source: src/ResolvExpr/AlternativeFinder.cc @ 7726839

//
// Cforall Version 1.0.0 Copyright (C) 2015 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// AlternativeFinder.cc --
//
// Author           : Richard C. Bilson
// Created On       : Sat May 16 23:52:08 2015
// Last Modified By : Peter A. Buhr
// Last Modified On : Thu Nov  1 21:00:56 2018
// Update Count     : 35
//

#include <algorithm>               // for copy
#include <cassert>                 // for strict_dynamic_cast, assert, assertf
#include <cstddef>                 // for size_t
#include <iostream>                // for operator<<, cerr, ostream, endl
#include <iterator>                // for back_insert_iterator, back_inserter
#include <list>                    // for _List_iterator, list, _List_const_...
#include <map>                     // for _Rb_tree_iterator, map, _Rb_tree_c...
#include <memory>                  // for allocator_traits<>::value_type, unique_ptr
#include <utility>                 // for pair
#include <vector>                  // for vector

#include "CompilationState.h"      // for resolvep
#include "Alternative.h"           // for AltList, Alternative
#include "AlternativeFinder.h"
#include "Common/SemanticError.h"  // for SemanticError
#include "Common/utility.h"        // for deleteAll, printAll, CodeLocation
#include "Cost.h"                  // for Cost, Cost::zero, operator<<, Cost...
#include "ExplodedActual.h"        // for ExplodedActual
#include "InitTweak/InitTweak.h"   // for getFunctionName
#include "RenameVars.h"            // for RenameVars, global_renamer
#include "ResolveAssertions.h"     // for resolveAssertions
#include "ResolveTypeof.h"         // for resolveTypeof
#include "Resolver.h"              // for resolveStmtExpr
#include "SymTab/Indexer.h"        // for Indexer
#include "SymTab/Mangler.h"        // for Mangler
#include "SymTab/Validate.h"       // for validateType
#include "SynTree/Constant.h"      // for Constant
#include "SynTree/Declaration.h"   // for DeclarationWithType, TypeDecl, Dec...
#include "SynTree/Expression.h"    // for Expression, CastExpr, NameExpr
#include "SynTree/Initializer.h"   // for SingleInit, operator<<, Designation
#include "SynTree/SynTree.h"       // for UniqueId
#include "SynTree/Type.h"          // for Type, FunctionType, PointerType
#include "Tuples/Explode.h"        // for explode
#include "Tuples/Tuples.h"         // for isTtype, handleTupleAssignment
#include "Unify.h"                 // for unify
#include "typeops.h"               // for adjustExprType, polyCost, castCost

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

using std::move;

/// copies any copyable type
template<typename T>
T copy(const T& x) { return x; }

namespace ResolvExpr {
        struct AlternativeFinder::Finder : public WithShortCircuiting {
                Finder( AlternativeFinder & altFinder ) : altFinder( altFinder ), indexer( altFinder.indexer ), alternatives( altFinder.alternatives ), env( altFinder.env ), targetType( altFinder.targetType )  {}

                void previsit( BaseSyntaxNode * ) { visit_children = false; }

                void postvisit( ApplicationExpr * applicationExpr );
                void postvisit( UntypedExpr * untypedExpr );
                void postvisit( AddressExpr * addressExpr );
                void postvisit( LabelAddressExpr * labelExpr );
                void postvisit( CastExpr * castExpr );
                void postvisit( VirtualCastExpr * castExpr );
                void postvisit( UntypedMemberExpr * memberExpr );
                void postvisit( MemberExpr * memberExpr );
                void postvisit( NameExpr * variableExpr );
                void postvisit( VariableExpr * variableExpr );
                void postvisit( ConstantExpr * constantExpr );
                void postvisit( SizeofExpr * sizeofExpr );
                void postvisit( AlignofExpr * alignofExpr );
                void postvisit( UntypedOffsetofExpr * offsetofExpr );
                void postvisit( OffsetofExpr * offsetofExpr );
                void postvisit( OffsetPackExpr * offsetPackExpr );
                void postvisit( AttrExpr * attrExpr );
                void postvisit( LogicalExpr * logicalExpr );
                void postvisit( ConditionalExpr * conditionalExpr );
                void postvisit( CommaExpr * commaExpr );
                void postvisit( ImplicitCopyCtorExpr  * impCpCtorExpr );
                void postvisit( ConstructorExpr  * ctorExpr );
                void postvisit( RangeExpr  * rangeExpr );
                void postvisit( UntypedTupleExpr * tupleExpr );
                void postvisit( TupleExpr * tupleExpr );
                void postvisit( TupleIndexExpr * tupleExpr );
                void postvisit( TupleAssignExpr * tupleExpr );
                void postvisit( UniqueExpr * unqExpr );
                void postvisit( StmtExpr * stmtExpr );
                void postvisit( UntypedInitExpr * initExpr );
                void postvisit( InitExpr * initExpr );
                void postvisit( DeletedExpr * delExpr );
                void postvisit( GenericExpr * genExpr );

                /// Adds alternatives for anonymous members
                void addAnonConversions( const Alternative & alt );
                /// Adds alternatives for member expressions, given the aggregate, conversion cost for that aggregate, and name of the member
                template< typename StructOrUnionType > void addAggMembers( StructOrUnionType *aggInst, Expression *expr, const Alternative &alt, const Cost &newCost, const std::string & name );
                /// Adds alternatives for member expressions where the left side has tuple type
                void addTupleMembers( TupleType *tupleType, Expression *expr, const Alternative &alt, const Cost &newCost, Expression *member );
                /// Adds alternatives for offsetof expressions, given the base type and name of the member
                template< typename StructOrUnionType > void addOffsetof( StructOrUnionType *aggInst, const std::string &name );
                /// Takes a final result and checks if its assertions can be satisfied
                template<typename OutputIterator>
                void validateFunctionAlternative( const Alternative &func, ArgPack& result, const std::vector<ArgPack>& results, OutputIterator out );
                /// Finds matching alternatives for a function, given a set of arguments
                template<typename OutputIterator>
                void makeFunctionAlternatives( const Alternative &func, FunctionType *funcType, const ExplodedArgs& args, OutputIterator out );
                /// Sets up parameter inference for an output alternative
                template< typename OutputIterator >
                void inferParameters( Alternative &newAlt, OutputIterator out );
        private:
                AlternativeFinder & altFinder;
                const SymTab::Indexer &indexer;
                AltList & alternatives;
                const TypeEnvironment &env;
                Type *& targetType;
        };

        Cost sumCost( const AltList &in ) {
                Cost total = Cost::zero;
                for ( AltList::const_iterator i = in.begin(); i != in.end(); ++i ) {
                        total += i->cost;
                }
                return total;
        }

        void printAlts( const AltList &list, std::ostream &os, unsigned int indentAmt ) {
                Indenter indent = { Indenter::tabsize, indentAmt };
                for ( AltList::const_iterator i = list.begin(); i != list.end(); ++i ) {
                        i->print( os, indent );
                        os << std::endl;
                }
        }

        namespace {
                void makeExprList( const AltList &in, std::list< Expression* > &out ) {
                        for ( AltList::const_iterator i = in.begin(); i != in.end(); ++i ) {
                                out.push_back( i->expr->clone() );
                        }
                }

                struct PruneStruct {
                        bool isAmbiguous;
                        AltList::iterator candidate;
                        PruneStruct() {}
                        PruneStruct( AltList::iterator candidate ): isAmbiguous( false ), candidate( candidate ) {}
                };

                /// Prunes a list of alternatives down to those that have the minimum conversion cost for a given return type; skips ambiguous interpretations
                template< typename InputIterator, typename OutputIterator >
                void pruneAlternatives( InputIterator begin, InputIterator end, OutputIterator out ) {
                        // select the alternatives that have the minimum conversion cost for a particular set of result types
                        std::map< std::string, PruneStruct > selected;
                        for ( AltList::iterator candidate = begin; candidate != end; ++candidate ) {
                                PruneStruct current( candidate );
                                std::string mangleName;
                                {
                                        Type * newType = candidate->expr->get_result()->clone();
                                        candidate->env.apply( newType );
                                        mangleName = SymTab::Mangler::mangle( newType );
                                        delete newType;
                                }
                                std::map< std::string, PruneStruct >::iterator mapPlace = selected.find( mangleName );
                                if ( mapPlace != selected.end() ) {
                                        if ( candidate->cost < mapPlace->second.candidate->cost ) {
                                                PRINT(
                                                        std::cerr << "cost " << candidate->cost << " beats " << mapPlace->second.candidate->cost << std::endl;
                                                )
                                                selected[ mangleName ] = current;
                                        } else if ( candidate->cost == mapPlace->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( candidate->expr ) ) {
                                                        // do nothing
                                                        PRINT( std::cerr << "candidate is deleted" << std::endl; )
                                                } else if ( findDeletedExpr( mapPlace->second.candidate->expr ) ) {
                                                        PRINT( std::cerr << "current is deleted" << std::endl; )
                                                        selected[ mangleName ] = current;
                                                } else {
                                                        PRINT(
                                                                std::cerr << "marking ambiguous" << std::endl;
                                                        )
                                                        mapPlace->second.isAmbiguous = true;
                                                }
                                        } else {
                                                PRINT(
                                                        std::cerr << "cost " << candidate->cost << " loses to " << mapPlace->second.candidate->cost << std::endl;
                                                )
                                        }
                                } else {
                                        selected[ mangleName ] = current;
                                }
                        }

