source: src/ResolvExpr/Unify.cc@ bc4bea8

//
// 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.
//
// Unify.cc --
//
// Author           : Richard C. Bilson
// Created On       : Sun May 17 12:27:10 2015
// Last Modified By : Aaron B. Moss
// Last Modified On : Mon Jun 18 11:58:00 2018
// Update Count     : 43
//

#include "Unify.h"

#include <cassert>                  // for assertf, assert
#include <iterator>                 // for back_insert_iterator, back_inserter
#include <map>                      // for _Rb_tree_const_iterator, _Rb_tree_i...
#include <memory>                   // for unique_ptr
#include <set>                      // for set
#include <string>                   // for string, operator==, operator!=, bas...
#include <utility>                  // for pair, move
#include <vector>

#include "AST/Decl.hpp"
#include "AST/Node.hpp"
#include "AST/Pass.hpp"
#include "AST/Type.hpp"
#include "AST/TypeEnvironment.hpp"
#include "Common/PassVisitor.h"     // for PassVisitor
#include "FindOpenVars.h"           // for findOpenVars
#include "Parser/LinkageSpec.h"     // for C
#include "SynTree/Constant.h"       // for Constant
#include "SynTree/Declaration.h"    // for TypeDecl, TypeDecl::Data, Declarati...
#include "SynTree/Expression.h"     // for TypeExpr, Expression, ConstantExpr
#include "SynTree/Mutator.h"        // for Mutator
#include "SynTree/Type.h"           // for Type, TypeInstType, FunctionType
#include "SynTree/Visitor.h"        // for Visitor
#include "Tuples/Tuples.h"          // for isTtype
#include "TypeEnvironment.h"        // for EqvClass, AssertionSet, OpenVarSet
#include "typeops.h"                // for flatten, occurs, commonType

namespace ast {
        class SymbolTable;
}

namespace SymTab {
class Indexer;
}  // namespace SymTab

// #define DEBUG

namespace ResolvExpr {

        struct Unify_old : public WithShortCircuiting {
                Unify_old( Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, const OpenVarSet &openVars, WidenMode widen, const SymTab::Indexer &indexer );

                bool get_result() const { return result; }

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

                void postvisit( VoidType * voidType );
                void postvisit( BasicType * basicType );
                void postvisit( PointerType * pointerType );
                void postvisit( ArrayType * arrayType );
                void postvisit( ReferenceType * refType );
                void postvisit( FunctionType * functionType );
                void postvisit( StructInstType * aggregateUseType );
                void postvisit( UnionInstType * aggregateUseType );
                void postvisit( EnumInstType * aggregateUseType );
                void postvisit( TraitInstType * aggregateUseType );
                void postvisit( TypeInstType * aggregateUseType );
                void postvisit( TupleType * tupleType );
                void postvisit( VarArgsType * varArgsType );
                void postvisit( ZeroType * zeroType );
                void postvisit( OneType * oneType );

          private:
                template< typename RefType > void handleRefType( RefType *inst, Type *other );
                template< typename RefType > void handleGenericRefType( RefType *inst, Type *other );

                bool result;
                Type *type2;                            // inherited
                TypeEnvironment &env;
                AssertionSet &needAssertions;
                AssertionSet &haveAssertions;
                const OpenVarSet &openVars;
                WidenMode widen;
                const SymTab::Indexer &indexer;
        };

        /// Attempts an inexact unification of type1 and type2.
        /// Returns false if no such unification; if the types can be unified, sets common (unless they unify exactly and have identical type qualifiers)
        bool unifyInexact( Type *type1, Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, const OpenVarSet &openVars, WidenMode widen, const SymTab::Indexer &indexer, Type *&common );
        bool unifyExact( Type *type1, Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, const OpenVarSet &openVars, WidenMode widen, const SymTab::Indexer &indexer );

        bool unifyExact( 
                const ast::Type * type1, const ast::Type * type2, ast::TypeEnvironment & env, 
                ast::AssertionSet & need, ast::AssertionSet & have, const ast::OpenVarSet & open, 
                WidenMode widen, const ast::SymbolTable & symtab );

        bool typesCompatible( Type *first, Type *second, const SymTab::Indexer &indexer, const TypeEnvironment &env ) {
                TypeEnvironment newEnv;
                OpenVarSet openVars, closedVars; // added closedVars
                AssertionSet needAssertions, haveAssertions;
                Type *newFirst = first->clone(), *newSecond = second->clone();
                env.apply( newFirst );
                env.apply( newSecond );

