertionSet & have, const ast::OpenVarSet & open )
+		: parent( 0 ), expr(), cost( Cost::zero ), env( env ), need( need ), have( have ),
+		  open( open ), nextArg( 0 ), tupleStart( 0 ), nextExpl( 0 ), explAlt( 0 ) {}
+
+		ArgPack(
+			std::size_t parent, const ast::Expr * expr, ast::TypeEnvironment && env,
+			ast::AssertionSet && need, ast::AssertionSet && have, ast::OpenVarSet && open,
+			unsigned nextArg, unsigned tupleStart = 0, Cost cost = Cost::zero,
+			unsigned nextExpl = 0, unsigned explAlt = 0 )
+		: parent(parent), expr( expr ), cost( cost ), env( std::move( env ) ), need( std::move( need ) ),
+		  have( std::move( have ) ), open( std::move( open ) ), nextArg( nextArg ), tupleStart( tupleStart ),
+		  nextExpl( nextExpl ), explAlt( explAlt ) {}
+
+		ArgPack(
+			const ArgPack & o, ast::TypeEnvironment && env, ast::AssertionSet && need,
+			ast::AssertionSet && have, ast::OpenVarSet && open, unsigned nextArg, Cost added )
+		: parent( o.parent ), expr( o.expr ), cost( o.cost + added ), env( std::move( env ) ),
+		  need( std::move( need ) ), have( std::move( have ) ), open( std::move( open ) ), nextArg( nextArg ),
+		  tupleStart( o.tupleStart ), nextExpl( 0 ), explAlt( 0 ) {}
+
+		/// true if this pack is in the middle of an exploded argument
+		bool hasExpl() const { return nextExpl > 0; }
+
+		/// Gets the list of exploded candidates for this pack
+		const ExplodedArg & getExpl( const ExplodedArgs_new & args ) const {
+			return args[ nextArg-1 ][ explAlt ];
+		}
+
+		/// Ends a tuple expression, consolidating the appropriate args
+		void endTuple( const std::vector< ArgPack > & packs ) {
+			// add all expressions in tuple to list, summing cost
+			std::deque< const ast::Expr * > exprs;
+			const ArgPack * pack = this;
+			if ( expr ) { exprs.emplace_front( expr ); }
+			while ( pack->tupleStart == 0 ) {
+				pack = &packs[pack->parent];
+				exprs.emplace_front( pack->expr );
+				cost += pack->cost;
+			}
+			// reset pack to appropriate tuple
+			std::vector< ast::ptr< ast::Expr > > exprv( exprs.begin(), exprs.end() );
+			expr = new ast::TupleExpr{ expr->location, std::move( exprv ) };
+			tupleStart = pack->tupleStart - 1;
+			parent = pack->parent;
+		}
+	};
+
+	/// Instantiates an argument to match a parameter, returns false if no matching results left
+	bool instantiateArgument(
+		const ast::Type * paramType, const ast::Init * init, const ExplodedArgs_new & args,
+		std::vector< ArgPack > & results, std::size_t & genStart, const ast::SymbolTable & symtab,
+		unsigned nTuples = 0
+	) {
+		if ( auto tupleType = dynamic_cast< const ast::TupleType * >( paramType ) ) {
+			// paramType is a TupleType -- group args into a TupleExpr
+			++nTuples;
+			for ( const ast::Type * type : *tupleType ) {
+				// xxx - dropping initializer changes behaviour from previous, but seems correct
+				// ^^^ need to handle the case where a tuple has a default argument
+				if ( ! instantiateArgument(
+					type, nullptr, args, results, genStart, symtab, nTuples ) ) return false;
+				nTuples = 0;
+			}
+			// re-constitute tuples for final generation
+			for ( auto i = genStart; i < results.size(); ++i ) {
+				results[i].endTuple( results );
+			}
+			return true;
+		} else if ( const ast::TypeInstType * ttype = Tuples::isTtype( paramType ) ) {
+			// paramType is a ttype, consumes all remaining arguments
+
+			// completed tuples; will be spliced to end of results to finish
+			std::vector< ArgPack > finalResults{};
+
+			// iterate until all results completed
+			std::size_t genEnd;
+			++nTuples;
+			do {
+				genEnd = results.size();
+
+				// add another argument to results
+				for ( std::size_t i = genStart; i < genEnd; ++i ) {
+					unsigned nextArg = results[i].nextArg;
+
+					// use next element of exploded tuple if present
+					if ( results[i].hasExpl() ) {
+						const ExplodedArg & expl = results[i].getExpl( args );
+
+						unsigned nextExpl = results[i].nextExpl + 1;
+						if ( nextExpl == expl.exprs.size() ) { nextExpl = 0; }
+
+						results.emplace_back(
+							i, expl.exprs[ results[i].nextExpl ], copy( results[i].env ),
+							copy( results[i].need ), copy( results[i].have ),
+							copy( results[i].open ), nextArg, nTuples, Cost::zero, nextExpl,
+							results[i].explAlt );
+
+						continue;
+					}
+
+					// finish result when out of arguments
+					if ( nextArg >= args.size() ) {
+						ArgPack newResult{
+							results[i].env, results[i].need, results[i].have, results[i].open };
+						newResult.nextArg = nextArg;
+						const ast::Type * argType = nullptr;
+
+						if ( nTuples > 0 || ! results[i].expr ) {
+							// first iteration or no expression to clone,
+							// push empty tuple expression
+							newResult.parent = i;
+							newResult.expr = new ast::TupleExpr{ CodeLocation{}, {} };
+							argType = newResult.expr->result;
+						} else {
+							// clone result to collect tuple
+							newResult.parent = results[i].parent;
+							newResult.cost = results[i].cost;
+							newResult.tupleStart = results[i].tupleStart;
+							newResult.expr = results[i].expr;
+							argType = newResult.expr->result;
+
+							if ( results[i].tupleStart > 0 && Tuples::isTtype( argType ) ) {
+								// the case where a ttype value is passed directly is special,
+								// e.g. for argument forwarding purposes
+								// xxx - what if passing multiple arguments, last of which is
+								//       ttype?