                        // accept the alternatives that were unambiguous
                        for ( std::map< std::string, PruneStruct >::iterator target = selected.begin(); target != selected.end(); ++target ) {
                                if ( ! target->second.isAmbiguous ) {
                                        Alternative &alt = *target->second.candidate;
                                        alt.env.applyFree( alt.expr->get_result() );
                                        *out++ = alt;
                                }
                        }
                }

                void renameTypes( Expression *expr ) {
                        renameTyVars( expr->result );
                }
        } // namespace

        void referenceToRvalueConversion( Expression *& expr, Cost & cost ) {
                if ( dynamic_cast< ReferenceType * >( expr->get_result() ) ) {
                        // cast away reference from expr
                        expr = new CastExpr( expr, expr->get_result()->stripReferences()->clone() );
                        cost.incReference();
                }
        }

        template< typename InputIterator, typename OutputIterator >
        void AlternativeFinder::findSubExprs( InputIterator begin, InputIterator end, OutputIterator out ) {
                while ( begin != end ) {
                        AlternativeFinder finder( indexer, env );
                        finder.findWithAdjustment( *begin );
                        // XXX  either this
                        //Designators::fixDesignations( finder, (*begin++)->get_argName() );
                        // or XXX this
                        begin++;
                        PRINT(
                                std::cerr << "findSubExprs" << std::endl;
                                printAlts( finder.alternatives, std::cerr );
                        )
                        *out++ = finder;
                }
        }

        AlternativeFinder::AlternativeFinder( const SymTab::Indexer &indexer, const TypeEnvironment &env )
                : indexer( indexer ), env( env ) {
        }

        void AlternativeFinder::find( Expression *expr, ResolvMode mode ) {
                PassVisitor<Finder> finder( *this );
                expr->accept( finder );
                if ( mode.failFast && alternatives.empty() ) {
                        PRINT(
                                std::cerr << "No reasonable alternatives for expression " << expr << std::endl;
                        )
                        SemanticError( expr, "No reasonable alternatives for expression " );
                }
                if ( mode.resolveAssns || mode.prune ) {
                        // trim candidates just to those where the assertions resolve
                        // - necessary pre-requisite to pruning
                        AltList candidates;
                        for ( unsigned i = 0; i < alternatives.size(); ++i ) {
                                resolveAssertions( alternatives[i], indexer, candidates );
                        }
                        // fail early if none such
                        if ( mode.failFast && candidates.empty() ) {
                                std::ostringstream stream;
                                stream << "No resolvable alternatives for expression " << expr << "\n"
                                       << "Alternatives with failing assertions are:\n";
                                printAlts( alternatives, stream, 1 );
                                SemanticError( expr->location, stream.str() );
                        }
                        // reset alternatives
                        alternatives = std::move( candidates );
                }
                if ( mode.prune ) {
                        auto oldsize = alternatives.size();
                        PRINT(
                                std::cerr << "alternatives before prune:" << std::endl;
                                printAlts( alternatives, std::cerr );
                        )
                        AltList pruned;
                        pruneAlternatives( alternatives.begin(), alternatives.end(), back_inserter( pruned ) );
                        if ( mode.failFast && pruned.empty() ) {
                                std::ostringstream stream;
                                AltList winners;
                                findMinCost( alternatives.begin(), alternatives.end(), back_inserter( winners ) );
                                stream << "Cannot choose between " << winners.size() << " alternatives for expression\n";
                                expr->print( stream );
                                stream << " Alternatives are:\n";
                                printAlts( winners, stream, 1 );
                                SemanticError( expr->location, stream.str() );
                        }
                        alternatives = move(pruned);
                        PRINT(
                                std::cerr << "there are " << oldsize << " alternatives before elimination" << std::endl;
                        )
                        PRINT(
                                std::cerr << "there are " << alternatives.size() << " alternatives after elimination" << std::endl;
                        )
                }
                // adjust types after pruning so that types substituted by pruneAlternatives are correctly adjusted
                if ( mode.adjust ) {
                        for ( Alternative& i : alternatives ) {
                                adjustExprType( i.expr->get_result(), i.env, indexer );
                        }
                }

                // Central location to handle gcc extension keyword, etc. for all expression types.
                for ( Alternative &iter: alternatives ) {
                        iter.expr->set_extension( expr->get_extension() );
                        iter.expr->location = expr->location;
                } // for
        }

        void AlternativeFinder::findWithAdjustment( Expression *expr ) {
                find( expr, ResolvMode::withAdjustment() );
        }

        void AlternativeFinder::findWithoutPrune( Expression * expr ) {
                find( expr, ResolvMode::withoutPrune() );
        }

        void AlternativeFinder::maybeFind( Expression * expr ) {
                find( expr, ResolvMode::withoutFailFast() );
        }

        void AlternativeFinder::Finder::addAnonConversions( const Alternative & alt ) {
                // 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
                std::unique_ptr<Expression> aggrExpr( alt.expr->clone() );
                alt.env.apply( aggrExpr->result );
                Type * aggrType = aggrExpr->result;
                if ( dynamic_cast< ReferenceType * >( aggrType ) ) {
                        aggrType = aggrType->stripReferences();
                        aggrExpr.reset( new CastExpr( aggrExpr.release(), aggrType->clone() ) );
                }

                if ( StructInstType *structInst = dynamic_cast< StructInstType* >( aggrExpr->result ) ) {
                        addAggMembers( structInst, aggrExpr.get(), alt, alt.cost+Cost::safe, "" );
                } else if ( UnionInstType *unionInst = dynamic_cast< UnionInstType* >( aggrExpr->result ) ) {
                        addAggMembers( unionInst, aggrExpr.get(), alt, alt.cost+Cost::safe, "" );
                } // if
        }

        template< typename StructOrUnionType >
        void AlternativeFinder::Finder::addAggMembers( StructOrUnionType *aggInst, Expression *expr, const Alternative& alt, const Cost &newCost, const std::string & name ) {
                std::list< Declaration* > members;
                aggInst->lookup( name, members );

                for ( Declaration * decl : members ) {
                        if ( DeclarationWithType *dwt = dynamic_cast< DeclarationWithType* >( decl ) ) {
                                // addAnonAlternatives uses vector::push_back, which invalidates references to existing elements, so
                                // can't construct in place and use vector::back
                                Alternative newAlt{ alt, new MemberExpr{ dwt, expr->clone() }, newCost };
                                renameTypes( newAlt.expr );
                                addAnonConversions( newAlt ); // add anonymous member interpretations whenever an aggregate value type is seen as a member expression.
                                alternatives.push_back( std::move(newAlt) );
                        } else {
                                assert( false );
                        }
                }
        }

        void AlternativeFinder::Finder::addTupleMembers( TupleType *tupleType, Expression *expr,                        const Alternative &alt, const Cost &newCost, Expression *member ) {
                if ( ConstantExpr * constantExpr = dynamic_cast< ConstantExpr * >( member ) ) {
                        // get the value of the constant expression as an int, must be between 0 and the length of the tuple type to have meaning
                        auto val = constantExpr->intValue();
                        std::string tmp;
                        if ( val >= 0 && (unsigned long long)val < tupleType->size() ) {
                                alternatives.push_back( Alternative{ 
                                        alt, new TupleIndexExpr( expr->clone(), val ), newCost } );
                        } // if
                } // if
        }

        void AlternativeFinder::Finder::postvisit( ApplicationExpr *applicationExpr ) {
                alternatives.push_back( Alternative{ applicationExpr->clone(), env } );
        }

        Cost computeConversionCost( Type * actualType, Type * formalType, const SymTab::Indexer &indexer, const TypeEnvironment & env ) {
                PRINT(
                        std::cerr << std::endl << "converting ";
                        actualType->print( std::cerr, 8 );
                        std::cerr << std::endl << " to ";
                        formalType->print( std::cerr, 8 );
                        std::cerr << std::endl << "environment is: ";
                        env.print( std::cerr, 8 );
                        std::cerr << std::endl;
                )
                Cost convCost = conversionCost( actualType, formalType, indexer, env );
                PRINT(
                        std::cerr << std::endl << "cost is " << convCost << std::endl;
                )
                if ( convCost == Cost::infinity ) {
                        return convCost;
                }
                convCost.incPoly( polyCost( formalType, env, indexer ) + polyCost( actualType, env, indexer ) );
                PRINT(
                        std::cerr << "cost with polycost is " << convCost << std::endl;
                )
                return convCost;
        }

        Cost computeExpressionConversionCost( Expression *& actualExpr, Type * formalType, const SymTab::Indexer &indexer, const TypeEnvironment & env ) {
                Cost convCost = computeConversionCost( actualExpr->result, formalType, indexer, env );

                // 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 ) {
                        Type *newType = formalType->clone();
                        env.apply( newType );
                        actualExpr = new CastExpr( actualExpr, 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 (the formal type) 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 just nothing.