                // do we need to do this? Seems like we do, types should be able to be compatible if they
                // have free variables that can unify
                findOpenVars( newFirst, openVars, closedVars, needAssertions, haveAssertions, false );
                findOpenVars( newSecond, openVars, closedVars, needAssertions, haveAssertions, true );

                bool result = unifyExact( newFirst, newSecond, newEnv, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                delete newFirst;
                delete newSecond;
                return result;
        }

        bool typesCompatible( 
                        const ast::Type * first, const ast::Type * second, const ast::SymbolTable & symtab, 
                        const ast::TypeEnvironment & env ) {
                ast::TypeEnvironment newEnv;
                ast::OpenVarSet open, closed;
                ast::AssertionSet need, have;

                ast::ptr<ast::Type> newFirst{ first }, newSecond{ second };
                env.apply( newFirst );
                env.apply( newSecond );

                findOpenVars( newFirst, open, closed, need, have, FirstClosed );
                findOpenVars( newSecond, open, closed, need, have, FirstOpen );

                return unifyExact( 
                        newFirst, newSecond, newEnv, need, have, open, WidenMode{ false, false }, symtab );
        }

        bool typesCompatibleIgnoreQualifiers( Type *first, Type *second, const SymTab::Indexer &indexer, const TypeEnvironment &env ) {
                TypeEnvironment newEnv;
                OpenVarSet openVars;
                AssertionSet needAssertions, haveAssertions;
                Type *newFirst = first->clone(), *newSecond = second->clone();
                env.apply( newFirst );
                env.apply( newSecond );
                newFirst->get_qualifiers() = Type::Qualifiers();
                newSecond->get_qualifiers() = Type::Qualifiers();
///   std::cerr << "first is ";
///   first->print( std::cerr );
///   std::cerr << std::endl << "second is ";
///   second->print( std::cerr );
///   std::cerr << std::endl << "newFirst is ";
///   newFirst->print( std::cerr );
///   std::cerr << std::endl << "newSecond is ";
///   newSecond->print( std::cerr );
///   std::cerr << std::endl;
                bool result = unifyExact( newFirst, newSecond, newEnv, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                delete newFirst;
                delete newSecond;
                return result;
        }

        bool typesCompatibleIgnoreQualifiers( 
                        const ast::Type * first, const ast::Type * second, const ast::SymbolTable & symtab, 
                        const ast::TypeEnvironment & env ) {
                ast::TypeEnvironment newEnv;
                ast::OpenVarSet open;
                ast::AssertionSet need, have;
                
                ast::ptr<ast::Type> newFirst{ first }, newSecond{ second };
                env.apply( newFirst );
                env.apply( newSecond );
                clear_qualifiers( newFirst );
                clear_qualifiers( newSecond );

                return unifyExact( 
                        newFirst, newSecond, newEnv, need, have, open, WidenMode{ false, false }, symtab );
        }

        bool unify( Type *type1, Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, OpenVarSet &openVars, const SymTab::Indexer &indexer ) {
                OpenVarSet closedVars;
                findOpenVars( type1, openVars, closedVars, needAssertions, haveAssertions, false );
                findOpenVars( type2, openVars, closedVars, needAssertions, haveAssertions, true );
                Type *commonType = 0;
                if ( unifyInexact( type1, type2, env, needAssertions, haveAssertions, openVars, WidenMode( true, true ), indexer, commonType ) ) {
                        if ( commonType ) {
                                delete commonType;
                        } // if
                        return true;
                } else {
                        return false;
                } // if
        }

        bool unify( Type *type1, Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, OpenVarSet &openVars, const SymTab::Indexer &indexer, Type *&commonType ) {
                OpenVarSet closedVars;
                findOpenVars( type1, openVars, closedVars, needAssertions, haveAssertions, false );
                findOpenVars( type2, openVars, closedVars, needAssertions, haveAssertions, true );
                return unifyInexact( type1, type2, env, needAssertions, haveAssertions, openVars, WidenMode( true, true ), indexer, commonType );
        }