+								// xxx - what would happen if unify was changed so that unifying
+								//       tuple
+								// types flattened both before unifying lists? then pass in
+								// TupleType (ttype) below.
+								--newResult.tupleStart;
+							} else {
+								// collapse leftover arguments into tuple
+								newResult.endTuple( results );
+								argType = newResult.expr->result;
+							}
+						}
+
+						// check unification for ttype before adding to final
+						if (
+							unify(
+								ttype, argType, newResult.env, newResult.need, newResult.have,
+								newResult.open )
+						) {
+							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 ) {
+			// Set need bindings for any unbound assertions
+			UniqueId crntResnSlot = 0; // matching ID for this expression's assertions
+			for ( auto & assn : newCand->need ) {
+				// skip already-matched assertions
+				if ( assn.second.resnSlot != 0 ) continue;
+				// assign slot for expression if needed
+				if ( crntResnSlot == 0 ) { crntResnSlot = ++globalResnSlot; }
+				// fix slot to assertion
+				assn.second.resnSlot = crntResnSlot;
+			}
+			// pair slot to expression
+			if ( crntResnSlot != 0 ) {
+				newCand->expr.get_and_mutate()->inferred.resnSlots().emplace_back( crntResnSlot );
+			}
+
+			// add to output list; assertion satisfaction will occur later
+			out.emplace_back( newCand );
+		}
+
+		/// Completes a function candidate with arguments located
+		void validateFunctionCandidate(
+			const CandidateRef & func, ArgPack & result, const std::vector< ArgPack > & results,
+			CandidateList & out
+		) {
+			ast::ApplicationExpr * appExpr =
+				new ast::ApplicationExpr{ func->expr->location, func->expr };
+			// sum cost and accumulate arguments
+			std::deque< const ast::Expr * > args;
+			Cost cost = func->cost;
+			const ArgPack * pack = &result;
+			while ( pack->expr ) {
+				args.emplace_front( pack->expr );
+				cost += pack->cost;
+				pack = &results[pack->parent];
+			}
+			std::vector< ast::ptr< ast::Expr > > vargs( args.begin(), args.end() );
+			appExpr->args = std::move( vargs );
+			// build and validate new candidate
+			auto newCand =
+				std::make_shared<Candidate>( appExpr, result.env, result.open, result.need, cost );
+			PRINT(
+				std::cerr << "instantiate function success: " << appExpr << std::endl;
+				std::cerr << "need assertions:" << std::endl;
+				ast::print( std::cerr, result.need, 2 );
+			)
+			inferParameters( newCand, out );
+		}
+
+		/// Builds a list of candidates for a function, storing them in out
+		void makeFunctionCandidates(
+			const CandidateRef & func, const ast::FunctionType * funcType,
+			const ExplodedArgs_new & args, CandidateList & out
+		) {
+			ast::OpenVarSet funcOpen;
+			ast::AssertionSet funcNeed, funcHave;
+			ast::TypeEnvironment funcEnv{ func->env };
+			makeUnifiableVars( funcType, funcOpen, funcNeed );
+			// add all type variables as open variables now so that those not used in the
+			// parameter list are still considered open
+			funcEnv.add( funcType->forall );
+
+			if ( targetType && ! targetType->isVoid() && ! funcType->returns.empty() ) {
+				// attempt to narrow based on expected target type
+				const ast::Type * returnType = funcType->returns.front();
+				if ( 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(
+							funcType->params[i], obj->init, args, results, genStart, symtab)) return;
+					}
+					goto endMatch;
+				}
+			}
+			for ( const auto & param : funcType->params ) {
+				// Try adding the arguments corresponding to the current parameter to the existing
+				// matches
+				// no default args for indirect calls
+				if ( ! instantiateArgument(
+					param, nullptr, args, results, genStart, symtab ) ) return;
+			}
+
+			endMatch:
+			if ( funcType->isVarArgs ) {
+				// append any unused arguments to vararg pack
+				std::size_t genEnd;
+				do {
+					genEnd = results.size();
+
+					// iterate results
+					for ( std::size_t i = genStart; i < genEnd; ++i ) {
+						unsigned nextArg = results[i].nextArg;
+
+						// use remainder of exploded tuple if present
+						if ( results[i].hasExpl() ) {
+							const ExplodedArg & expl = results[i].getExpl( args );
+
+							unsigned nextExpl = results[i].nextExpl + 1;
+							if ( nextExpl == expl.exprs.size() ) { nextExpl = 0; }
+
+							results.emplace_back(
+								i, expl.exprs[ results[i].nextExpl ], copy( results[i].env ),
+								copy( results[i].need ), copy( results[i].have ),
+								copy( results[i].open ), nextArg, 0, Cost::zero, nextExpl,
+								results[i].explAlt );
+
+							continue;
+						}
+
+						// finish result when out of arguments
+						if ( nextArg >= args.size() ) {
+							validateFunctionCandidate( func, results[i], results, out );
+
+							continue;
+						}
+
+						// add each possible next argument
+						for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
+							const ExplodedArg & expl = args[nextArg][j];
+
+							// fresh copies of parent parameters for this iteration
+							ast::TypeEnvironment env = results[i].env;
+							ast::OpenVarSet open = results[i].open;
+
+							env.addActual( expl.env, open );
+
+							// skip empty tuple arguments by (nearly) cloning parent into next gen
+							if ( expl.exprs.empty() ) {
+								results.emplace_back(
+									results[i], std::move( env ), copy( results[i].need ),
+									copy( results[i].have ), std::move( open ), nextArg + 1,
+									expl.cost );
+
+								continue;
+							}
+
+							// add new result
+							results.emplace_back(
+								i, expl.exprs.front(), std::move( env ), copy( results[i].need ),
+								copy( results[i].have ), std::move( open ), nextArg + 1, 0, expl.cost,
+								expl.exprs.size() == 1 ? 0 : 1, j );
+						}
+					}
+
+					genStart = genEnd;
+				} while( genEnd != results.size() );
+			} else {
+				// filter out the results that don't use all the arguments
+				for ( std::size_t i = genStart; i < results.size(); ++i ) {
+					ArgPack & result = results[i];
+					if ( ! result.hasExpl() && result.nextArg >= args.size() ) {
+						validateFunctionCandidate( func, result, results, out );
+					}
+				}
+			}
+		}
+
+		/// Adds implicit struct-conversions to the alternative list
+		void addAnonConversions( const CandidateRef & cand ) {
+			// adds anonymous member interpretations whenever an aggregate value type is seen.