                        // AlternativeFinder finder( indexer, env );
                        // finder.findWithAdjustment( actualExpr );
                        // assertf( finder.get_alternatives().size() > 0, "Somehow castable expression failed to find alternatives." );
                        // assertf( finder.get_alternatives().size() == 1, "Somehow got multiple alternatives for known cast expression." );
                        // Alternative & alt = finder.get_alternatives().front();
                        // delete actualExpr;
                        // actualExpr = alt.expr->clone();
                }
                return convCost;
        }

        Cost computeApplicationConversionCost( Alternative &alt, const SymTab::Indexer &indexer ) {
                ApplicationExpr *appExpr = strict_dynamic_cast< ApplicationExpr* >( alt.expr );
                PointerType *pointer = strict_dynamic_cast< PointerType* >( appExpr->function->result );
                FunctionType *function = strict_dynamic_cast< FunctionType* >( pointer->base );

                Cost convCost = Cost::zero;
                std::list< DeclarationWithType* >& formals = function->parameters;
                std::list< DeclarationWithType* >::iterator formal = formals.begin();
                std::list< Expression* >& actuals = appExpr->args;

                for ( Expression*& actualExpr : actuals ) {
                        Type * actualType = actualExpr->result;
                        PRINT(
                                std::cerr << "actual expression:" << std::endl;
                                actualExpr->print( std::cerr, 8 );
                                std::cerr << "--- results are" << std::endl;
                                actualType->print( std::cerr, 8 );
                        )
                        if ( formal == formals.end() ) {
                                if ( function->isVarArgs ) {
                                        convCost.incUnsafe();
                                        PRINT( std::cerr << "end of formals with varargs function: inc unsafe: " << convCost << std::endl; ; )
                                        // convert reference-typed expressions to value-typed expressions
                                        referenceToRvalueConversion( actualExpr, convCost );
                                        continue;
                                } else {
                                        return Cost::infinity;
                                }
                        }
                        if ( DefaultArgExpr * def = dynamic_cast< DefaultArgExpr * >( actualExpr ) ) {
                                // default arguments should be free - don't include conversion cost.
                                // Unwrap them here because they are not relevant to the rest of the system.
                                actualExpr = def->expr;
                                ++formal;
                                continue;
                        }
                        // mark conversion cost to formal and also specialization cost of formal type
                        Type * formalType = (*formal)->get_type();
                        convCost += computeExpressionConversionCost( actualExpr, formalType, indexer, alt.env );
                        convCost.decSpec( specCost( formalType ) );
                        ++formal; // can't be in for-loop update because of the continue
                }
                if ( formal != formals.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() );
                for ( TypeDecl* td : function->forall ) {
                        convCost.decSpec( td->assertions.size() );
                }

                return convCost;
        }

        /// Adds type variables to the open variable set and marks their assertions
        void makeUnifiableVars( Type *type, OpenVarSet &unifiableVars, AssertionSet &needAssertions ) {
                for ( Type::ForallList::const_iterator tyvar = type->forall.begin(); tyvar != type->forall.end(); ++tyvar ) {
                        unifiableVars[ (*tyvar)->get_name() ] = TypeDecl::Data{ *tyvar };
                        for ( std::list< DeclarationWithType* >::iterator assert = (*tyvar)->assertions.begin(); assert != (*tyvar)->assertions.end(); ++assert ) {
                                needAssertions[ *assert ].isUsed = true;
                        }
                }
        }

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

        template< typename OutputIterator >
        void AlternativeFinder::Finder::inferParameters( Alternative &newAlt, OutputIterator out ) {
                // Set need bindings for any unbound assertions
                UniqueId crntResnSlot = 0;  // matching ID for this expression's assertions
                for ( auto& assn : newAlt.need ) {
                        // skip already-matched assertions
                        if ( assn.info.resnSlot != 0 ) continue;
                        // assign slot for expression if needed
                        if ( crntResnSlot == 0 ) { crntResnSlot = ++globalResnSlot; }
                        // fix slot to assertion
                        assn.info.resnSlot = crntResnSlot;
                }
                // pair slot to expression
                if ( crntResnSlot != 0 ) { newAlt.expr->resnSlots.push_back( crntResnSlot ); }

                // add to output list, assertion resolution is deferred
                *out++ = newAlt;
        }

        /// Gets a default value from an initializer, nullptr if not present
        ConstantExpr* getDefaultValue( Initializer* init ) {
                if ( SingleInit* si = dynamic_cast<SingleInit*>( init ) ) {
                        if ( CastExpr* ce = dynamic_cast<CastExpr*>( si->value ) ) {
                                return dynamic_cast<ConstantExpr*>( ce->arg );
                        } else {
                                return dynamic_cast<ConstantExpr*>( si->value );
                        }
                }
                return nullptr;
        }

        /// State to iteratively build a match of parameter expressions to arguments
        struct ArgPack {
                std::size_t parent;                ///< Index of parent pack
                std::unique_ptr<Expression> expr;  ///< The argument stored here
                Cost cost;                         ///< The cost of this argument
                TypeEnvironment env;               ///< Environment for this pack
                AssertionSet need;                 ///< Assertions outstanding for this pack
                AssertionSet have;                 ///< Assertions found for this pack
                OpenVarSet openVars;               ///< 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(), openVars(), nextArg(0),
                          tupleStart(0), nextExpl(0), explAlt(0) {}

                ArgPack(const TypeEnvironment& env, const AssertionSet& need, const AssertionSet& have,
                                const OpenVarSet& openVars)
                        : parent(0), expr(), cost(Cost::zero), env(env), need(need), have(have),
                          openVars(openVars), nextArg(0), tupleStart(0), nextExpl(0), explAlt(0) {}

                ArgPack(std::size_t parent, Expression* expr, TypeEnvironment&& env, AssertionSet&& need,
                                AssertionSet&& have, OpenVarSet&& openVars, unsigned nextArg,
                                unsigned tupleStart = 0, Cost cost = Cost::zero, unsigned nextExpl = 0,
                                unsigned explAlt = 0 )
                        : parent(parent), expr(expr->clone()), cost(cost), env(move(env)), need(move(need)),
                          have(move(have)), openVars(move(openVars)), nextArg(nextArg), tupleStart(tupleStart),
                          nextExpl(nextExpl), explAlt(explAlt) {}

                ArgPack(const ArgPack& o, TypeEnvironment&& env, AssertionSet&& need, AssertionSet&& have,
                                OpenVarSet&& openVars, unsigned nextArg, Cost added )
                        : parent(o.parent), expr(o.expr ? o.expr->clone() : nullptr), cost(o.cost + added),
                          env(move(env)), need(move(need)), have(move(have)), openVars(move(openVars)),
                          nextArg(nextArg), tupleStart(o.tupleStart), nextExpl(0), explAlt(0) {}

                /// true iff this pack is in the middle of an exploded argument
                bool hasExpl() const { return nextExpl > 0; }

                /// Gets the list of exploded alternatives for this pack
                const ExplodedActual& getExpl( const ExplodedArgs& args ) const {
                        return args[nextArg-1][explAlt];
                }

                /// Ends a tuple expression, consolidating the appropriate actuals
                void endTuple( const std::vector<ArgPack>& packs ) {
                        // add all expressions in tuple to list, summing cost
                        std::list<Expression*> exprs;
                        const ArgPack* pack = this;
                        if ( expr ) { exprs.push_front( expr.release() ); }
                        while ( pack->tupleStart == 0 ) {
                                pack = &packs[pack->parent];
                                exprs.push_front( pack->expr->clone() );
                                cost += pack->cost;
                        }
                        // reset pack to appropriate tuple
                        expr.reset( new TupleExpr( exprs ) );
                        tupleStart = pack->tupleStart - 1;
                        parent = pack->parent;
                }
        };

        /// Instantiates an argument to match a formal, returns false if no results left
        bool instantiateArgument( Type* formalType, Initializer* initializer,
                        const ExplodedArgs& args, std::vector<ArgPack>& results, std::size_t& genStart,
                        const SymTab::Indexer& indexer, unsigned nTuples = 0 ) {
                if ( TupleType * tupleType = dynamic_cast<TupleType*>( formalType ) ) {
                        // formalType is a TupleType - group actuals into a TupleExpr
                        ++nTuples;
                        for ( 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, indexer, nTuples ) )
                                        return false;
                                nTuples = 0;
                        }
                        // re-consititute tuples for final generation
                        for ( auto i = genStart; i < results.size(); ++i ) {
                                results[i].endTuple( results );
                        }
                        return true;
                } else if ( TypeInstType * ttype = Tuples::isTtype( formalType ) ) {
                        // formalType is a ttype, consumes all remaining arguments
                        // xxx - mixing default arguments with variadic??