        bool unifyExact( Type *type1, Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, const OpenVarSet &openVars, WidenMode widen, const SymTab::Indexer &indexer ) {
#ifdef DEBUG
                TypeEnvironment debugEnv( env );
#endif
                if ( type1->get_qualifiers() != type2->get_qualifiers() ) {
                        return false;
                }

                bool result;
                TypeInstType *var1 = dynamic_cast< TypeInstType* >( type1 );
                TypeInstType *var2 = dynamic_cast< TypeInstType* >( type2 );
                OpenVarSet::const_iterator entry1, entry2;
                if ( var1 ) {
                        entry1 = openVars.find( var1->get_name() );
                } // if
                if ( var2 ) {
                        entry2 = openVars.find( var2->get_name() );
                } // if
                bool isopen1 = var1 && ( entry1 != openVars.end() );
                bool isopen2 = var2 && ( entry2 != openVars.end() );

                if ( isopen1 && isopen2 ) {
                        if ( entry1->second.kind != entry2->second.kind ) {
                                result = false;
                        } else {
                                result = env.bindVarToVar(
                                        var1, var2, TypeDecl::Data{ entry1->second, entry2->second }, needAssertions,
                                        haveAssertions, openVars, widen, indexer );
                        }
                } else if ( isopen1 ) {
                        result = env.bindVar( var1, type2, entry1->second, needAssertions, haveAssertions, openVars, widen, indexer );
                } else if ( isopen2 ) { // TODO: swap widen values in call, since type positions are flipped?
                        result = env.bindVar( var2, type1, entry2->second, needAssertions, haveAssertions, openVars, widen, indexer );
                } else {
                        PassVisitor<Unify_old> comparator( type2, env, needAssertions, haveAssertions, openVars, widen, indexer );
                        type1->accept( comparator );
                        result = comparator.pass.get_result();
                } // if
#ifdef DEBUG
                std::cerr << "============ unifyExact" << std::endl;
                std::cerr << "type1 is ";
                type1->print( std::cerr );
                std::cerr << std::endl << "type2 is ";
                type2->print( std::cerr );
                std::cerr << std::endl << "openVars are ";
                printOpenVarSet( openVars, std::cerr, 8 );
                std::cerr << std::endl << "input env is " << std::endl;
                debugEnv.print( std::cerr, 8 );
                std::cerr << std::endl << "result env is " << std::endl;
                env.print( std::cerr, 8 );
                std::cerr << "result is " << result << std::endl;
#endif
                return result;
        }

        bool unifyExact( Type *type1, Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, OpenVarSet &openVars, const SymTab::Indexer &indexer ) {
                return unifyExact( type1, type2, env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
        }

        bool unifyInexact( Type *type1, Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, const OpenVarSet &openVars, WidenMode widen, const SymTab::Indexer &indexer, Type *&common ) {
                Type::Qualifiers tq1 = type1->get_qualifiers(), tq2 = type2->get_qualifiers();
                type1->get_qualifiers() = Type::Qualifiers();
                type2->get_qualifiers() = Type::Qualifiers();
                bool result;
#ifdef DEBUG
                std::cerr << "unifyInexact type 1 is ";
                type1->print( std::cerr );
                std::cerr << " type 2 is ";
                type2->print( std::cerr );
                std::cerr << std::endl;
#endif
                if ( ! unifyExact( type1, type2, env, needAssertions, haveAssertions, openVars, widen, indexer ) ) {
#ifdef DEBUG
                        std::cerr << "unifyInexact: no exact unification found" << std::endl;
#endif
                        if ( ( common = commonType( type1, type2, widen.first, widen.second, indexer, env, openVars ) ) ) {
                                common->get_qualifiers() = tq1 | tq2;
#ifdef DEBUG
                                std::cerr << "unifyInexact: common type is ";
                                common->print( std::cerr );
                                std::cerr << std::endl;
#endif
                                result = true;
                        } else {
#ifdef DEBUG
                                std::cerr << "unifyInexact: no common type found" << std::endl;
#endif
                                result = false;
                        } // if
                } else {
                        if ( tq1 != tq2 ) {
                                if ( ( tq1 > tq2 || widen.first ) && ( tq2 > tq1 || widen.second ) ) {
                                        common = type1->clone();
                                        common->get_qualifiers() = tq1 | tq2;
                                        result = true;
                                } else {
                                        result = false;
                                } // if
                        } else {
                                common = type1->clone();
                                common->get_qualifiers() = tq1 | tq2;
                                result = true;
                        } // if
                } // if
                type1->get_qualifiers() = tq1;
                type2->get_qualifiers() = tq2;
                return result;
        }