+			// it's okay for the aggregate expression to have reference type -- cast it to the
+			// base type to treat the aggregate as the referenced value
+			ast::ptr< ast::Expr > aggrExpr( cand->expr );
+			ast::ptr< ast::Type > & aggrType = aggrExpr.get_and_mutate()->result;
+			cand->env.apply( aggrType );
+
+			if ( aggrType.as< ast::ReferenceType >() ) {
+				aggrExpr = new ast::CastExpr{ aggrExpr, aggrType->stripReferences() };
+			}
+
+			if ( auto structInst = aggrExpr->result.as< ast::StructInstType >() ) {
+				addAggMembers( structInst, aggrExpr, *cand, Cost::unsafe, "" );
+			} else if ( auto unionInst = aggrExpr->result.as< ast::UnionInstType >() ) {
+				addAggMembers( unionInst, aggrExpr, *cand, Cost::unsafe, "" );
+			}
+		}
+
+		/// Adds aggregate member interpretations
+		void addAggMembers(
+			const ast::BaseInstType * aggrInst, const ast::Expr * expr,
+			const Candidate & cand, const Cost & addedCost, const std::string & name
+		) {
+			for ( const ast::Decl * decl : aggrInst->lookup( name ) ) {
+				auto dwt = strict_dynamic_cast< const ast::DeclWithType * >( decl );
+				CandidateRef newCand = std::make_shared<Candidate>(
+					cand, new ast::MemberExpr{ expr->location, dwt, expr }, addedCost );
+				// add anonymous member interpretations whenever an aggregate value type is seen
+				// as a member expression
+				addAnonConversions( newCand );
+				candidates.emplace_back( std::move( newCand ) );
+			}
+		}
+
+		/// Adds tuple member interpretations
+		void addTupleMembers(
+			const ast::TupleType * tupleType, const ast::Expr * expr, const Candidate & cand,
+			const Cost & addedCost, const ast::Expr * member
+		) {
+			if ( auto constantExpr = dynamic_cast< const ast::ConstantExpr * >( member ) ) {
+				// get the value of the constant expression as an int, must be between 0 and the
+				// length of the tuple to have meaning
+				long long val = constantExpr->intValue();
+				if ( val >= 0 && (unsigned long long)val < tupleType->size() ) {
+					addCandidate(
+						cand, new ast::TupleIndexExpr{ expr->location, expr, (unsigned)val },
+						addedCost );
+				}
+			}
+		}
+
+		void postvisit( const ast::UntypedExpr * untypedExpr ) {
+			std::vector< CandidateFinder > argCandidates =
+				selfFinder.findSubExprs( untypedExpr->args );
+
+			// take care of possible tuple assignments
+			// if not tuple assignment, handled as normal function call
+			Tuples::handleTupleAssignment( selfFinder, untypedExpr, argCandidates );
+
+			CandidateFinder funcFinder( context, tenv );
+			if (auto nameExpr = untypedExpr->func.as<ast::NameExpr>()) {
+				auto kind = ast::SymbolTable::getSpecialFunctionKind(nameExpr->name);
+				if (kind != ast::SymbolTable::SpecialFunctionKind::NUMBER_OF_KINDS) {
+					assertf(!argCandidates.empty(), "special function call without argument");
+					for (auto & firstArgCand: argCandidates[0]) {
+						ast::ptr<ast::Type> argType = firstArgCand->expr->result;
+						firstArgCand->env.apply(argType);
+						// strip references
+						// xxx - is this correct?
+						while (argType.as<ast::ReferenceType>()) argType = argType.as<ast::ReferenceType>()->base;
+
+						// convert 1-tuple to plain type
+						if (auto tuple = argType.as<ast::TupleType>()) {
+							if (tuple->size() == 1) {
+								argType = tuple->types[0];
+							}
+						}
+
+						// if argType is an unbound type parameter, all special functions need to be searched.