                        // 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 ) {
                                        auto nextArg = results[i].nextArg;

                                        // use next element of exploded tuple if present
                                        if ( results[i].hasExpl() ) {
                                                const ExplodedActual& 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].get(), copy(results[i].env),
                                                        copy(results[i].need), copy(results[i].have),
                                                        copy(results[i].openVars), 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].openVars };
                                                newResult.nextArg = nextArg;
                                                Type* argType;

                                                if ( nTuples > 0 || ! results[i].expr ) {
                                                        // first iteration or no expression to clone,
                                                        // push empty tuple expression
                                                        newResult.parent = i;
                                                        std::list<Expression*> emptyList;
                                                        newResult.expr.reset( new TupleExpr( emptyList ) );
                                                        argType = newResult.expr->get_result();
                                                } else {
                                                        // clone result to collect tuple
                                                        newResult.parent = results[i].parent;
                                                        newResult.cost = results[i].cost;
                                                        newResult.tupleStart = results[i].tupleStart;
                                                        newResult.expr.reset( results[i].expr->clone() );
                                                        argType = newResult.expr->get_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->get_result();
                                                        }
                                                }

                                                // check unification for ttype before adding to final
                                                if ( unify( ttype, argType, newResult.env, newResult.need, newResult.have,
                                                                newResult.openVars, indexer ) ) {
                                                        finalResults.push_back( move(newResult) );
                                                }

                                                continue;
                                        }

                                        // add each possible next argument
                                        for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
                                                const ExplodedActual& expl = args[nextArg][j];

                                                // fresh copies of parent parameters for this iteration
                                                TypeEnvironment env = results[i].env;
                                                OpenVarSet openVars = results[i].openVars;

                                                env.addActual( expl.env, openVars );

                                                // skip empty tuple arguments by (near-)cloning parent into next gen
                                                if ( expl.exprs.empty() ) {
                                                        results.emplace_back(
                                                                results[i], move(env), copy(results[i].need),
                                                                copy(results[i].have), move(openVars), nextArg + 1, expl.cost );

                                                        continue;
                                                }

                                                // add new result
                                                results.emplace_back(
                                                        i, expl.exprs.front().get(), move(env), copy(results[i].need),
                                                        copy(results[i].have), move(openVars), 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.push_back( 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 ) {
                        auto nextArg = results[i].nextArg;

                        // use remainder of exploded tuple if present
                        if ( results[i].hasExpl() ) {
                                const ExplodedActual& expl = results[i].getExpl( args );
                                Expression* expr = expl.exprs[results[i].nextExpl].get();

                                TypeEnvironment env = results[i].env;
                                AssertionSet need = results[i].need, have = results[i].have;
                                OpenVarSet openVars = results[i].openVars;

                                Type* actualType = expr->get_result();

                                PRINT(
                                        std::cerr << "formal type is ";
                                        formalType->print( std::cerr );
                                        std::cerr << std::endl << "actual type is ";
                                        actualType->print( std::cerr );
                                        std::cerr << std::endl;
                                )

                                if ( unify( formalType, actualType, env, need, have, openVars, indexer ) ) {
                                        unsigned nextExpl = results[i].nextExpl + 1;
                                        if ( nextExpl == expl.exprs.size() ) {
                                                nextExpl = 0;
                                        }

                                        results.emplace_back(
                                                i, expr, move(env), move(need), move(have), move(openVars), nextArg,
                                                nTuples, Cost::zero, nextExpl, results[i].explAlt );
                                }

                                continue;
                        }

                        // use default initializers if out of arguments
                        if ( nextArg >= args.size() ) {
                                if ( ConstantExpr* cnstExpr = getDefaultValue( initializer ) ) {
                                        if ( Constant* cnst = dynamic_cast<Constant*>( cnstExpr->get_constant() ) ) {
                                                TypeEnvironment env = results[i].env;
                                                AssertionSet need = results[i].need, have = results[i].have;
                                                OpenVarSet openVars = results[i].openVars;

                                                if ( unify( formalType, cnst->get_type(), env, need, have, openVars,
                                                                indexer ) ) {
                                                        results.emplace_back(
                                                                i, new DefaultArgExpr( cnstExpr ), move(env), move(need), move(have),
                                                                move(openVars), nextArg, nTuples );
                                                }
                                        }
                                }

                                continue;
                        }

                        // Check each possible next argument
                        for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
                                const ExplodedActual& expl = args[nextArg][j];

                                // fresh copies of parent parameters for this iteration
                                TypeEnvironment env = results[i].env;
                                AssertionSet need = results[i].need, have = results[i].have;
                                OpenVarSet openVars = results[i].openVars;

                                env.addActual( expl.env, openVars );

                                // skip empty tuple arguments by (near-)cloning parent into next gen
                                if ( expl.exprs.empty() ) {
                                        results.emplace_back(
                                                results[i], move(env), move(need), move(have), move(openVars),
                                                nextArg + 1, expl.cost );

                                        continue;
                                }

                                // consider only first exploded actual
                                Expression* expr = expl.exprs.front().get();
                                Type* actualType = expr->result->clone();

                                PRINT(
                                        std::cerr << "formal type is ";
                                        formalType->print( std::cerr );
                                        std::cerr << std::endl << "actual type is ";
                                        actualType->print( std::cerr );
                                        std::cerr << std::endl;
                                )

                                // attempt to unify types
                                if ( unify( formalType, actualType, env, need, have, openVars, indexer ) ) {
                                        // add new result
                                        results.emplace_back(
                                                i, expr, move(env), move(need), move(have), move(openVars), nextArg + 1,
                                                nTuples, expl.cost, expl.exprs.size() == 1 ? 0 : 1, j );
                                }
                        }
                }

                // reset for next parameter
                genStart = genEnd;

                return genEnd != results.size();
        }

        template<typename OutputIterator>
        void AlternativeFinder::Finder::validateFunctionAlternative( const Alternative &func, ArgPack& result,
                        const std::vector<ArgPack>& results, OutputIterator out ) {
                ApplicationExpr *appExpr = new ApplicationExpr( func.expr->clone() );
                // sum cost and accumulate actuals
                std::list<Expression*>& args = appExpr->args;
                Cost cost = func.cost;
                const ArgPack* pack = &result;
                while ( pack->expr ) {
                        args.push_front( pack->expr->clone() );
                        cost += pack->cost;
                        pack = &results[pack->parent];
                }
                // build and validate new alternative
                Alternative newAlt{ appExpr, result.env, result.openVars, result.need, cost };
                PRINT(
                        std::cerr << "instantiate function success: " << appExpr << std::endl;
                        std::cerr << "need assertions:" << std::endl;
                        printAssertionSet( result.need, std::cerr, 8 );
                )
                inferParameters( newAlt, out );
        }

        template<typename OutputIterator>
        void AlternativeFinder::Finder::makeFunctionAlternatives( const Alternative &func,
                        FunctionType *funcType, const ExplodedArgs &args, OutputIterator out ) {
                OpenVarSet funcOpenVars;
                AssertionSet funcNeed, funcHave;
                TypeEnvironment funcEnv( func.env );
                makeUnifiableVars( funcType, funcOpenVars, 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->returnVals.empty() ) {
                        // attempt to narrow based on expected target type
                        Type * returnType = funcType->returnVals.front()->get_type();
                        if ( ! unify( returnType, targetType, funcEnv, funcNeed, funcHave, funcOpenVars,
                                        indexer ) ) {
                                // unification failed, don't pursue this function alternative
                                return;
                        }
                }

                // iteratively build matches, one parameter at a time
                std::vector<ArgPack> results;
                results.push_back( ArgPack{ funcEnv, funcNeed, funcHave, funcOpenVars } );
                std::size_t genStart = 0;

                for ( DeclarationWithType* formal : funcType->parameters ) {
                        ObjectDecl* obj = strict_dynamic_cast< ObjectDecl* >( formal );
                        if ( ! instantiateArgument(
                                        obj->type, obj->init, args, results, genStart, indexer ) )
                                return;
                }

                if ( funcType->get_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 ) {
                                        auto nextArg = results[i].nextArg;

                                        // use remainder of exploded tuple if present
                                        if ( results[i].hasExpl() ) {
                                                const ExplodedActual& 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].get(), copy(results[i].env),
                                                        copy(results[i].need), copy(results[i].have),
                                                        copy(results[i].openVars), nextArg, 0, Cost::zero, nextExpl,
                                                        results[i].explAlt );

                                                continue;
                                        }

                                        // finish result when out of arguments
                                        if ( nextArg >= args.size() ) {
                                                validateFunctionAlternative( func, results[i], results, out );

                                                continue;
                                        }

                                        // add each possible next argument
                                        for ( std::size_t j = 0; j < args[nextArg].size(); ++j ) {
                                                const ExplodedActual& expl = args[nextArg][j];

                                                // fresh copies of parent parameters for this iteration
                                                TypeEnvironment env = results[i].env;
                                                OpenVarSet openVars = results[i].openVars;

                                                env.addActual( expl.env, openVars );

                                                // skip empty tuple arguments by (near-)cloning parent into next gen
                                                if ( expl.exprs.empty() ) {
                                                        results.emplace_back(
                                                                results[i], move(env), copy(results[i].need),
                                                                copy(results[i].have), move(openVars), nextArg + 1, expl.cost );

                                                        continue;
                                                }

                                                // add new result
                                                results.emplace_back(
                                                        i, expl.exprs.front().get(), move(env), copy(results[i].need),
                                                        copy(results[i].have), move(openVars), nextArg + 1, 0,
                                                        expl.cost, expl.exprs.size() == 1 ? 0 : 1, j );
                                        }
                                }

