source: src/ResolvExpr/AlternativeFinder.cc@ 1a59641

//
// 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 : Aaron B. Moss
// Last Modified On : Mon Jun 11 16:40:00 2018
// Update Count     : 34
//

#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 <limits>                  // for numeric_limits<int>::max()
#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
#include <tuple>                   // for tuple
#include <utility>                 // for pair
#include <vector>                  // for vector

#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 "ResolveTypeof.h"         // for resolveTypeof
#include "Resolver.h"              // for resolveStmtExpr
#include "Common/FilterCombos.h"   // for filterCombos
#include "Common/GC.h"             // for new_static_root
#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

extern bool resolvep;
#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 Cost &newCost, const TypeEnvironment & env, const std::string & name );
                /// Adds alternatives for member expressions where the left side has tuple type
                void addTupleMembers( TupleType * tupleType, Expression *expr, const Cost &newCost, const TypeEnvironment & env, 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 );
                /// Checks if assertion parameters match for a new alternative
                template< typename OutputIterator >
                void inferParameters( const AssertionSet &need, AssertionSet &have, Alternative &&newAlt, OpenVarSet &openVars, 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 );
                                }
                                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.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->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
                Expression* aggrExpr = alt.expr->clone();
                alt.env.apply( aggrExpr->result );
                Type * aggrType = aggrExpr->result;
                if ( dynamic_cast< ReferenceType * >( aggrType ) ) {
                        aggrType = aggrType->stripReferences();
                        aggrExpr = new CastExpr{ aggrExpr, aggrType->clone() };
                }

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

        template< typename StructOrUnionType >
        void AlternativeFinder::Finder::addAggMembers( StructOrUnionType *aggInst, Expression *expr, const Cost &newCost, const TypeEnvironment & env, 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( new MemberExpr( dwt, expr->clone() ), env, 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 Cost &newCost, const TypeEnvironment & env, 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( new TupleIndexExpr( expr->clone(), val ), env, newCost ) );
                        } // if
                } // if
        }

        void AlternativeFinder::Finder::postvisit( ApplicationExpr *applicationExpr ) {
                alternatives.push_back( Alternative( applicationExpr, env, Cost::zero ) );
        }

        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->get_function()->get_result() );
                FunctionType *function = strict_dynamic_cast< FunctionType* >( pointer->get_base() );

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

                for ( std::list< Expression* >::iterator actualExpr = actuals.begin(); actualExpr != actuals.end(); ++actualExpr ) {
                        Type * actualType = (*actualExpr)->get_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->get_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;
                        }
                        Type * formalType = (*formal)->get_type();
                        convCost += computeExpressionConversionCost( *actualExpr, formalType, indexer, alt.env );
                        ++formal; // can't be in for-loop update because of the continue
                }
                if ( formal != formals.end() ) {
                        return Cost::infinity;
                }

                #if 1 // cost of assertions accounted for in function creation
                for ( InferredParams::const_iterator assert = appExpr->get_inferParams().begin(); assert != appExpr->get_inferParams().end(); ++assert ) {
                        convCost += computeConversionCost( assert->second.actualType, assert->second.formalType, indexer, alt.env );
                }
                #endif

                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;
                        }
///     needAssertions.insert( needAssertions.end(), (*tyvar)->get_assertions().begin(), (*tyvar)->get_assertions().end() );
                }
        }

        static const int recursionLimit = /*10*/ 4;  ///< Limit to depth of recursion satisfaction

        void addToIndexer( AssertionSet &assertSet, SymTab::Indexer &indexer ) {
                for ( AssertionSet::iterator i = assertSet.begin(); i != assertSet.end(); ++i ) {
                        if ( i->second.isUsed ) {
                                indexer.addId( i->first );
                        }
                }
        }

        template< typename ForwardIterator, typename OutputIterator >
        void inferRecursive( ForwardIterator begin, ForwardIterator end, const Alternative &newAlt, OpenVarSet &openVars, const SymTab::Indexer &decls, const AssertionSet &newNeed, int level, const SymTab::Indexer &indexer, OutputIterator out ) {
                if ( begin == end ) {
                        if ( newNeed.empty() ) {
                                PRINT(
                                        std::cerr << "all assertions satisfied, output alternative: ";
                                        newAlt.print( std::cerr );
                                        std::cerr << std::endl;
                                );
                                *out++ = newAlt;
                                return;
                        } else if ( level >= recursionLimit ) {
                                SemanticError( newAlt.expr->location, "Too many recursive assertions" );
                        } else {
                                AssertionSet newerNeed;
                                PRINT(
                                        std::cerr << "recursing with new set:" << std::endl;
                                        printAssertionSet( newNeed, std::cerr, 8 );
                                )
                                inferRecursive( newNeed.begin(), newNeed.end(), newAlt, openVars, decls, newerNeed, level+1, indexer, out );
                                return;
                        }
                }