+						if (isUnboundType(argType)) {
+							funcFinder.otypeKeys.clear();
+							break;
+						}
+
+						if (argType.as<ast::PointerType>()) funcFinder.otypeKeys.insert(Mangle::Encoding::pointer);						
+						// else if (const ast::EnumInstType * enumInst = argType.as<ast::EnumInstType>()) {
+						// 	const ast::EnumDecl * enumDecl = enumInst->base; // Here
+						// 	if ( const ast::Type* enumType = enumDecl->base ) {
+						// 		// instance of enum (T) is a instance of type (T) 
+						// 		funcFinder.otypeKeys.insert(Mangle::mangle(enumType, Mangle::NoGenericParams | Mangle::Type));
+						// 	} else {
+						// 		// instance of an untyped enum is techically int
+						// 		funcFinder.otypeKeys.insert(Mangle::mangle(enumDecl, Mangle::NoGenericParams | Mangle::Type));
+						// 	}
+						// }
+						else funcFinder.otypeKeys.insert(Mangle::mangle(argType, Mangle::NoGenericParams | Mangle::Type));
+					}
+				}
+			}
+			// if candidates are already produced, do not fail
+			// xxx - is it possible that handleTupleAssignment and main finder both produce candidates?
+			// this means there exists ctor/assign functions with a tuple as first parameter.
+			ResolvMode mode = {
+				true, // adjust
+				!untypedExpr->func.as<ast::NameExpr>(), // prune if not calling by name
+				selfFinder.candidates.empty() // failfast if other options are not found
+			};
+			funcFinder.find( untypedExpr->func, mode );
+			// short-circuit if no candidates
+			// if ( funcFinder.candidates.empty() ) return;
+
+			reason.code = NoMatch;
+
+			// find function operators
+			ast::ptr< ast::Expr > opExpr = new ast::NameExpr{ untypedExpr->location, "?()" }; // ??? why not ?{}
+			CandidateFinder opFinder( context, tenv );
+			// okay if there aren't any function operations
+			opFinder.find( opExpr, ResolvMode::withoutFailFast() );
+			PRINT(
+				std::cerr << "known function ops:" << std::endl;
+				print( std::cerr, opFinder.candidates, 1 );
+			)
+
+			// pre-explode arguments
+			ExplodedArgs_new argExpansions;
+			for ( const CandidateFinder & args : argCandidates ) {
+				argExpansions.emplace_back();
+				auto & argE = argExpansions.back();
+				for ( const CandidateRef & arg : args ) { argE.emplace_back( *arg, symtab ); }
+			}
+
+			// Find function matches
+			CandidateList found;
+			SemanticErrorException errors;
+			for ( CandidateRef & func : funcFinder ) {
+				try {
+					PRINT(
+						std::cerr << "working on alternative:" << std::endl;
+						print( std::cerr, *func, 2 );
+					)
+
+					// check if the type is a pointer to function
+					const ast::Type * funcResult = func->expr->result->stripReferences();
+					if ( auto pointer = dynamic_cast< const ast::PointerType * >( funcResult ) ) {
+						if ( auto function = pointer->base.as< ast::FunctionType >() ) {
+							// if (!selfFinder.allowVoid && function->returns.empty()) continue;
+							CandidateRef newFunc{ new Candidate{ *func } };
+							newFunc->expr =
+								referenceToRvalueConversion( newFunc->expr, newFunc->cost );
+							makeFunctionCandidates( newFunc, function, argExpansions, found );
+						}
+					} else if (
+						auto inst = dynamic_cast< const ast::TypeInstType * >( funcResult )
+					) {
+						if ( const ast::EqvClass * clz = func->env.lookup( *inst ) ) {
+							if ( auto function = clz->bound.as< ast::FunctionType >() ) {
+								CandidateRef newFunc{ new Candidate{ *func } };
+								newFunc->expr =
+									referenceToRvalueConversion( newFunc->expr, newFunc->cost );
+								makeFunctionCandidates( newFunc, function, argExpansions, found );
+							}
+						}
+					}
+				} catch ( SemanticErrorException & e ) { errors.append( e ); }
+			}
+
+			// Find matches on function operators `?()`
+			if ( ! opFinder.candidates.empty() ) {
+				// add exploded function alternatives to front of argument list
+				std::vector< ExplodedArg > funcE;
+				funcE.reserve( funcFinder.candidates.size() );
+				for ( const CandidateRef & func : funcFinder ) {
+					funcE.emplace_back( *func, symtab );
+				}
+				argExpansions.emplace_front( std::move( funcE ) );
+
+				for ( const CandidateRef & op : opFinder ) {
+					try {
+						// check if type is pointer-to-function
+						const ast::Type * opResult = op->expr->result->stripReferences();
+						if ( auto pointer = dynamic_cast< const ast::PointerType * >( opResult ) ) {
+							if ( auto function = pointer->base.as< ast::FunctionType >() ) {
+								CandidateRef newOp{ new Candidate{ *op} };
+								newOp->expr =
+									referenceToRvalueConversion( newOp->expr, newOp->cost );
+								makeFunctionCandidates( newOp, function, argExpansions, found );
+							}
+						}
+					} catch ( SemanticErrorException & e ) { errors.append( e ); }
+				}
+			}
+
+			// Implement SFINAE; resolution errors are only errors if there aren't any non-error
+			// candidates