                                genStart = genEnd;
                        } while ( genEnd != results.size() );
                } else {
                        // filter out 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() ) {
                                        validateFunctionAlternative( func, result, results, out );
                                }
                        }
                }
        }

        void AlternativeFinder::Finder::postvisit( UntypedExpr *untypedExpr ) {
                AlternativeFinder funcFinder( indexer, env );
                funcFinder.findWithAdjustment( untypedExpr->function );
                // if there are no function alternatives, then proceeding is a waste of time.
                // xxx - findWithAdjustment throws, so this check and others like it shouldn't be necessary.
                if ( funcFinder.alternatives.empty() ) return;

                std::vector< AlternativeFinder > argAlternatives;
                altFinder.findSubExprs( untypedExpr->begin_args(), untypedExpr->end_args(),
                        back_inserter( argAlternatives ) );

                // take care of possible tuple assignments
                // if not tuple assignment, assignment is taken care of as a normal function call
                Tuples::handleTupleAssignment( altFinder, untypedExpr, argAlternatives );

                // find function operators
                static NameExpr *opExpr = new NameExpr( "?()" );
                AlternativeFinder funcOpFinder( indexer, env );
                // it's ok if there aren't any defined function ops
                funcOpFinder.maybeFind( opExpr );
                PRINT(
                        std::cerr << "known function ops:" << std::endl;
                        printAlts( funcOpFinder.alternatives, std::cerr, 1 );
                )

                // pre-explode arguments
                ExplodedArgs argExpansions;
                argExpansions.reserve( argAlternatives.size() );

                for ( const AlternativeFinder& arg : argAlternatives ) {
                        argExpansions.emplace_back();
                        auto& argE = argExpansions.back();
                        // argE.reserve( arg.alternatives.size() );

                        for ( const Alternative& actual : arg ) {
                                argE.emplace_back( actual, indexer );
                        }
                }

                AltList candidates;
                SemanticErrorException errors;
                for ( AltList::iterator func = funcFinder.alternatives.begin(); func != funcFinder.alternatives.end(); ++func ) {
                        try {
                                PRINT(
                                        std::cerr << "working on alternative: " << std::endl;
                                        func->print( std::cerr, 8 );
                                )
                                // check if the type is pointer to function
                                if ( PointerType *pointer = dynamic_cast< PointerType* >( func->expr->result->stripReferences() ) ) {
                                        if ( FunctionType *function = dynamic_cast< FunctionType* >( pointer->base ) ) {
                                                Alternative newFunc( *func );
                                                referenceToRvalueConversion( newFunc.expr, newFunc.cost );
                                                makeFunctionAlternatives( newFunc, function, argExpansions,
                                                        std::back_inserter( candidates ) );
                                        }
                                } else if ( TypeInstType *typeInst = dynamic_cast< TypeInstType* >( func->expr->result->stripReferences() ) ) { // handle ftype (e.g. *? on function pointer)
                                        if ( const EqvClass *eqvClass = func->env.lookup( typeInst->name ) ) {
                                                if ( FunctionType *function = dynamic_cast< FunctionType* >( eqvClass->type ) ) {
                                                        Alternative newFunc( *func );
                                                        referenceToRvalueConversion( newFunc.expr, newFunc.cost );
                                                        makeFunctionAlternatives( newFunc, function, argExpansions,
                                                                std::back_inserter( candidates ) );
                                                } // if
                                        } // if
                                }
                        } catch ( SemanticErrorException &e ) {
                                errors.append( e );
                        }
                } // for

                // try each function operator ?() with each function alternative
                if ( ! funcOpFinder.alternatives.empty() ) {
                        // add exploded function alternatives to front of argument list
                        std::vector<ExplodedActual> funcE;
                        funcE.reserve( funcFinder.alternatives.size() );
                        for ( const Alternative& actual : funcFinder ) {
                                funcE.emplace_back( actual, indexer );
                        }
                        argExpansions.insert( argExpansions.begin(), move(funcE) );

                        for ( AltList::iterator funcOp = funcOpFinder.alternatives.begin();
                                        funcOp != funcOpFinder.alternatives.end(); ++funcOp ) {
                                try {
                                        // check if type is a pointer to function
                                        if ( PointerType* pointer = dynamic_cast<PointerType*>(
                                                        funcOp->expr->result->stripReferences() ) ) {
                                                if ( FunctionType* function =
                                                                dynamic_cast<FunctionType*>( pointer->base ) ) {
                                                        Alternative newFunc( *funcOp );
                                                        referenceToRvalueConversion( newFunc.expr, newFunc.cost );
                                                        makeFunctionAlternatives( newFunc, function, argExpansions,
                                                                std::back_inserter( candidates ) );
                                                }
                                        }
                                } catch ( SemanticErrorException &e ) {
                                        errors.append( e );
                                }
                        }
                }

                // Implement SFINAE; resolution errors are only errors if there aren't any non-erroneous resolutions
                if ( candidates.empty() && ! errors.isEmpty() ) { throw errors; }

                // compute conversionsion costs
                for ( Alternative& withFunc : candidates ) {
                        Cost cvtCost = computeApplicationConversionCost( withFunc, indexer );

                        PRINT(
                                ApplicationExpr *appExpr = strict_dynamic_cast< ApplicationExpr* >( withFunc.expr );
                                PointerType *pointer = strict_dynamic_cast< PointerType* >( appExpr->function->result );
                                FunctionType *function = strict_dynamic_cast< FunctionType* >( pointer->base );
                                std::cerr << "Case +++++++++++++ " << appExpr->function << std::endl;
                                std::cerr << "formals are:" << std::endl;
                                printAll( function->parameters, std::cerr, 8 );
                                std::cerr << "actuals are:" << std::endl;
                                printAll( appExpr->args, std::cerr, 8 );
                                std::cerr << "bindings are:" << std::endl;
                                withFunc.env.print( std::cerr, 8 );
                                std::cerr << "cost is: " << withFunc.cost << std::endl;
                                std::cerr << "cost of conversion is:" << cvtCost << std::endl;
                        )
                        if ( cvtCost != Cost::infinity ) {
                                withFunc.cvtCost = cvtCost;
                                alternatives.push_back( withFunc );
                        } // if
                } // for

                candidates = move(alternatives);

                // use a new list so that alternatives are not examined by addAnonConversions twice.
                AltList winners;
                findMinCost( candidates.begin(), candidates.end(), std::back_inserter( winners ) );

                // function may return struct or union value, in which case we need to add alternatives
                // for implicit conversions to each of the anonymous members, must happen after findMinCost
                // since anon conversions are never the cheapest expression
                for ( const Alternative & alt : winners ) {
                        addAnonConversions( alt );
                }
                spliceBegin( alternatives, winners );

                if ( alternatives.empty() && targetType && ! targetType->isVoid() ) {
                        // xxx - this is a temporary hack. If resolution is unsuccessful with a target type, try again without a
                        // target type, since it will sometimes succeed when it wouldn't easily with target type binding. For example,
                        //   forall( otype T ) lvalue T ?[?]( T *, ptrdiff_t );
                        //   const char * x = "hello world";
                        //   unsigned char ch = x[0];
                        // Fails with simple return type binding. First, T is bound to unsigned char, then (x: const char *) is unified
                        // with unsigned char *, which fails because pointer base types must be unified exactly. The new resolver should
                        // fix this issue in a more robust way.
                        targetType = nullptr;
                        postvisit( untypedExpr );
                }
        }

        bool isLvalue( Expression *expr ) {
                // xxx - recurse into tuples?
                return expr->result && ( expr->result->get_lvalue() || dynamic_cast< ReferenceType * >( expr->result ) );
        }

        void AlternativeFinder::Finder::postvisit( AddressExpr *addressExpr ) {
                AlternativeFinder finder( indexer, env );
                finder.find( addressExpr->get_arg() );
                for ( Alternative& alt : finder.alternatives ) {
                        if ( isLvalue( alt.expr ) ) {
                                alternatives.push_back(
                                        Alternative{ alt, new AddressExpr( alt.expr->clone() ), alt.cost } );
                        } // if
                } // for
        }

        void AlternativeFinder::Finder::postvisit( LabelAddressExpr * expr ) {
                alternatives.push_back( Alternative{ expr->clone(), env } );
        }

        Expression * restructureCast( Expression * argExpr, Type * toType, bool isGenerated ) {
                if ( argExpr->get_result()->size() > 1 && ! toType->isVoid() && ! dynamic_cast<ReferenceType *>( toType ) ) {
                        // Argument expression is a tuple and the target type is not void and not a reference type.
                        // 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 UniqueExprs ensure that
                        // side effects will still be done).
                        if ( Tuples::maybeImpureIgnoreUnique( argExpr ) ) {
                                // expressions which may contain side effects require a single unique instance of the expression.
                                argExpr = new UniqueExpr( argExpr );
                        }
                        std::list< Expression * > componentExprs;
                        for ( unsigned int i = 0; i < toType->size(); i++ ) {
                                // cast each component
                                TupleIndexExpr * idx = new TupleIndexExpr( argExpr->clone(), i );
                                componentExprs.push_back( restructureCast( idx, toType->getComponent( i ), isGenerated ) );
                        }
                        delete argExpr;
                        assert( componentExprs.size() > 0 );
                        // produce the tuple of casts
                        return new TupleExpr( componentExprs );
                } else {
                        // handle normally
                        CastExpr * ret = new CastExpr( argExpr, toType->clone() );
                        ret->isGenerated = isGenerated;
                        return ret;
                }
        }

        void AlternativeFinder::Finder::postvisit( CastExpr *castExpr ) {
                Type *& toType = castExpr->get_result();
                assert( toType );
                toType = resolveTypeof( toType, indexer );
                SymTab::validateType( toType, &indexer );
                adjustExprType( toType, env, indexer );