                ForwardIterator cur = begin++;
                if ( ! cur->second.isUsed ) {
                        inferRecursive( begin, end, newAlt, openVars, decls, newNeed, level, indexer, out );
                        return; // xxx - should this continue? previously this wasn't here, and it looks like it should be
                }
                DeclarationWithType *curDecl = cur->first;

                PRINT(
                        std::cerr << "inferRecursive: assertion is ";
                        curDecl->print( std::cerr );
                        std::cerr << std::endl;
                )
                std::list< SymTab::Indexer::IdData > candidates;
                decls.lookupId( curDecl->get_name(), candidates );
///   if ( candidates.empty() ) { std::cerr << "no candidates!" << std::endl; }
                for ( const auto & data : candidates ) {
                        DeclarationWithType * candidate = data.id;
                        PRINT(
                                std::cerr << "inferRecursive: candidate is ";
                                candidate->print( std::cerr );
                                std::cerr << std::endl;
                        )

                        AssertionSet newHave, newerNeed( newNeed );
                        TypeEnvironment newEnv( newAlt.env );
                        OpenVarSet newOpenVars( openVars );
                        Type *adjType = candidate->get_type()->clone();
                        adjustExprType( adjType, newEnv, indexer );
                        renameTyVars( adjType );
                        PRINT(
                                std::cerr << "unifying ";
                                curDecl->get_type()->print( std::cerr );
                                std::cerr << " with ";
                                adjType->print( std::cerr );
                                std::cerr << std::endl;
                        )
                        if ( unify( curDecl->get_type(), adjType, newEnv, newerNeed, newHave, newOpenVars, indexer ) ) {
                                PRINT(
                                        std::cerr << "success!" << std::endl;
                                )
                                SymTab::Indexer newDecls( decls );
                                addToIndexer( newHave, newDecls );
                                Alternative newerAlt( newAlt );
                                newerAlt.env = newEnv;
                                assertf( candidate->get_uniqueId(), "Assertion candidate does not have a unique ID: %s", toString( candidate ).c_str() );

                                // everything with an empty idChain was pulled in by the current assertion.
                                // add current assertion's idChain + current assertion's ID so that the correct inferParameters can be found.
                                for ( auto & a : newerNeed ) {
                                        if ( a.second.idChain.empty() ) {
                                                a.second.idChain = cur->second.idChain;
                                                a.second.idChain.push_back( curDecl->get_uniqueId() );
                                        }
                                }

                                Expression *varExpr = data.combine( newerAlt.cvtCost );
                                varExpr->set_result( adjType->clone() );
                                PRINT(
                                        std::cerr << "satisfying assertion " << curDecl->get_uniqueId() << " ";
                                        curDecl->print( std::cerr );
                                        std::cerr << " with declaration " << candidate->get_uniqueId() << " ";
                                        candidate->print( std::cerr );
                                        std::cerr << std::endl;
                                )
                                // follow the current assertion's ID chain to find the correct set of inferred parameters to add the candidate to (i.e. the set of inferred parameters belonging to the entity which requested the assertion parameter).
                                InferredParams * inferParameters = &newerAlt.expr->get_inferParams();
                                for ( UniqueId id : cur->second.idChain ) {
                                        inferParameters = (*inferParameters)[ id ].inferParams.get();
                                }
                                
                                (*inferParameters)[ curDecl->get_uniqueId() ] = ParamEntry( candidate->get_uniqueId(), adjType, curDecl->get_type(), varExpr );
                                inferRecursive( begin, end, newerAlt, newOpenVars, newDecls, newerNeed, level, indexer, out );
                        }
                }
        }