        Unify_old::Unify_old( Type *type2, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, const OpenVarSet &openVars, WidenMode widen, const SymTab::Indexer &indexer )
                : result( false ), type2( type2 ), env( env ), needAssertions( needAssertions ), haveAssertions( haveAssertions ), openVars( openVars ), widen( widen ), indexer( indexer ) {
        }

        void Unify_old::postvisit( __attribute__((unused)) VoidType *voidType) {
                result = dynamic_cast< VoidType* >( type2 );
        }

        void Unify_old::postvisit(BasicType *basicType) {
                if ( BasicType *otherBasic = dynamic_cast< BasicType* >( type2 ) ) {
                        result = basicType->get_kind() == otherBasic->get_kind();
                } // if
        }

        void markAssertionSet( AssertionSet &assertions, DeclarationWithType *assert ) {
///   std::cerr << "assertion set is" << std::endl;
///   printAssertionSet( assertions, std::cerr, 8 );
///   std::cerr << "looking for ";
///   assert->print( std::cerr );
///   std::cerr << std::endl;
                AssertionSet::iterator i = assertions.find( assert );
                if ( i != assertions.end() ) {
///     std::cerr << "found it!" << std::endl;
                        i->second.isUsed = true;
                } // if
        }

        void markAssertions( AssertionSet &assertion1, AssertionSet &assertion2, Type *type ) {
                for ( std::list< TypeDecl* >::const_iterator tyvar = type->get_forall().begin(); tyvar != type->get_forall().end(); ++tyvar ) {
                        for ( std::list< DeclarationWithType* >::const_iterator assert = (*tyvar)->get_assertions().begin(); assert != (*tyvar)->get_assertions().end(); ++assert ) {
                                markAssertionSet( assertion1, *assert );
                                markAssertionSet( assertion2, *assert );
                        } // for
                } // for
        }

        void Unify_old::postvisit(PointerType *pointerType) {
                if ( PointerType *otherPointer = dynamic_cast< PointerType* >( type2 ) ) {
                        result = unifyExact( pointerType->get_base(), otherPointer->get_base(), env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        markAssertions( haveAssertions, needAssertions, pointerType );
                        markAssertions( haveAssertions, needAssertions, otherPointer );
                } // if
        }

        void Unify_old::postvisit(ReferenceType *refType) {
                if ( ReferenceType *otherRef = dynamic_cast< ReferenceType* >( type2 ) ) {
                        result = unifyExact( refType->get_base(), otherRef->get_base(), env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        markAssertions( haveAssertions, needAssertions, refType );
                        markAssertions( haveAssertions, needAssertions, otherRef );
                } // if
        }

        void Unify_old::postvisit(ArrayType *arrayType) {
                ArrayType *otherArray = dynamic_cast< ArrayType* >( type2 );
                // to unify, array types must both be VLA or both not VLA
                // and must both have a dimension expression or not have a dimension
                if ( otherArray && arrayType->get_isVarLen() == otherArray->get_isVarLen() ) {

                        if ( ! arrayType->get_isVarLen() && ! otherArray->get_isVarLen() &&
                                arrayType->get_dimension() != 0 && otherArray->get_dimension() != 0 ) {
                                ConstantExpr * ce1 = dynamic_cast< ConstantExpr * >( arrayType->get_dimension() );
                                ConstantExpr * ce2 = dynamic_cast< ConstantExpr * >( otherArray->get_dimension() );
                                // see C11 Reference Manual 6.7.6.2.6
                                // two array types with size specifiers that are integer constant expressions are
                                // compatible if both size specifiers have the same constant value
                                if ( ce1 && ce2 ) {
                                        Constant * c1 = ce1->get_constant();
                                        Constant * c2 = ce2->get_constant();

                                        if ( c1->get_value() != c2->get_value() ) {
                                                // does not unify if the dimension is different
                                                return;
                                        }
                                }
                        }