                AlternativeFinder finder( indexer, env );
                finder.targetType = toType;
                finder.findWithAdjustment( castExpr->arg );

                AltList candidates;
                for ( Alternative & alt : finder.alternatives ) {
                        AssertionSet needAssertions( alt.need.begin(), alt.need.end() );
                        AssertionSet haveAssertions;
                        OpenVarSet openVars{ alt.openVars };

                        alt.env.extractOpenVars( openVars );

                        // It's possible that a cast can throw away some values in a multiply-valued expression.  (An example is a
                        // cast-to-void, which casts from 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 = alt.expr->result->size() - castExpr->result->size();
                        if ( discardedValues < 0 ) continue;
                        // xxx - may need to go into tuple types and extract relevant types and use unifyList. Note that currently, this does not
                        // allow casting a tuple to an atomic type (e.g. (int)([1, 2, 3]))
                        // unification run for side-effects
                        unify( castExpr->result, alt.expr->result, alt.env, needAssertions,
                                haveAssertions, openVars, indexer );
                        Cost thisCost = castCost( alt.expr->result, castExpr->result, indexer,
                                alt.env );
                        PRINT(
                                std::cerr << "working on cast with result: " << castExpr->result << std::endl;
                                std::cerr << "and expr type: " << alt.expr->result << std::endl;
                                std::cerr << "env: " << alt.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 );
                                Alternative newAlt{ 
                                        restructureCast( alt.expr->clone(), toType, castExpr->isGenerated ), 
                                        alt.env, openVars, needAssertions, alt.cost, alt.cost + thisCost };
                                inferParameters( newAlt, back_inserter( candidates ) );
                        } // if
                } // for

                // findMinCost selects the alternatives with the lowest "cost" members, but has the side effect of copying the
                // cvtCost member to the cost member (since the old cost is now irrelevant).  Thus, calling findMinCost twice
                // selects first based on argument cost, then on conversion cost.
                AltList minArgCost;
                findMinCost( candidates.begin(), candidates.end(), std::back_inserter( minArgCost ) );
                findMinCost( minArgCost.begin(), minArgCost.end(), std::back_inserter( alternatives ) );
        }

        void AlternativeFinder::Finder::postvisit( VirtualCastExpr * castExpr ) {
                assertf( castExpr->get_result(), "Implicit virtual cast targets not yet supported." );
                AlternativeFinder finder( indexer, env );
                // don't prune here, since it's guaranteed all alternatives will have the same type
                finder.findWithoutPrune( castExpr->get_arg() );
                for ( Alternative & alt : finder.alternatives ) {
                        alternatives.push_back( Alternative{
                                alt, new VirtualCastExpr{ alt.expr->clone(), castExpr->get_result()->clone() },
                                alt.cost } );
                }
        }

        namespace {
                /// Gets name from untyped member expression (member must be NameExpr)
                const std::string& get_member_name( UntypedMemberExpr *memberExpr ) {
                        if ( dynamic_cast< ConstantExpr * >( memberExpr->get_member() ) ) {
                                SemanticError( memberExpr, "Indexed access to struct fields unsupported: " );
                        } // if
                        NameExpr * nameExpr = dynamic_cast< NameExpr * >( memberExpr->get_member() );
                        assert( nameExpr );
                        return nameExpr->get_name();
                }
        }

        void AlternativeFinder::Finder::postvisit( UntypedMemberExpr *memberExpr ) {
                AlternativeFinder funcFinder( indexer, env );
                funcFinder.findWithAdjustment( memberExpr->get_aggregate() );
                for ( AltList::const_iterator agg = funcFinder.alternatives.begin(); agg != funcFinder.alternatives.end(); ++agg ) {
                        // 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 cost = agg->cost;
                        Expression * aggrExpr = agg->expr->clone();
                        referenceToRvalueConversion( aggrExpr, cost );
                        std::unique_ptr<Expression> guard( aggrExpr );

                        // find member of the given type
                        if ( StructInstType *structInst = dynamic_cast< StructInstType* >( aggrExpr->get_result() ) ) {
                                addAggMembers( structInst, aggrExpr, *agg, cost, get_member_name(memberExpr) );
                        } else if ( UnionInstType *unionInst = dynamic_cast< UnionInstType* >( aggrExpr->get_result() ) ) {
                                addAggMembers( unionInst, aggrExpr, *agg, cost, get_member_name(memberExpr) );
                        } else if ( TupleType * tupleType = dynamic_cast< TupleType * >( aggrExpr->get_result() ) ) {
                                addTupleMembers( tupleType, aggrExpr, *agg, cost, memberExpr->get_member() );
                        } // if
                } // for
        }

        void AlternativeFinder::Finder::postvisit( MemberExpr *memberExpr ) {
                alternatives.push_back( Alternative{ memberExpr->clone(), env } );
        }

        void AlternativeFinder::Finder::postvisit( NameExpr *nameExpr ) {
                std::list< SymTab::Indexer::IdData > declList;
                indexer.lookupId( nameExpr->name, declList );
                PRINT( std::cerr << "nameExpr is " << nameExpr->name << std::endl; )
                for ( auto & data : declList ) {
                        Cost cost = Cost::zero;
                        Expression * newExpr = data.combine( cost );

                        // addAnonAlternatives uses vector::push_back, which invalidates references to existing elements, so
                        // can't construct in place and use vector::back
                        Alternative newAlt{ newExpr, env, OpenVarSet{}, AssertionList{}, Cost::zero, cost };
                        PRINT(
                                std::cerr << "decl is ";
                                data.id->print( std::cerr );
                                std::cerr << std::endl;
                                std::cerr << "newExpr is ";
                                newExpr->print( std::cerr );
                                std::cerr << std::endl;
                        )
                        renameTypes( newAlt.expr );
                        addAnonConversions( newAlt ); // add anonymous member interpretations whenever an aggregate value type is seen as a name expression.
                        alternatives.push_back( std::move(newAlt) );
                } // for
        }

        void AlternativeFinder::Finder::postvisit( VariableExpr *variableExpr ) {
                // not sufficient to clone here, because variable's type may have changed
                // since the VariableExpr was originally created.
                alternatives.push_back( Alternative{ new VariableExpr{ variableExpr->var }, env } );
        }

        void AlternativeFinder::Finder::postvisit( ConstantExpr *constantExpr ) {
                alternatives.push_back( Alternative{ constantExpr->clone(), env } );
        }

        void AlternativeFinder::Finder::postvisit( SizeofExpr *sizeofExpr ) {
                if ( sizeofExpr->get_isType() ) {
                        Type * newType = sizeofExpr->get_type()->clone();
                        alternatives.push_back( Alternative{ 
                                new SizeofExpr{ resolveTypeof( newType, indexer ) }, env } );
                } else {
                        // find all alternatives for the argument to sizeof
                        AlternativeFinder finder( indexer, env );
                        finder.find( sizeofExpr->get_expr() );
                        // find the lowest cost alternative among the alternatives, otherwise ambiguous
                        AltList winners;
                        findMinCost( finder.alternatives.begin(), finder.alternatives.end(), back_inserter( winners ) );
                        if ( winners.size() != 1 ) {
                                SemanticError( sizeofExpr->get_expr(), "Ambiguous expression in sizeof operand: " );
                        } // if
                        // return the lowest cost alternative for the argument
                        Alternative &choice = winners.front();
                        referenceToRvalueConversion( choice.expr, choice.cost );
                        alternatives.push_back( Alternative{ 
                                choice, new SizeofExpr( choice.expr->clone() ), Cost::zero } );
                } // if
        }

        void AlternativeFinder::Finder::postvisit( AlignofExpr *alignofExpr ) {
                if ( alignofExpr->get_isType() ) {
                        Type * newType = alignofExpr->get_type()->clone();
                        alternatives.push_back( Alternative{ 
                                new AlignofExpr{ resolveTypeof( newType, indexer ) }, env } );
                } else {
                        // find all alternatives for the argument to sizeof
                        AlternativeFinder finder( indexer, env );
                        finder.find( alignofExpr->get_expr() );
                        // find the lowest cost alternative among the alternatives, otherwise ambiguous
                        AltList winners;
                        findMinCost( finder.alternatives.begin(), finder.alternatives.end(), back_inserter( winners ) );
                        if ( winners.size() != 1 ) {
                                SemanticError( alignofExpr->get_expr(), "Ambiguous expression in alignof operand: " );
                        } // if
                        // return the lowest cost alternative for the argument
                        Alternative &choice = winners.front();
                        referenceToRvalueConversion( choice.expr, choice.cost );
                        alternatives.push_back( Alternative{ 
                                choice, new AlignofExpr{ choice.expr->clone() }, Cost::zero } );
                } // if
        }