#if 1
        namespace {
                /// Information required to defer resolution of an expression
                struct AssertionPack {
                        SymTab::Indexer::IdData cdata;  ///< Satisfying declaration
                        Type* adjType;                  ///< Satisfying type
                        TypeEnvironment env;            ///< Post-unification environment
                        AssertionSet have;              ///< Post-unification have-set
                        AssertionSet need;              ///< Post-unification need-set
                        OpenVarSet openVars;            ///< Post-unification open-var set

                        AssertionPack( const SymTab::Indexer::IdData& cdata, Type* adjType, 
                                        TypeEnvironment&& env, AssertionSet&& have, AssertionSet&& need, 
                                        OpenVarSet&& openVars ) 
                                : cdata(cdata), adjType(adjType), env(std::move(env)), have(std::move(have)), 
                                  need(std::move(need)), openVars(std::move(openVars)) {}
                        
                        class iterator {
                        friend AssertionPack;
                        public:
                                const AssertionPack& pack;
                        private:
                                AssertionSet::iterator it;

                                iterator(const AssertionPack& p, AssertionSet::iterator i) : pack{p}, it{i} {}
                        public:
                                iterator& operator++ () { ++it; return *this; }
                                iterator operator++ (int) { iterator tmp = *this; ++it; return tmp; }
                                AssertionSet::value_type& operator* () { return *it; }
                                AssertionSet::value_type* operator-> () { return it.operator->(); }
                                bool operator== (const iterator& o) const { return this == &o && it == o.it; }
                                bool operator!= (const iterator& o) const { return !(*this == o); }
                        };

                        iterator begin() { return { *this, need.begin() }; }
                        iterator end() { return { *this, need.end() }; }
                };

                /// List of deferred assertion resolutions for the same type
                using DeferList = std::vector<AssertionPack>;

                /// Single deferred item
                struct DeferElement {
                        const DeclarationWithType* curDecl;
                        const AssertionSetValue& assnInfo;
                        const AssertionPack& match;
                };

                /// Wrapper for the DeferList of a single assertion resolution
                struct DeferItem {
                        DeclarationWithType* curDecl;
                        AssertionSetValue assnInfo;
                        DeferList matches;

                        DeferItem(DeclarationWithType* cd, const AssertionSetValue& ai, DeferList&& m )
                                : curDecl{cd}, assnInfo{ai}, matches(std::move(m)) {}

                        bool empty() const { return matches.empty(); }

                        DeferList::size_type size() const { return matches.size(); }

                        DeferElement operator[] ( unsigned i ) const {
                                return { curDecl, assnInfo, matches[i] };
                        }
                };

                /// Flag for state iteration
                enum CopyFlag { IterateState };

                /// Intermediate state for assertion resolution
                struct AssertionResnState {
                        Alternative alt;                  ///< Alternative being built from
                        AssertionSet need;                ///< Assertions to find
                        AssertionSet newNeed;             ///< New assertions found from current assertions
                        OpenVarSet openVars;              ///< Open variables in current context
                        std::vector<DeferItem> deferred;  ///< Possible deferred assertion resolutions
                        SymTab::Indexer& indexer;         ///< Name lookup

                        /// field-wise constructor (some fields defaulted)
                        AssertionResnState(Alternative&& alt, const AssertionSet& need, 
                                const OpenVarSet& openVars, SymTab::Indexer& indexer ) 
                                : alt{std::move(alt)}, need{need}, newNeed{}, openVars{openVars}, deferred{}, 
                                  indexer{indexer} {}
                        
                        /// construct iterated state object from previous state
                        AssertionResnState(AssertionResnState&& o, CopyFlag) 
                                : alt{std::move(o.alt)}, need{std::move(o.newNeed)}, newNeed{}, 
                                  openVars{std::move(o.openVars)}, deferred{}, indexer{o.indexer} {}
                        
                        /// copy resn state updated with specified data
                        AssertionResnState(const AssertionResnState& o, const AssertionSet& need, 
                                const OpenVarSet& openVars, TypeEnvironment& env )
                                : alt{o.alt}, need{}, newNeed{o.newNeed}, openVars{openVars}, deferred{},
                                  indexer{o.indexer} {
                                newNeed.insert( need.begin(), need.end() );
                                alt.env = env;
                        }
                };

                /// Binds a single assertion from a compatible AssertionPack, updating resolution state 
                /// as appropriate.
                void bindAssertion( const DeclarationWithType* curDecl, const AssertionSetValue& assnInfo, 
                                Alternative& alt, AssertionPack& match ) {