                        result = unifyExact( arrayType->get_base(), otherArray->get_base(), env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                } // if
        }

        template< typename Iterator, typename Func >
        std::unique_ptr<Type> combineTypes( Iterator begin, Iterator end, Func & toType ) {
                std::list< Type * > types;
                for ( ; begin != end; ++begin ) {
                        // it's guaranteed that a ttype variable will be bound to a flat tuple, so ensure that this results in a flat tuple
                        flatten( toType( *begin ), back_inserter( types ) );
                }
                return std::unique_ptr<Type>( new TupleType( Type::Qualifiers(), types ) );
        }

        template< typename Iterator1, typename Iterator2 >
        bool unifyDeclList( Iterator1 list1Begin, Iterator1 list1End, Iterator2 list2Begin, Iterator2 list2End, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, const OpenVarSet &openVars, const SymTab::Indexer &indexer ) {
                auto get_type = [](DeclarationWithType * dwt){ return dwt->get_type(); };
                for ( ; list1Begin != list1End && list2Begin != list2End; ++list1Begin, ++list2Begin ) {
                        Type * t1 = (*list1Begin)->get_type();
                        Type * t2 = (*list2Begin)->get_type();
                        bool isTtype1 = Tuples::isTtype( t1 );
                        bool isTtype2 = Tuples::isTtype( t2 );
                        // xxx - assumes ttype must be last parameter
                        // xxx - there may be a nice way to refactor this, but be careful because the argument positioning might matter in some cases.
                        if ( isTtype1 && ! isTtype2 ) {
                                // combine all of the things in list2, then unify
                                return unifyExact( t1, combineTypes( list2Begin, list2End, get_type ).get(), env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        } else if ( isTtype2 && ! isTtype1 ) {
                                // combine all of the things in list1, then unify
                                return unifyExact( combineTypes( list1Begin, list1End, get_type ).get(), t2, env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        } else if ( ! unifyExact( t1, t2, env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer ) ) {
                                return false;
                        } // if
                } // for
                // may get to the end of one argument list before the end of the other. This is only okay when the other is a ttype
                if ( list1Begin != list1End ) {
                        // try unifying empty tuple type with ttype
                        Type * t1 = (*list1Begin)->get_type();
                        if ( Tuples::isTtype( t1 ) ) {
                                return unifyExact( t1, combineTypes( list2Begin, list2End, get_type ).get(), env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        } else return false;
                } else if ( list2Begin != list2End ) {
                        // try unifying empty tuple type with ttype
                        Type * t2 = (*list2Begin)->get_type();
                        if ( Tuples::isTtype( t2 ) ) {
                                return unifyExact( combineTypes( list1Begin, list1End, get_type ).get(), t2, env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        } else return false;
                } else {
                        return true;
                } // if
        }

        /// Finds ttypes and replaces them with their expansion, if known.
        /// This needs to be done so that satisfying ttype assertions is easier.
        /// If this isn't done then argument lists can have wildly different
        /// size and structure, when they should be compatible.
        struct TtypeExpander_old : public WithShortCircuiting {
                TypeEnvironment & tenv;
                TtypeExpander_old( TypeEnvironment & tenv ) : tenv( tenv ) {}
                void premutate( TypeInstType * ) { visit_children = false; }
                Type * postmutate( TypeInstType * typeInst ) {
                        if ( const EqvClass *eqvClass = tenv.lookup( typeInst->get_name() ) ) {
                                // expand ttype parameter into its actual type
                                if ( eqvClass->data.kind == TypeDecl::Ttype && eqvClass->type ) {
                                        delete typeInst;
                                        return eqvClass->type->clone();
                                }
                        }
                        return typeInst;
                }
        };

        /// flattens a list of declarations, so that each tuple type has a single declaration.
        /// makes use of TtypeExpander to ensure ttypes are flat as well.
        void flattenList( std::list< DeclarationWithType * > src, std::list< DeclarationWithType * > & dst, TypeEnvironment & env ) {
                dst.clear();
                for ( DeclarationWithType * dcl : src ) {
                        PassVisitor<TtypeExpander_old> expander( env );
                        dcl->acceptMutator( expander );
                        std::list< Type * > types;
                        flatten( dcl->get_type(), back_inserter( types ) );
                        for ( Type * t : types ) {
                                // outermost const, volatile, _Atomic qualifiers in parameters should not play a role in the unification of function types, since they do not determine whether a function is callable.
                                // Note: MUST consider at least mutex qualifier, since functions can be overloaded on outermost mutex and a mutex function has different requirements than a non-mutex function.
                                t->get_qualifiers() -= Type::Qualifiers(Type::Const | Type::Volatile | Type::Atomic);