+			if ( found.empty() && ! errors.isEmpty() ) { throw errors; }
+
+			// 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 );
+			}
+		}
+
+		/// true if expression is an lvalue
+		static bool isLvalue( const ast::Expr * x ) {
+			return x->result && ( x->get_lvalue() || x->result.as< ast::ReferenceType >() );
+		}
+
+		void postvisit( const ast::AddressExpr * addressExpr ) {
+			CandidateFinder finder( context, tenv );
+			finder.find( addressExpr->arg );
+
+			if( finder.candidates.empty() ) return;
+
+			reason.code = NoMatch;
+
+			for ( CandidateRef & r : finder.candidates ) {
+				if ( ! isLvalue( r->expr ) ) continue;
+				addCandidate( *r, new ast::AddressExpr{ addressExpr->location, r->expr } );
+			}
+		}
+
+		void postvisit( const ast::LabelAddressExpr * labelExpr ) {
+			addCandidate( labelExpr, tenv );
+		}
+
+		void postvisit( const ast::CastExpr * castExpr ) {
+			ast::ptr< ast::Type > toType = castExpr->result;
+			assert( toType );
+			toType = resolveTypeof( toType, context );
+			toType = adjustExprType( toType, tenv, symtab );
+
+			CandidateFinder finder( context, tenv, toType );
+			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 postvisit( const ast::VirtualCastExpr * castExpr ) {
+			assertf( castExpr->result, "Implicit virtual cast targets not yet supported." );
+			CandidateFinder finder( context, tenv );
+			// don't prune here, all alternatives guaranteed to have same type
+			finder.find( castExpr->arg, ResolvMode::withoutPrune() );
+			for ( CandidateRef & r : finder.candidates ) {
+				addCandidate(
+					*r,
+					new ast::VirtualCastExpr{ castExpr->location, r->expr, castExpr->result } );
+			}
+		}
+
+		void postvisit( const ast::KeywordCastExpr * castExpr ) {
+			const auto & loc = castExpr->location;
+			assertf( castExpr->result, "Cast target should have been set in Validate." );
+			auto ref = castExpr->result.strict_as<ast::ReferenceType>();
+			auto inst = ref->base.strict_as<ast::StructInstType>();
+			auto target = inst->base.get();
+
+			CandidateFinder finder( context, tenv );
+
+			auto pick_alternatives = [target, this](CandidateList & found, bool expect_ref) {
+				for(auto & cand : found) {
+					const ast::Type * expr = cand->expr->result.get();
+					if(expect_ref) {
+						auto res = dynamic_cast<const ast::ReferenceType*>(expr);
+						if(!res) { continue; }
+						expr = res->base.get();
+					}
+
+					if(auto insttype = dynamic_cast<const ast::TypeInstType*>(expr)) {
+						auto td = cand->env.lookup(*insttype);
+						if(!td) { continue; }
+						expr = td->bound.get();
+					}
+
+					if(auto base = dynamic_cast<const ast::StructInstType*>(expr)) {
+						if(base->base == target) {
+							candidates.push_back( std::move(cand) );
+							reason.code = NoReason;
+						}
+					}
+				}
+			};
+
+			try {
+				// Attempt 1 : turn (thread&)X into (thread$&)X.__thrd
+				// Clone is purely for memory management
+				std::unique_ptr<const ast::Expr> tech1 { new ast::UntypedMemberExpr(loc, new ast::NameExpr(loc, castExpr->concrete_target.field), castExpr->arg) };
+
+				// don't prune here, since it's guaranteed all alternatives will have the same type
+				finder.find( tech1.get(), ResolvMode::withoutPrune() );
+				pick_alternatives(finder.candidates, false);
+
+				return;
+			} catch(SemanticErrorException & ) {}
+
+			// Fallback : turn (thread&)X into (thread$&)get_thread(X)
+			std::unique_ptr<const ast::Expr> fallback { ast::UntypedExpr::createDeref(loc,  new ast::UntypedExpr(loc, new ast::NameExpr(loc, castExpr->concrete_target.getter), { castExpr->arg })) };
+			// don't prune here, since it's guaranteed all alternatives will have the same type
+			finder.find( fallback.get(), ResolvMode::withoutPrune() );
+
+			pick_alternatives(finder.candidates, true);
+
+			// Whatever happens here, we have no more fallbacks
+		}
+
+		void postvisit( const ast::UntypedMemberExpr * memberExpr ) {
+			CandidateFinder aggFinder( context, tenv );
+			aggFinder.find( memberExpr->aggregate, ResolvMode::withAdjustment() );
+			for ( CandidateRef & agg : aggFinder.candidates ) {
+				// it's okay for the aggregate expression to have reference type -- cast it to the
+				// base type to treat the aggregate as the referenced value
+				Cost addedCost = Cost::zero;
+				agg->expr = referenceToRvalueConversion( agg->expr, addedCost );
+
+				// find member of the given type
+				if ( auto structInst = agg->expr->result.as< ast::StructInstType >() ) {
+					addAggMembers(
+						structInst, agg->expr, *agg, addedCost, getMemberName( memberExpr ) );
+				} else if ( auto unionInst = agg->expr->result.as< ast::UnionInstType >() ) {
+					addAggMembers(
+						unionInst, agg->expr, *agg, addedCost, getMemberName( memberExpr ) );
+				} else if ( auto tupleType = agg->expr->result.as< ast::TupleType >() ) {
+					addTupleMembers( tupleType, agg->expr, *agg, addedCost, memberExpr->member );
+				}
+			}