        template< typename StructOrUnionType >
        void AlternativeFinder::Finder::addOffsetof( StructOrUnionType *aggInst, const std::string &name ) {
                std::list< Declaration* > members;
                aggInst->lookup( name, members );
                for ( std::list< Declaration* >::const_iterator i = members.begin(); i != members.end(); ++i ) {
                        if ( DeclarationWithType *dwt = dynamic_cast< DeclarationWithType* >( *i ) ) {
                                alternatives.push_back( Alternative{ 
                                        new OffsetofExpr{ aggInst->clone(), dwt }, env } );
                                renameTypes( alternatives.back().expr );
                        } else {
                                assert( false );
                        }
                }
        }

        void AlternativeFinder::Finder::postvisit( UntypedOffsetofExpr *offsetofExpr ) {
                AlternativeFinder funcFinder( indexer, env );
                // xxx - resolveTypeof?
                if ( StructInstType *structInst = dynamic_cast< StructInstType* >( offsetofExpr->get_type() ) ) {
                        addOffsetof( structInst, offsetofExpr->member );
                } else if ( UnionInstType *unionInst = dynamic_cast< UnionInstType* >( offsetofExpr->get_type() ) ) {
                        addOffsetof( unionInst, offsetofExpr->member );
                }
        }

        void AlternativeFinder::Finder::postvisit( OffsetofExpr *offsetofExpr ) {
                alternatives.push_back( Alternative{ offsetofExpr->clone(), env } );
        }

        void AlternativeFinder::Finder::postvisit( OffsetPackExpr *offsetPackExpr ) {
                alternatives.push_back( Alternative{ offsetPackExpr->clone(), env } );
        }

        namespace {
                void resolveAttr( SymTab::Indexer::IdData data, FunctionType *function, Type *argType, const TypeEnvironment &env, AlternativeFinder & finder ) {
                        // assume no polymorphism
                        // assume no implicit conversions
                        assert( function->get_parameters().size() == 1 );
                        PRINT(
                                std::cerr << "resolvAttr: funcDecl is ";
                                data.id->print( std::cerr );
                                std::cerr << " argType is ";
                                argType->print( std::cerr );
                                std::cerr << std::endl;
                        )
                        const SymTab::Indexer & indexer = finder.get_indexer();
                        AltList & alternatives = finder.get_alternatives();
                        if ( typesCompatibleIgnoreQualifiers( argType, function->get_parameters().front()->get_type(), indexer, env ) ) {
                                Cost cost = Cost::zero;
                                Expression * newExpr = data.combine( cost );
                                alternatives.push_back( Alternative{ 
                                        new AttrExpr{ newExpr, argType->clone() }, env, OpenVarSet{}, 
                                        AssertionList{}, Cost::zero, cost } );
                                for ( DeclarationWithType * retVal : function->returnVals ) {
                                        alternatives.back().expr->result = retVal->get_type()->clone();
                                } // for
                        } // if
                }
        }

        void AlternativeFinder::Finder::postvisit( AttrExpr *attrExpr ) {
                // assume no 'pointer-to-attribute'
                NameExpr *nameExpr = dynamic_cast< NameExpr* >( attrExpr->get_attr() );
                assert( nameExpr );
                std::list< SymTab::Indexer::IdData > attrList;
                indexer.lookupId( nameExpr->get_name(), attrList );
                if ( attrExpr->get_isType() || attrExpr->get_expr() ) {
                        for ( auto & data : attrList ) {
                                DeclarationWithType * id = data.id;
                                // check if the type is function
                                if ( FunctionType *function = dynamic_cast< FunctionType* >( id->get_type() ) ) {
                                        // assume exactly one parameter
                                        if ( function->get_parameters().size() == 1 ) {
                                                if ( attrExpr->get_isType() ) {
                                                        resolveAttr( data, function, attrExpr->get_type(), env, altFinder);
                                                } else {
                                                        AlternativeFinder finder( indexer, env );
                                                        finder.find( attrExpr->get_expr() );
                                                        for ( AltList::iterator choice = finder.alternatives.begin(); choice != finder.alternatives.end(); ++choice ) {
                                                                if ( choice->expr->get_result()->size() == 1 ) {
                                                                        resolveAttr(data, function, choice->expr->get_result(), choice->env, altFinder );
                                                                } // fi
                                                        } // for
                                                } // if
                                        } // if
                                } // if
                        } // for
                } else {
                        for ( auto & data : attrList ) {
                                Cost cost = Cost::zero;
                                Expression * newExpr = data.combine( cost );
                                alternatives.push_back( Alternative{ 
                                        newExpr, env, OpenVarSet{}, AssertionList{}, Cost::zero, cost } );
                                renameTypes( alternatives.back().expr );
                        } // for
                } // if
        }

        void AlternativeFinder::Finder::postvisit( LogicalExpr *logicalExpr ) {
                AlternativeFinder firstFinder( indexer, env );
                firstFinder.findWithAdjustment( logicalExpr->get_arg1() );
                if ( firstFinder.alternatives.empty() ) return;
                AlternativeFinder secondFinder( indexer, env );
                secondFinder.findWithAdjustment( logicalExpr->get_arg2() );
                if ( secondFinder.alternatives.empty() ) return;
                for ( const Alternative & first : firstFinder.alternatives ) {
                        for ( const Alternative & second : secondFinder.alternatives ) {
                                TypeEnvironment compositeEnv{ first.env };
                                compositeEnv.simpleCombine( second.env );
                                OpenVarSet openVars{ first.openVars };
                                mergeOpenVars( openVars, second.openVars );
                                AssertionSet need;
                                cloneAll( first.need, need );
                                cloneAll( second.need, need );

                                LogicalExpr *newExpr = new LogicalExpr{ 
                                        first.expr->clone(), second.expr->clone(), logicalExpr->get_isAnd() };
                                alternatives.push_back( Alternative{ 
                                        newExpr, std::move(compositeEnv), std::move(openVars), 
                                        AssertionList( need.begin(), need.end() ), first.cost + second.cost } );
                        }
                }
        }

        void AlternativeFinder::Finder::postvisit( ConditionalExpr *conditionalExpr ) {
                // find alternatives for condition
                AlternativeFinder firstFinder( indexer, env );
                firstFinder.findWithAdjustment( conditionalExpr->arg1 );
                if ( firstFinder.alternatives.empty() ) return;
                // find alternatives for true expression
                AlternativeFinder secondFinder( indexer, env );
                secondFinder.findWithAdjustment( conditionalExpr->arg2 );
                if ( secondFinder.alternatives.empty() ) return;
                // find alterantives for false expression
                AlternativeFinder thirdFinder( indexer, env );
                thirdFinder.findWithAdjustment( conditionalExpr->arg3 );
                if ( thirdFinder.alternatives.empty() ) return;
                for ( const Alternative & first : firstFinder.alternatives ) {
                        for ( const Alternative & second : secondFinder.alternatives ) {
                                for ( const Alternative & third : thirdFinder.alternatives ) {
                                        TypeEnvironment compositeEnv{ first.env };
                                        compositeEnv.simpleCombine( second.env );
                                        compositeEnv.simpleCombine( third.env );
                                        OpenVarSet openVars{ first.openVars };
                                        mergeOpenVars( openVars, second.openVars );
                                        mergeOpenVars( openVars, third.openVars );
                                        AssertionSet need;
                                        cloneAll( first.need, need );
                                        cloneAll( second.need, need );
                                        cloneAll( third.need, need );
                                        AssertionSet have;
                                        
                                        // unify true and false types, then infer parameters to produce new alternatives
                                        Type* commonType = nullptr;
                                        if ( unify( second.expr->result, third.expr->result, compositeEnv, 
                                                        need, have, openVars, indexer, commonType ) ) {
                                                ConditionalExpr *newExpr = new ConditionalExpr{ 
                                                        first.expr->clone(), second.expr->clone(), third.expr->clone() };
                                                newExpr->result = commonType ? commonType : second.expr->result->clone();
                                                // convert both options to the conditional result type
                                                Cost cost = first.cost + second.cost + third.cost;
                                                cost += computeExpressionConversionCost( 
                                                        newExpr->arg2, newExpr->result, indexer, compositeEnv );
                                                cost += computeExpressionConversionCost( 
                                                        newExpr->arg3, newExpr->result, indexer, compositeEnv );
                                                // output alternative
                                                Alternative newAlt{ 
                                                        newExpr, std::move(compositeEnv), std::move(openVars), 
                                                        AssertionList( need.begin(), need.end() ), cost };
                                                inferParameters( newAlt, back_inserter( alternatives ) );
                                        } // if
                                } // for
                        } // for
                } // for
        }

        void AlternativeFinder::Finder::postvisit( CommaExpr *commaExpr ) {
                TypeEnvironment newEnv( env );
                Expression *newFirstArg = resolveInVoidContext( commaExpr->get_arg1(), indexer, newEnv );
                AlternativeFinder secondFinder( indexer, newEnv );
                secondFinder.findWithAdjustment( commaExpr->get_arg2() );
                for ( const Alternative & alt : secondFinder.alternatives ) {
                        alternatives.push_back( Alternative{ 
                                alt, new CommaExpr{ newFirstArg->clone(), alt.expr->clone() }, alt.cost } );
                } // for
                delete newFirstArg;
        }

        void AlternativeFinder::Finder::postvisit( RangeExpr * rangeExpr ) {
                // resolve low and high, accept alternatives whose low and high types unify
                AlternativeFinder firstFinder( indexer, env );
                firstFinder.findWithAdjustment( rangeExpr->low );
                if ( firstFinder.alternatives.empty() ) return;
                AlternativeFinder secondFinder( indexer, env );
                secondFinder.findWithAdjustment( rangeExpr->high );
                if ( secondFinder.alternatives.empty() ) return;
                for ( const Alternative & first : firstFinder.alternatives ) {
                        for ( const Alternative & second : secondFinder.alternatives ) {
                                TypeEnvironment compositeEnv{ first.env };
                                compositeEnv.simpleCombine( second.env );
                                OpenVarSet openVars{ first.openVars };
                                mergeOpenVars( openVars, second.openVars );
                                AssertionSet need;
                                cloneAll( first.need, need );
                                cloneAll( second.need, need );
                                AssertionSet have;