                                dst.push_back( new ObjectDecl( "", Type::StorageClasses(), LinkageSpec::C, nullptr, t, nullptr ) );
                        }
                        delete dcl;
                }
        }

        void Unify_old::postvisit(FunctionType *functionType) {
                FunctionType *otherFunction = dynamic_cast< FunctionType* >( type2 );
                if ( otherFunction && functionType->get_isVarArgs() == otherFunction->get_isVarArgs() ) {
                        // flatten the parameter lists for both functions so that tuple structure
                        // doesn't affect unification. Must be a clone so that the types don't change.
                        std::unique_ptr<FunctionType> flatFunc( functionType->clone() );
                        std::unique_ptr<FunctionType> flatOther( otherFunction->clone() );
                        flattenList( flatFunc->get_parameters(), flatFunc->get_parameters(), env );
                        flattenList( flatOther->get_parameters(), flatOther->get_parameters(), env );

                        // sizes don't have to match if ttypes are involved; need to be more precise wrt where the ttype is to prevent errors
                        if ( 
                                        (flatFunc->parameters.size() == flatOther->parameters.size() && 
                                                flatFunc->returnVals.size() == flatOther->returnVals.size()) 
                                        || flatFunc->isTtype() 
                                        || flatOther->isTtype() 
                        ) {
                                if ( unifyDeclList( flatFunc->parameters.begin(), flatFunc->parameters.end(), flatOther->parameters.begin(), flatOther->parameters.end(), env, needAssertions, haveAssertions, openVars, indexer ) ) {
                                        if ( unifyDeclList( flatFunc->returnVals.begin(), flatFunc->returnVals.end(), flatOther->returnVals.begin(), flatOther->returnVals.end(), env, needAssertions, haveAssertions, openVars, indexer ) ) {

                                                // the original types must be used in mark assertions, since pointer comparisons are used
                                                markAssertions( haveAssertions, needAssertions, functionType );
                                                markAssertions( haveAssertions, needAssertions, otherFunction );

                                                result = true;
                                        } // if
                                } // if
                        } // if
                } // if
        }

        template< typename RefType >
        void Unify_old::handleRefType( RefType *inst, Type *other ) {
                // check that other type is compatible and named the same
                RefType *otherStruct = dynamic_cast< RefType* >( other );
                result = otherStruct && inst->name == otherStruct->name;
        }

        template< typename RefType >
        void Unify_old::handleGenericRefType( RefType *inst, Type *other ) {
                // Check that other type is compatible and named the same
                handleRefType( inst, other );
                if ( ! result ) return;
                // Check that parameters of types unify, if any
                std::list< Expression* > params = inst->parameters;
                std::list< Expression* > otherParams = ((RefType*)other)->parameters;

                std::list< Expression* >::const_iterator it = params.begin(), jt = otherParams.begin();
                for ( ; it != params.end() && jt != otherParams.end(); ++it, ++jt ) {
                        TypeExpr *param = dynamic_cast< TypeExpr* >(*it);
                        assertf(param, "Aggregate parameters should be type expressions");
                        TypeExpr *otherParam = dynamic_cast< TypeExpr* >(*jt);
                        assertf(otherParam, "Aggregate parameters should be type expressions");

                        Type* paramTy = param->get_type();
                        Type* otherParamTy = otherParam->get_type();

                        bool tupleParam = Tuples::isTtype( paramTy );
                        bool otherTupleParam = Tuples::isTtype( otherParamTy );

                        if ( tupleParam && otherTupleParam ) {
                                ++it; ++jt;  // skip ttype parameters for break
                        } else if ( tupleParam ) {
                                // bundle other parameters into tuple to match
                                std::list< Type * > binderTypes;

                                do {
                                        binderTypes.push_back( otherParam->get_type()->clone() );
                                        ++jt;

                                        if ( jt == otherParams.end() ) break;