+		}
+
+		void postvisit( const ast::MemberExpr * memberExpr ) {
+			addCandidate( memberExpr, tenv );
+		}
+
+		void postvisit( const ast::NameExpr * nameExpr ) {
+			std::vector< ast::SymbolTable::IdData > declList;
+			if (!selfFinder.otypeKeys.empty()) {
+				auto kind = ast::SymbolTable::getSpecialFunctionKind(nameExpr->name);
+				assertf(kind != ast::SymbolTable::SpecialFunctionKind::NUMBER_OF_KINDS, "special lookup with non-special target: %s", nameExpr->name.c_str());
+
+				for (auto & otypeKey: selfFinder.otypeKeys) {
+					auto result = symtab.specialLookupId(kind, otypeKey);
+					declList.insert(declList.end(), std::make_move_iterator(result.begin()), std::make_move_iterator(result.end()));
+				}
+			}
+			else {
+				declList = symtab.lookupId( nameExpr->name );
+			}
+			PRINT( std::cerr << "nameExpr is " << nameExpr->name << std::endl; )
+
+			if( declList.empty() ) return;
+
+			reason.code = NoMatch;
+
+			for ( auto & data : declList ) {
+				Cost cost = Cost::zero;
+				ast::Expr * newExpr = data.combine( nameExpr->location, cost );
+
+				CandidateRef newCand = std::make_shared<Candidate>(
+					newExpr, copy( tenv ), ast::OpenVarSet{}, ast::AssertionSet{}, cost );
+
+				if (newCand->expr->env) {
+					newCand->env.add(*newCand->expr->env);
+					auto mutExpr = newCand->expr.get_and_mutate();
+					mutExpr->env  = nullptr;
+					newCand->expr = mutExpr;
+				}
+
+				PRINT(
+					std::cerr << "decl is ";
+					ast::print( std::cerr, data.id );
+					std::cerr << std::endl;
+					std::cerr << "newExpr is ";
+					ast::print( std::cerr, newExpr );
+					std::cerr << std::endl;
+				)
+				newCand->expr = ast::mutate_field(
+					newCand->expr.get(), &ast::Expr::result,
+					renameTyVars( newCand->expr->result ) );
+				// add anonymous member interpretations whenever an aggregate value type is seen
+				// as a name expression
+				addAnonConversions( newCand );
+				candidates.emplace_back( std::move( newCand ) );
+			}
+		}
+
+		void postvisit( const ast::VariableExpr * variableExpr ) {
+			// not sufficient to just pass `variableExpr` here, type might have changed since
+			// creation
+			addCandidate(
+				new ast::VariableExpr{ variableExpr->location, variableExpr->var }, tenv );
+		}
+
+		void postvisit( const ast::ConstantExpr * constantExpr ) {
+			addCandidate( constantExpr, tenv );
+		}
+
+		void postvisit( const ast::SizeofExpr * sizeofExpr ) {
+			if ( sizeofExpr->type ) {
+				addCandidate(
+					new ast::SizeofExpr{
+						sizeofExpr->location, resolveTypeof( sizeofExpr->type, context ) },
+					tenv );
+			} else {
+				// find all candidates for the argument to sizeof
+				CandidateFinder finder( context, tenv );
+				finder.find( sizeofExpr->expr );
+				// find the lowest-cost candidate, otherwise ambiguous
+				CandidateList winners = findMinCost( finder.candidates );
+				if ( winners.size() != 1 ) {
+					SemanticError(
+						sizeofExpr->expr.get(), "Ambiguous expression in sizeof operand: " );
+				}
+				// return the lowest-cost candidate
+				CandidateRef & choice = winners.front();
+				choice->expr = referenceToRvalueConversion( choice->expr, choice->cost );
+				choice->cost = Cost::zero;
+				addCandidate( *choice, new ast::SizeofExpr{ sizeofExpr->location, choice->expr } );
+			}
+		}
+
+		void postvisit( const ast::AlignofExpr * alignofExpr ) {
+			if ( alignofExpr->type ) {
+				addCandidate(
+					new ast::AlignofExpr{
+						alignofExpr->location, resolveTypeof( alignofExpr->type, context ) },
+					tenv );
+			} else {
+				// find all candidates for the argument to alignof
+				CandidateFinder finder( context, tenv );
+				finder.find( alignofExpr->expr );
+				// find the lowest-cost candidate, otherwise ambiguous
+				CandidateList winners = findMinCost( finder.candidates );
+				if ( winners.size() != 1 ) {
+					SemanticError(
+						alignofExpr->expr.get(), "Ambiguous expression in alignof operand: " );
+				}
+				// return the lowest-cost candidate
+				CandidateRef & choice = winners.front();
+				choice->expr = referenceToRvalueConversion( choice->expr, choice->cost );
+				choice->cost = Cost::zero;
+				addCandidate(
+					*choice, new ast::AlignofExpr{ alignofExpr->location, choice->expr } );
+			}
+		}
+
+		void postvisit( const ast::UntypedOffsetofExpr * offsetofExpr ) {
+			const ast::BaseInstType * aggInst;
+			if (( aggInst = offsetofExpr->type.as< ast::StructInstType >() )) ;
+			else if (( aggInst = offsetofExpr->type.as< ast::UnionInstType >() )) ;
+			else return;
+
+			for ( const ast::Decl * member : aggInst->lookup( offsetofExpr->member ) ) {
+				auto dwt = strict_dynamic_cast< const ast::DeclWithType * >( member );
+				addCandidate(
+					new ast::OffsetofExpr{ offsetofExpr->location, aggInst, dwt }, tenv );
+			}
+		}
+
+		void postvisit( const ast::OffsetofExpr * offsetofExpr ) {
+			addCandidate( offsetofExpr, tenv );
+		}
+