                                Type* commonType = nullptr;
                                if ( unify( first.expr->result, second.expr->result, compositeEnv, need, have, 
                                                openVars, indexer, commonType ) ) {
                                        RangeExpr * newExpr = 
                                                new RangeExpr{ first.expr->clone(), second.expr->clone() };
                                        newExpr->result = commonType ? commonType : first.expr->result->clone();
                                        Alternative newAlt{ 
                                                newExpr, std::move(compositeEnv), std::move(openVars), 
                                                AssertionList( need.begin(), need.end() ), first.cost + second.cost };
                                        inferParameters( newAlt, back_inserter( alternatives ) );
                                } // if
                        } // for
                } // for
        }

        void AlternativeFinder::Finder::postvisit( UntypedTupleExpr *tupleExpr ) {
                std::vector< AlternativeFinder > subExprAlternatives;
                altFinder.findSubExprs( tupleExpr->get_exprs().begin(), tupleExpr->get_exprs().end(),
                        back_inserter( subExprAlternatives ) );
                std::vector< AltList > possibilities;
                combos( subExprAlternatives.begin(), subExprAlternatives.end(),
                        back_inserter( possibilities ) );
                for ( const AltList& alts : possibilities ) {
                        std::list< Expression * > exprs;
                        makeExprList( alts, exprs );

                        TypeEnvironment compositeEnv;
                        OpenVarSet openVars;
                        AssertionSet need;
                        for ( const Alternative& alt : alts ) {
                                compositeEnv.simpleCombine( alt.env );
                                mergeOpenVars( openVars, alt.openVars );
                                cloneAll( alt.need, need );
                        }
                        
                        alternatives.push_back( Alternative{ 
                                new TupleExpr{ exprs }, std::move(compositeEnv), std::move(openVars), 
                                AssertionList( need.begin(), need.end() ), sumCost( alts ) } );
                } // for
        }

        void AlternativeFinder::Finder::postvisit( TupleExpr *tupleExpr ) {
                alternatives.push_back( Alternative{ tupleExpr->clone(), env } );
        }

        void AlternativeFinder::Finder::postvisit( ImplicitCopyCtorExpr * impCpCtorExpr ) {
                alternatives.push_back( Alternative{ impCpCtorExpr->clone(), env } );
        }

        void AlternativeFinder::Finder::postvisit( ConstructorExpr * ctorExpr ) {
                AlternativeFinder finder( indexer, env );
                // don't prune here, since it's guaranteed all alternatives will have the same type
                // (giving the alternatives different types is half of the point of ConstructorExpr nodes)
                finder.findWithoutPrune( ctorExpr->get_callExpr() );
                for ( Alternative & alt : finder.alternatives ) {
                        alternatives.push_back( Alternative{ 
                                alt, new ConstructorExpr( alt.expr->clone() ), alt.cost } );
                }
        }

        void AlternativeFinder::Finder::postvisit( TupleIndexExpr *tupleExpr ) {
                alternatives.push_back( Alternative{ tupleExpr->clone(), env } );
        }

        void AlternativeFinder::Finder::postvisit( TupleAssignExpr *tupleAssignExpr ) {
                alternatives.push_back( Alternative{ tupleAssignExpr->clone(), env } );
        }

        void AlternativeFinder::Finder::postvisit( UniqueExpr *unqExpr ) {
                AlternativeFinder finder( indexer, env );
                finder.findWithAdjustment( unqExpr->get_expr() );
                for ( Alternative & alt : finder.alternatives ) {
                        // ensure that the id is passed on to the UniqueExpr alternative so that the expressions are "linked"
                        UniqueExpr * newUnqExpr = new UniqueExpr( alt.expr->clone(), unqExpr->get_id() );
                        alternatives.push_back( Alternative{ alt, newUnqExpr, alt.cost } );
                }
        }

        void AlternativeFinder::Finder::postvisit( StmtExpr *stmtExpr ) {
                StmtExpr * newStmtExpr = stmtExpr->clone();
                ResolvExpr::resolveStmtExpr( newStmtExpr, indexer );
                // xxx - this env is almost certainly wrong, and needs to somehow contain the combined environments from all of the statements in the stmtExpr...
                alternatives.push_back( Alternative{ newStmtExpr, env } );
        }

        void AlternativeFinder::Finder::postvisit( UntypedInitExpr *initExpr ) {
                // handle each option like a cast
                AltList candidates;
                PRINT(
                        std::cerr << "untyped init expr: " << initExpr << std::endl;
                )
                // O(N^2) checks of d-types with e-types
                for ( InitAlternative & initAlt : initExpr->get_initAlts() ) {
                        Type * toType = resolveTypeof( initAlt.type->clone(), indexer );
                        SymTab::validateType( toType, &indexer );
                        adjustExprType( toType, env, indexer );
                        // Ideally the call to findWithAdjustment could be moved out of the loop, but unfortunately it currently has to occur inside or else
                        // polymorphic return types are not properly bound to the initialization type, since return type variables are only open for the duration of resolving
                        // the UntypedExpr. This is only actually an issue in initialization contexts that allow more than one possible initialization type, but it is still suboptimal.
                        AlternativeFinder finder( indexer, env );
                        finder.targetType = toType;
                        finder.findWithAdjustment( initExpr->expr );
                        for ( Alternative & alt : finder.get_alternatives() ) {
                                TypeEnvironment newEnv( alt.env );
                                AssertionSet need;
                                cloneAll( alt.need, need );
                                AssertionSet have;
                                OpenVarSet openVars( alt.openVars );  
                                // xxx - find things in env that don't have a "representative type" and claim 
                                // those are open vars?
                                PRINT(
                                        std::cerr << "  @ " << toType << " " << initAlt.designation << std::endl;
                                )
                                // It's possible that a cast can throw away some values in a multiply-valued 
                                // expression. (An example is a cast-to-void, which casts from 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 = alt.expr->result->size() - toType->size();
                                if ( discardedValues < 0 ) continue;
                                // xxx - may need to go into tuple types and extract relevant types and use 
                                // unifyList. Note that currently, this does not allow casting a tuple to an 
                                // atomic type (e.g. (int)([1, 2, 3]))
                                
                                // unification run for side-effects
                                unify( toType, alt.expr->result, newEnv, need, have, openVars, indexer );
                                // xxx - do some inspecting on this line... why isn't result bound to initAlt.type?

                                Cost thisCost = castCost( alt.expr->result, toType, indexer, newEnv );
                                if ( thisCost != Cost::infinity ) {
                                        // count one safe conversion for each value that is thrown away
                                        thisCost.incSafe( discardedValues );
                                        Alternative newAlt{ 
                                                new InitExpr{ 
                                                        restructureCast( alt.expr->clone(), toType, true ), initAlt.designation->clone() }, 
                                                std::move(newEnv), std::move(openVars), 
                                                AssertionList( need.begin(), need.end() ), alt.cost, thisCost };
                                        inferParameters( newAlt, back_inserter( candidates ) );
                                }
                        }
                }

                // findMinCost selects the alternatives with the lowest "cost" members, but has the side effect of copying the
                // cvtCost member to the cost member (since the old cost is now irrelevant).  Thus, calling findMinCost twice
                // selects first based on argument cost, then on conversion cost.
                AltList minArgCost;
                findMinCost( candidates.begin(), candidates.end(), std::back_inserter( minArgCost ) );
                findMinCost( minArgCost.begin(), minArgCost.end(), std::back_inserter( alternatives ) );
        }

        void AlternativeFinder::Finder::postvisit( InitExpr * ) {
                assertf( false, "AlternativeFinder should never see a resolved InitExpr." );
        }

        void AlternativeFinder::Finder::postvisit( DeletedExpr * ) {
                assertf( false, "AlternativeFinder should never see a DeletedExpr." );
        }

        void AlternativeFinder::Finder::postvisit( GenericExpr * ) {
                assertf( false, "_Generic is not yet supported." );
        }
} // namespace ResolvExpr

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