                        DeclarationWithType* candidate = match.cdata.id;
                        assertf( candidate->get_uniqueId(), "Assertion candidate does not have a unique ID: %s", toString( candidate ).c_str() );
                        for ( auto& a : match.need ) {
                                if ( a.second.idChain.empty() ) {
                                        a.second.idChain = assnInfo.idChain;
                                        a.second.idChain.push_back( curDecl->get_uniqueId() );
                                }
                        }

                        Expression* varExpr = match.cdata.combine( alt.cvtCost );
                        varExpr->result = match.adjType;

                        // follow the current assertion's ID chain to find the correct set of inferred 
                        // parameters to add the candidate o (i.e. the set of inferred parameters belonging 
                        // to the entity which requested the assertion parameter)
                        InferredParams* inferParams = &alt.expr->inferParams;
                        for ( UniqueId id : assnInfo.idChain ) {
                                inferParams = (*inferParams)[ id ].inferParams.get();
                        }

                        (*inferParams)[ curDecl->get_uniqueId() ] = ParamEntry{
                                candidate->get_uniqueId(), match.adjType, curDecl->get_type(), varExpr };
                }

                /// Resolves a single assertion, returning false if no satisfying assertion, binding it 
                /// if there is exactly one satisfying assertion, or adding to the defer-list if there 
                /// is more than one
                bool resolveAssertion( DeclarationWithType* curDecl, AssertionSetValue& assnInfo, 
                                AssertionResnState& resn ) {
                        // skip unused assertions
                        if ( ! assnInfo.isUsed ) return true;

                        // lookup candidates for this assertion
                        std::list< SymTab::Indexer::IdData > candidates;
                        resn.indexer.lookupId( curDecl->name, candidates );

                        // find the ones that unify with the desired type
                        DeferList matches;
                        for ( const auto& cdata : candidates ) {
                                DeclarationWithType* candidate = cdata.id;

                                // build independent unification context for candidate
                                AssertionSet have, newNeed;
                                TypeEnvironment newEnv{ resn.alt.env };
                                OpenVarSet newOpenVars{ resn.openVars };
                                Type* adjType = candidate->get_type()->clone();
                                adjustExprType( adjType, newEnv, resn.indexer );
                                renameTyVars( adjType );

                                if ( unify( curDecl->get_type(), adjType, newEnv, 
                                                newNeed, have, newOpenVars, resn.indexer ) ) {
                                        matches.emplace_back( cdata, adjType, std::move(newEnv), std::move(have), 
                                                std::move(newNeed), std::move(newOpenVars) );
                                }
                        }

                        // Break if no suitable assertion
                        if ( matches.empty() ) return false;

                        // Defer if too many suitable assertions
                        if ( matches.size() > 1 ) {
                                resn.deferred.emplace_back( curDecl, assnInfo, std::move(matches) );
                                return true;
                        }

                        // otherwise bind current match in ongoing scope
                        AssertionPack& match = matches.front();
                        addToIndexer( match.have, resn.indexer );
                        resn.newNeed.insert( match.need.begin(), match.need.end() );
                        resn.openVars = std::move(match.openVars);
                        resn.alt.env = std::move(match.env);

                        bindAssertion( curDecl, assnInfo, resn.alt, match );
                        return true;
                }

                /// Combo iterator that combines interpretations into an interpretation list, merging 
                /// their environments. Rejects an appended interpretation if the environments cannot 
                /// be merged.
                class InterpretationEnvMerger {
                        /// Current list of merged interpretations
                        std::vector<DeferElement> crnt;
                        /// Stack of environments, to support backtracking
                        std::vector<TypeEnvironment> envs;
                        /// Open variables for environment combination
                        std::vector<OpenVarSet> varSets;
                        /// Indexer to use for environment merges
                        const SymTab::Indexer& indexer;
                public:
                        /// The merged environment/open variables and the list of interpretations
                        struct OutType {
                                TypeEnvironment env;
                                OpenVarSet openVars;
                                std::vector<DeferElement> assns;

                                OutType( const TypeEnvironment& env, const OpenVarSet& openVars, 
                                        const std::vector<DeferElement>& assns )
                                        : env(env), openVars(openVars), assns(assns) {}
                        };

                        InterpretationEnvMerger( const TypeEnvironment& env, const OpenVarSet& openVars, 
                                const SymTab::Indexer& indexer ) : crnt{}, envs{}, varSets{}, indexer{indexer} {
                                envs.push_back( env );
                                varSets.push_back( openVars );
                        }