+		void postvisit( const ast::OffsetPackExpr * offsetPackExpr ) {
+			addCandidate( offsetPackExpr, tenv );
+		}
+
+		void postvisit( const ast::LogicalExpr * logicalExpr ) {
+			CandidateFinder finder1( context, tenv );
+			finder1.find( logicalExpr->arg1, ResolvMode::withAdjustment() );
+			if ( finder1.candidates.empty() ) return;
+
+			CandidateFinder finder2( context, tenv );
+			finder2.find( logicalExpr->arg2, ResolvMode::withAdjustment() );
+			if ( finder2.candidates.empty() ) return;
+
+			reason.code = NoMatch;
+
+			for ( const CandidateRef & r1 : finder1.candidates ) {
+				for ( const CandidateRef & r2 : finder2.candidates ) {
+					ast::TypeEnvironment env{ r1->env };
+					env.simpleCombine( r2->env );
+					ast::OpenVarSet open{ r1->open };
+					mergeOpenVars( open, r2->open );
+					ast::AssertionSet need;
+					mergeAssertionSet( need, r1->need );
+					mergeAssertionSet( need, r2->need );
+
+					addCandidate(
+						new ast::LogicalExpr{
+							logicalExpr->location, r1->expr, r2->expr, logicalExpr->isAnd },
+						std::move( env ), std::move( open ), std::move( need ), r1->cost + r2->cost );
+				}
+			}
+		}
+
+		void postvisit( const ast::ConditionalExpr * conditionalExpr ) {
+			// candidates for condition
+			CandidateFinder finder1( context, tenv );
+			finder1.find( conditionalExpr->arg1, ResolvMode::withAdjustment() );
+			if ( finder1.candidates.empty() ) return;
+
+			// candidates for true result
+			CandidateFinder finder2( context, tenv );
+			finder2.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 postvisit( const ast::CommaExpr * commaExpr ) {
+			ast::TypeEnvironment env{ tenv };
+			ast::ptr< ast::Expr > arg1 = resolveInVoidContext( commaExpr->arg1, context, env );
+
+			CandidateFinder finder2( context, env );
+			finder2.find( commaExpr->arg2, ResolvMode::withAdjustment() );
+
+			for ( const CandidateRef & r2 : finder2.candidates ) {
+				addCandidate( *r2, new ast::CommaExpr{ commaExpr->location, arg1, r2->expr } );
+			}
+		}
+
+		void postvisit( const ast::ImplicitCopyCtorExpr * ctorExpr ) {
+			addCandidate( ctorExpr, tenv );
+		}
+
+		void postvisit( const ast::ConstructorExpr * ctorExpr ) {
+			CandidateFinder finder( context, tenv );
+			finder.allowVoid = true;
+			finder.find( ctorExpr->callExpr, ResolvMode::withoutPrune() );
+			for ( CandidateRef & r : finder.candidates ) {
+				addCandidate( *r, new ast::ConstructorExpr{ ctorExpr->location, r->expr } );
+			}
+		}
+
+		void postvisit( const ast::RangeExpr * rangeExpr ) {
+			// resolve low and high, accept candidates where low and high types unify
+			CandidateFinder finder1( context, tenv );
+			finder1.find( rangeExpr->low, ResolvMode::withAdjustment() );
+			if ( finder1.candidates.empty() ) return;
+
+			CandidateFinder finder2( context, tenv );
+			finder2.find( rangeExpr->high, ResolvMode::withAdjustment() );
+			if ( finder2.candidates.empty() ) return;
+
+			reason.code = NoMatch;
+
+			for ( const CandidateRef & r1 : finder1.candidates ) {
+				for ( const CandidateRef & r2 : finder2.candidates ) {
+					ast::TypeEnvironment env{ r1->env };
+					env.simpleCombine( r2->env );
+					ast::OpenVarSet open{ r1->open };
+					mergeOpenVars( open, r2->open );
+					ast::AssertionSet need;
+					mergeAssertionSet( need, r1->need );
+					mergeAssertionSet( need, r2->need );
+					ast::AssertionSet have;
+
+					ast::ptr< ast::Type > common;
+					if (
+						unify(
+							r1->expr->result, r2->expr->result, env, need, have, open,
+							common )
+					) {
+						// generate new expression
+						ast::RangeExpr * newExpr =
+							new ast::RangeExpr{ rangeExpr->location, r1->expr, r2->expr };
+						newExpr->result = common ? common : r1->expr->result;
+						// add candidate
+						CandidateRef newCand = std::make_shared<Candidate>(
+							newExpr, std::move( env ), std::move( open ), std::move( need ),
+							r1->cost + r2->cost );
+						inferParameters( newCand, candidates );
+					}
+				}
+			}
+		}
+
+		void postvisit( const ast::UntypedTupleExpr * tupleExpr ) {
+			std::vector< CandidateFinder > subCandidates =
+				selfFinder.findSubExprs( tupleExpr->exprs );
+			std::vector< CandidateList > possibilities;
+			combos( subCandidates.begin(), subCandidates.end(), back_inserter( possibilities ) );
+
+			for ( const CandidateList & subs : possibilities ) {
+				std::vector< ast::ptr< ast::Expr > > exprs;
+				exprs.reserve( subs.size() );
+				for ( const CandidateRef & sub : subs ) { exprs.emplace_back( sub->expr ); }
+
+				ast::TypeEnvironment env;
+				ast::OpenVarSet open;
+				ast::AssertionSet need;
+				for ( const CandidateRef & sub : subs ) {
+					env.simpleCombine( sub->env );
+					mergeOpenVars( open, sub->open );
+					mergeAssertionSet( need, sub->need );
+				}
+
+				addCandidate(
+					new ast::TupleExpr{ tupleExpr->location, std::move( exprs ) },