                        ComboResult append( DeferElement i ) {
                                TypeEnvironment env = envs.back();
                                OpenVarSet openVars = varSets.back();
                                mergeOpenVars( openVars, i.match.openVars );

                                if ( ! env.combine( i.match.env, openVars, indexer ) )
                                        return ComboResult::REJECT_THIS;
                                
                                crnt.push_back( i );
                                envs.push_back( env );
                                varSets.push_back( openVars );
                                return ComboResult::ACCEPT;
                        }

                        void backtrack() {
                                crnt.pop_back();
                                envs.pop_back();
                                varSets.pop_back();
                        }

                        OutType finalize() { return { envs.back(), varSets.back(), crnt }; }
                };
        }
#endif

        template< typename OutputIterator >
        void AlternativeFinder::Finder::inferParameters( const AssertionSet &need, AssertionSet &have, Alternative &&newAlt, OpenVarSet &openVars, OutputIterator out ) {
//      PRINT(
//          std::cerr << "inferParameters: assertions needed are" << std::endl;
//          printAll( need, std::cerr, 8 );
//          )
                SymTab::Indexer decls( indexer );
                // PRINT(
                //      std::cerr << "============= original indexer" << std::endl;
                //      indexer.print( std::cerr );
                //      std::cerr << "============= new indexer" << std::endl;
                //      decls.print( std::cerr );
                // )
                addToIndexer( have, decls );
#if 1
                using ResnList = std::vector<AssertionResnState>;
                ResnList resns{ AssertionResnState{ std::move(newAlt), need, openVars, decls } };
                ResnList new_resns{};

                // resolve assertions in breadth-first-order up to a limited number of levels deep
                for ( int level = 0; level < recursionLimit; ++level ) {
                        // scan over all mutually-compatible resolutions
                        for ( auto& resn : resns ) {
                                // make initial pass at matching assertions
                                for ( auto& assn : resn.need ) {
                                        if ( ! resolveAssertion( assn.first, assn.second, resn ) ) {
                                                // fail early if any assertion fails to resolve
                                                goto nextResn;
                                        }
                                }

                                if ( ! resn.deferred.empty() ) {
                                        // resolve deferred assertions by mutual compatibility

                                        std::vector<InterpretationEnvMerger::OutType> compatible = filterCombos( 
                                                resn.deferred, 
                                                InterpretationEnvMerger{ resn.alt.env, resn.openVars, resn.indexer } );
                                        
                                        for ( auto& compat : compatible ) {
                                                AssertionResnState new_resn = resn;

                                                // add compatible assertions to new resolution state
                                                for ( DeferElement el : compat.assns ) {
                                                        AssertionPack match = el.match;
                                                        addToIndexer( match.have, new_resn.indexer );
                                                        resn.newNeed.insert( match.need.begin(), match.need.end() );
                                                        
                                                        bindAssertion( el.curDecl, el.assnInfo, new_resn.alt, match );
                                                }

                                                // set mutual environment into resolution state
                                                new_resn.alt.env = std::move(compat.env);
                                                new_resn.openVars = std::move(compat.openVars);

                                                // add successful match or push back next state
                                                if ( new_resn.newNeed.empty() ) {
                                                        *out++ = new_resn.alt;
                                                } else {
                                                        new_resns.emplace_back( std::move(new_resn), IterateState );
                                                }
                                        }
                                } else {
                                        // add successful match or push back next state
                                        if ( resn.newNeed.empty() ) {
                                                *out++ = resn.alt;
                                        } else {
                                                new_resns.emplace_back( std::move(resn), IterateState );
                                        }
                                }
                        nextResn:; }
                        
                        // finish or reset for next round
                        if ( new_resns.empty() ) return;
                        resns.swap( new_resns );
                        new_resns.clear();
                }
                // if reaches here, exceeded recursion limit
                SemanticError( newAlt.expr->location, "Too many recursive assertions" );

#else           
                AssertionSet newNeed;
                PRINT(
                        std::cerr << "env is: " << std::endl;
                        newAlt.env.print( std::cerr, 0 );
                        std::cerr << std::endl;
                )

                inferRecursive( need.begin(), need.end(), newAlt, openVars, decls, newNeed, 0, indexer, out );
//      PRINT(
//          std::cerr << "declaration 14 is ";
//          Declaration::declFromId
//          *out++ = newAlt;
//          )
#endif
        }

        /// 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
                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(nullptr), 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, Cost initCost = Cost::zero)
                        : parent(0), expr(nullptr), cost(initCost), 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), 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), 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 ); }
                        while ( pack->tupleStart == 0 ) {
                                pack = &packs[pack->parent];
                                exprs.push_front( pack->expr );
                                cost += pack->cost;
                        }
                        // reset pack to appropriate tuple
                        expr = 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], 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 = 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 = results[i].expr;
                                                        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(), 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];

                                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();
                                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();
        }

#if !1
        namespace {

                struct CountSpecs : public WithVisitorRef<CountSpecs>, WithShortCircuiting {

                        void postvisit(PointerType*) {
                                // mark specialization of base type
                                if ( count >= 0 ) ++count;
                        }

                        void postvisit(ArrayType*) {
                                // mark specialization of base type
                                if ( count >= 0 ) ++count;
                        }

                        void postvisit(ReferenceType*) {
                                // mark specialization of base type -- xxx maybe not?
                                if ( count >= 0 ) ++count;
                        }

                private:
                        // takes minimum non-negative count over parameter/return list
                        void takeminover( int& mincount, std::list<DeclarationWithType*>& dwts ) {
                                for ( DeclarationWithType* dwt : dwts ) {
                                        count = -1;
                                        maybeAccept( dwt->get_type(), *visitor );
                                        if ( count != -1 && count < mincount ) mincount = count;
                                }
                        }

                public:
                        void previsit(FunctionType*) {
                                // override default child visiting behaviour
                                visit_children = false;
                        }

                        void postvisit(FunctionType* fty) {
                                // take minimal set value of count over ->returnVals and ->parameters
                                int mincount = std::numeric_limits<int>::max();
                                takeminover( mincount, fty->parameters );
                                takeminover( mincount, fty->returnVals );
                                // add another level to mincount if set
                                count = mincount < std::numeric_limits<int>::max() ? mincount + 1 : -1;
                        }

                private:
                        // returns minimum non-negative count + 1 over type parameters (-1 if none such)
                        int minover( std::list<Expression*>& parms ) {
                                int mincount = std::numeric_limits<int>::max();
                                for ( Expression* parm : parms ) {
                                        count = -1;
                                        maybeAccept( parm->get_result(), *visitor );
                                        if ( count != -1 && count < mincount ) mincount = count;
                                }
                                return mincount < std::numeric_limits<int>::max() ? mincount + 1 : -1;
                        }
                        
                public:
                        void previsit(StructInstType*) {
                                // override default child behaviour
                                visit_children = false;
                        }

                        void postvisit(StructInstType* sity) {
                                // look for polymorphic parameters
                                count = minover( sity->parameters );
                        }

                        void previsit(UnionInstType*) {
                                // override default child behaviour
                                visit_children = false;
                        }

                        void postvisit(UnionInstType* uity) {
                                // look for polymorphic parameters
                                count = minover( uity->parameters );
                        }

                        void postvisit(TypeInstType*) {
                                // note polymorphic type (which may be specialized)
                                // xxx - maybe account for open/closed type variables
                                count = 0;
                        }

                        void previsit(TupleType*) {
                                // override default child behaviour
                                visit_children = false;
                        }

                        void postvisit(TupleType* tty) {
                                // take minimum non-negative count
                                int mincount = std::numeric_limits<int>::max();
                                for ( Type* ty : tty->types ) {
                                        count = -1;
                                        maybeAccept( ty, *visitor );
                                        if ( count != -1 && count < mincount ) mincount = count;
                                }
                                // xxx - maybe don't increment, tuple flattening doesn't necessarily specialize
                                count = mincount < std::numeric_limits<int>::max() ? mincount + 1: -1;
                        }

                        int get_count() const { return count >= 0 ? count : 0; }
                private:
                        int count = -1;
                };

                /// Counts the specializations in the types in a function parameter or return list
                int countSpecs( std::list<DeclarationWithType*>& dwts ) {
                        int k = 0;
                        for ( DeclarationWithType* dwt : dwts ) {
                                PassVisitor<CountSpecs> counter;
                                maybeAccept( dwt->get_type(), *counter.pass.visitor );
                                k += counter.pass.get_count();
                        }
                        return k;
                }