+					std::move( env ), std::move( open ), std::move( need ), sumCost( subs ) );
+			}
+		}
+
+		void postvisit( const ast::TupleExpr * tupleExpr ) {
+			addCandidate( tupleExpr, tenv );
+		}
+
+		void postvisit( const ast::TupleIndexExpr * tupleExpr ) {
+			addCandidate( tupleExpr, tenv );
+		}
+
+		void postvisit( const ast::TupleAssignExpr * tupleExpr ) {
+			addCandidate( tupleExpr, tenv );
+		}
+
+		void postvisit( const ast::UniqueExpr * unqExpr ) {
+			CandidateFinder finder( context, tenv );
+			finder.find( unqExpr->expr, ResolvMode::withAdjustment() );
+			for ( CandidateRef & r : finder.candidates ) {
+				// ensure that the the id is passed on so that the expressions are "linked"
+				addCandidate( *r, new ast::UniqueExpr{ unqExpr->location, r->expr, unqExpr->id } );
+			}
+		}
+
+		void postvisit( const ast::StmtExpr * stmtExpr ) {
+			addCandidate( resolveStmtExpr( stmtExpr, context ), tenv );
+		}
+
+		void postvisit( const ast::UntypedInitExpr * initExpr ) {
+			// handle each option like a cast
+			CandidateList matches;
+			PRINT(
+				std::cerr << "untyped init expr: " << initExpr << std::endl;
+			)
+			// O(n^2) checks of d-types with e-types
+			for ( const ast::InitAlternative & initAlt : initExpr->initAlts ) {
+				// calculate target type
+				const ast::Type * toType = resolveTypeof( initAlt.type, context );
+				toType = adjustExprType( toType, tenv, symtab );
+				// The call to find must occur inside this loop, otherwise polymorphic return
+				// types are not bound to the initialization type, since return type variables are
+				// only open for the duration of resolving the UntypedExpr.
+				CandidateFinder finder( context, tenv, toType );
+				finder.find( initExpr->expr, ResolvMode::withAdjustment() );
+
+				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);
+		}
+
+		void postvisit( const ast::InitExpr * ) {
+			assertf( false, "CandidateFinder should never see a resolved InitExpr." );
+		}
+
+		void postvisit( const ast::DeletedExpr * ) {
+			assertf( false, "CandidateFinder should never see a DeletedExpr." );
+		}
+
+		void postvisit( const ast::GenericExpr * ) {
+			assertf( false, "_Generic is not yet supported." );
+		}
+	};
+
+	// size_t Finder::traceId = Stats::Heap::new_stacktrace_id("Finder");
+	/// Prunes a list of candidates down to those that have the minimum conversion cost for a given
+	/// return type. Skips ambiguous candidates.
+
+} // anonymous namespace
+
+bool CandidateFinder::pruneCandidates( CandidateList & candidates, CandidateList & out, std::vector<std::string> & errors ) {
+	struct PruneStruct {
+		CandidateRef candidate;
+		bool ambiguous;
+
+		PruneStruct() = default;
+		PruneStruct( const CandidateRef & c ) : candidate( c ), ambiguous( false ) {}
+	};
+
+	// find lowest-cost candidate for each type
+	std::unordered_map< std::string, PruneStruct > selected;
+	// attempt to skip satisfyAssertions on more expensive alternatives if better options have been found
+	std::sort(candidates.begin(), candidates.end(), [](const CandidateRef & x, const CandidateRef & y){return x->cost < y->cost;});
+	for ( CandidateRef & candidate : candidates ) {
+		std::string mangleName;
+		{
+			ast::ptr< ast::Type > newType = candidate->expr->result;
+			assertf(candidate->expr->result, "Result of expression %p for candidate is null", candidate->expr.get());
+			candidate->env.apply( newType );
+			mangleName = Mangle::mangle( newType );
+		}
+
+		auto found = selected.find( mangleName );
+		if (found != selected.end() && found->second.candidate->cost < candidate->cost) {
+			PRINT(
+				std::cerr << "cost " << candidate->cost << " loses to "
+					<< found->second.candidate->cost << std::endl;
+			)
+			continue;
+		}
+
+		// xxx - when do satisfyAssertions produce more than 1 result?
+		// this should only happen when initial result type contains
+		// unbound type parameters, then it should never be pruned by
+		// the previous step, since renameTyVars guarantees the mangled name
+		// is unique.
+		CandidateList satisfied;
+		bool needRecomputeKey = false;
+		if (candidate->need.empty()) {
+			satisfied.emplace_back(candidate);
+		}
+		else {
+			satisfyAssertions(candidate, context.symtab, satisfied, errors);
+			needRecomputeKey = true;
+		}
+
+		for (auto & newCand : satisfied) {
+			// recomputes type key, if satisfyAssertions changed it
+			if (needRecomputeKey)
+			{
+				ast::ptr< ast::Type > newType = newCand->expr->result;
+				assertf(newCand->expr->result, "Result of expression %p for candidate is null", newCand->expr.get());
+				newCand->env.apply( newType );
+				mangleName = Mangle::mangle( newType );
+			}
+			auto found = selected.find( mangleName );
+			if ( found != selected.end() ) {
+				// 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 );
+	}
+	*/
+
+	// if ( mode.prune ) {
+	// 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;
+}
+
 } // namespace ResolvExpr
 