                /// Calculates the inherent costs in a function declaration; varCost for the number of 
                /// type variables and specCost for type assertions, as well as PolyType specializations 
                /// in the parameter and return lists.
                Cost declCost( FunctionType* funcType ) {
                        Cost k = Cost::zero;

                        // add cost of type variables
                        k.incVar( funcType->forall.size() );

                        // subtract cost of type assertions
                        for ( TypeDecl* td : funcType->forall ) {
                                k.decSpec( td->assertions.size() );
                        }

                        // count specialized polymorphic types in parameter/return lists
                        k.decSpec( countSpecs( funcType->parameters ) );
                        k.decSpec( countSpecs( funcType->returnVals ) );

                        return k;
                }
        }
#endif

        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 );
                // 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 );
                        cost += pack->cost;
                        pack = &results[pack->parent];
                }
                // build and validate new alternative
                Alternative newAlt( appExpr, result.env, cost );
                PRINT(
                        std::cerr << "instantiate function success: " << appExpr << std::endl;
                        std::cerr << "need assertions:" << std::endl;
                        printAssertionSet( result.need, std::cerr, 8 );
                )
                inferParameters( result.need, result.have, std::move(newAlt), result.openVars, 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;
                        }
                }

                // calculate declaration cost of function (+vars-spec)
#if !1
                Cost funcCost = declCost( funcType );
#else
                Cost funcCost = Cost::zero;
#endif

                // iteratively build matches, one parameter at a time
                std::vector<ArgPack> results;
                results.push_back( ArgPack{ funcEnv, funcNeed, funcHave, funcOpenVars, funcCost } );
                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], 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(), 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 auto *opExpr = new_static_root<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->get_result()->stripReferences() ) ) {
                                        if ( FunctionType *function = dynamic_cast< FunctionType* >( pointer->get_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 ( ClassRef eqvClass = func->env.lookup( typeInst->name ) ) {
                                                if ( FunctionType *function = dynamic_cast< FunctionType* >( eqvClass.get_bound().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->get_result()->stripReferences() ) ) {
                                                if ( FunctionType* function =
                                                                dynamic_cast<FunctionType*>( pointer->get_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->get_function()->get_result() );
                                FunctionType *function = strict_dynamic_cast< FunctionType* >( pointer->get_base() );
                                std::cerr << "Case +++++++++++++ " << appExpr->get_function() << std::endl;
                                std::cerr << "formals are:" << std::endl;
                                printAll( function->get_parameters(), std::cerr, 8 );
                                std::cerr << "actuals are:" << std::endl;
                                printAll( appExpr->get_args(), std::cerr, 8 );
                                std::cerr << "bindings are:" << std::endl;
                                withFunc.env.print( std::cerr, 8 );
                                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 implicitconversions 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->get_result()->get_lvalue() || dynamic_cast< ReferenceType * >( expr->get_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{ new AddressExpr( alt.expr ), alt.env, alt.cost } );
                        } // if
                } // for
        }

        void AlternativeFinder::Finder::postvisit( LabelAddressExpr * expr ) {
                alternatives.push_back( Alternative{ expr, env, Cost::zero } );
        }

        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 ) );
                        }
                        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, haveAssertions;
                        OpenVarSet 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,
                                        alt.cost, thisCost );
                                inferParameters( needAssertions, haveAssertions, std::move(newAlt), openVars,
                                        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(), "Implicate 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(
                                new VirtualCastExpr( alt.expr, castExpr->get_result()->clone() ),
                                alt.env, alt.cost ) );
                }
        }

        namespace {
                /// Gets name from untyped member expression (member must be NameExpr)
                const std::string& get_member_name( UntypedMemberExpr *memberExpr ) {
                        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 );

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

        void AlternativeFinder::Finder::postvisit( MemberExpr *memberExpr ) {
                alternatives.push_back( Alternative( memberExpr, env, Cost::zero ) );
        }

        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, 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, Cost::zero ) );
        }

        void AlternativeFinder::Finder::postvisit( ConstantExpr *constantExpr ) {
                alternatives.push_back( Alternative( constantExpr, env, Cost::zero ) );
        }

        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, Cost::zero ) );
                } 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( new SizeofExpr( choice.expr ), choice.env, 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, Cost::zero ) );
                } 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( new AlignofExpr( choice.expr ), choice.env, 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, Cost::zero ) );
                                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, env, Cost::zero ) );
        }

        void AlternativeFinder::Finder::postvisit( OffsetPackExpr *offsetPackExpr ) {
                alternatives.push_back( Alternative( offsetPackExpr, env, Cost::zero ) );
        }

        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, 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, 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;
                                compositeEnv.simpleCombine( first.env );
                                compositeEnv.simpleCombine( second.env );

                                LogicalExpr *newExpr = new LogicalExpr( first.expr, second.expr, logicalExpr->get_isAnd() );
                                alternatives.push_back( Alternative( newExpr, compositeEnv, 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;
                                        compositeEnv.simpleCombine( first.env );
                                        compositeEnv.simpleCombine( second.env );
                                        compositeEnv.simpleCombine( third.env );

                                        // unify true and false types, then infer parameters to produce new alternatives
                                        OpenVarSet openVars;
                                        AssertionSet needAssertions, haveAssertions;
                                        Alternative newAlt( 0, compositeEnv, first.cost + second.cost + third.cost );
                                        Type* commonType = nullptr;
                                        if ( unify( second.expr->result, third.expr->result, newAlt.env, needAssertions, haveAssertions, openVars, indexer, commonType ) ) {
                                                ConditionalExpr *newExpr = new ConditionalExpr( first.expr, second.expr, third.expr );
                                                newExpr->result = commonType ? commonType : second.expr->result->clone();
                                                // convert both options to the conditional result type
                                                newAlt.cost += computeExpressionConversionCost( newExpr->arg2, newExpr->result, indexer, newAlt.env );
                                                newAlt.cost += computeExpressionConversionCost( newExpr->arg3, newExpr->result, indexer, newAlt.env );
                                                newAlt.expr = newExpr;
                                                inferParameters( needAssertions, haveAssertions, std::move(newAlt), openVars, 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( new CommaExpr( newFirstArg, alt.expr ), alt.env, alt.cost ) );
                } // for
        }

        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;
                                compositeEnv.simpleCombine( first.env );
                                compositeEnv.simpleCombine( second.env );
                                OpenVarSet openVars;
                                AssertionSet needAssertions, haveAssertions;
                                Alternative newAlt( 0, compositeEnv, first.cost + second.cost );
                                Type* commonType = nullptr;
                                if ( unify( first.expr->result, second.expr->result, newAlt.env, needAssertions, haveAssertions, openVars, indexer, commonType ) ) {
                                        RangeExpr * newExpr = new RangeExpr( first.expr, second.expr );
                                        newExpr->result = commonType ? commonType : first.expr->result->clone();
                                        newAlt.expr = newExpr;
                                        inferParameters( needAssertions, haveAssertions, std::move(newAlt), openVars, 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;
                        simpleCombineEnvironments( alts.begin(), alts.end(), compositeEnv );
                        alternatives.push_back(
                                Alternative{ new TupleExpr( exprs ), compositeEnv, sumCost( alts ) } );
                } // for
        }

        void AlternativeFinder::Finder::postvisit( TupleExpr *tupleExpr ) {
                alternatives.push_back( Alternative( tupleExpr, env, Cost::zero ) );
        }

        void AlternativeFinder::Finder::postvisit( ImplicitCopyCtorExpr * impCpCtorExpr ) {
                alternatives.push_back( Alternative( impCpCtorExpr, env, Cost::zero ) );
        }

        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( new ConstructorExpr( alt.expr ), alt.env, alt.cost ) );
                }
        }

        void AlternativeFinder::Finder::postvisit( TupleIndexExpr *tupleExpr ) {
                alternatives.push_back( Alternative( tupleExpr, env, Cost::zero ) );
        }

        void AlternativeFinder::Finder::postvisit( TupleAssignExpr *tupleAssignExpr ) {
                alternatives.push_back( Alternative( tupleAssignExpr, env, Cost::zero ) );
        }

        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, unqExpr->get_id() );
                        alternatives.push_back( Alternative( newUnqExpr, alt.env, 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, Cost::zero ) );
        }

        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 needAssertions, haveAssertions;
                                OpenVarSet openVars;  // 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, needAssertions, haveAssertions, 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, toType, true ), initAlt.designation ), newEnv, alt.cost, thisCost );
                                        inferParameters( needAssertions, haveAssertions, std::move(newAlt), openVars, 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: //