                                        otherParam = dynamic_cast< TypeExpr* >(*jt);
                                        assertf(otherParam, "Aggregate parameters should be type expressions");
                                } while (true);

                                otherParamTy = new TupleType{ paramTy->get_qualifiers(), binderTypes };
                                ++it;  // skip ttype parameter for break
                        } else if ( otherTupleParam ) {
                                // bundle parameters into tuple to match other
                                std::list< Type * > binderTypes;

                                do {
                                        binderTypes.push_back( param->get_type()->clone() );
                                        ++it;

                                        if ( it == params.end() ) break;

                                        param = dynamic_cast< TypeExpr* >(*it);
                                        assertf(param, "Aggregate parameters should be type expressions");
                                } while (true);

                                paramTy = new TupleType{ otherParamTy->get_qualifiers(), binderTypes };
                                ++jt;  // skip ttype parameter for break
                        }

                        if ( ! unifyExact( paramTy, otherParamTy, env, needAssertions, haveAssertions, openVars, WidenMode(false, false), indexer ) ) {
                                result = false;
                                return;
                        }

                        // ttype parameter should be last
                        if ( tupleParam || otherTupleParam ) break;
                }
                result = ( it == params.end() && jt == otherParams.end() );
        }

        void Unify_old::postvisit(StructInstType *structInst) {
                handleGenericRefType( structInst, type2 );
        }

        void Unify_old::postvisit(UnionInstType *unionInst) {
                handleGenericRefType( unionInst, type2 );
        }

        void Unify_old::postvisit(EnumInstType *enumInst) {
                handleRefType( enumInst, type2 );
        }

        void Unify_old::postvisit(TraitInstType *contextInst) {
                handleRefType( contextInst, type2 );
        }

        void Unify_old::postvisit(TypeInstType *typeInst) {
                assert( openVars.find( typeInst->get_name() ) == openVars.end() );
                TypeInstType *otherInst = dynamic_cast< TypeInstType* >( type2 );
                if ( otherInst && typeInst->get_name() == otherInst->get_name() ) {
                        result = true;
///   } else {
///     NamedTypeDecl *nt = indexer.lookupType( typeInst->get_name() );
///     if ( nt ) {
///       TypeDecl *type = dynamic_cast< TypeDecl* >( nt );
///       assert( type );
///       if ( type->get_base() ) {
///         result = unifyExact( type->get_base(), typeInst, env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
///       }
///     }
                } // if
        }

        template< typename Iterator1, typename Iterator2 >
        bool unifyList( Iterator1 list1Begin, Iterator1 list1End, Iterator2 list2Begin, Iterator2 list2End, TypeEnvironment &env, AssertionSet &needAssertions, AssertionSet &haveAssertions, const OpenVarSet &openVars, const SymTab::Indexer &indexer ) {
                auto get_type = [](Type * t) { return t; };
                for ( ; list1Begin != list1End && list2Begin != list2End; ++list1Begin, ++list2Begin ) {
                        Type * t1 = *list1Begin;
                        Type * t2 = *list2Begin;
                        bool isTtype1 = Tuples::isTtype( t1 );
                        bool isTtype2 = Tuples::isTtype( t2 );
                        // xxx - assumes ttype must be last parameter
                        // xxx - there may be a nice way to refactor this, but be careful because the argument positioning might matter in some cases.
                        if ( isTtype1 && ! isTtype2 ) {
                                // combine all of the things in list2, then unify
                                return unifyExact( t1, combineTypes( list2Begin, list2End, get_type ).get(), env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        } else if ( isTtype2 && ! isTtype1 ) {
                                // combine all of the things in list1, then unify
                                return unifyExact( combineTypes( list1Begin, list1End, get_type ).get(), t2, env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        } else if ( ! unifyExact( t1, t2, env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer ) ) {
                                return false;
                        } // if

                } // for
                if ( list1Begin != list1End ) {
                        // try unifying empty tuple type with ttype
                        Type * t1 = *list1Begin;
                        if ( Tuples::isTtype( t1 ) ) {
                                return unifyExact( t1, combineTypes( list2Begin, list2End, get_type ).get(), env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        } else return false;
                } else if ( list2Begin != list2End ) {
                        // try unifying empty tuple type with ttype
                        Type * t2 = *list2Begin;
                        if ( Tuples::isTtype( t2 ) ) {
                                return unifyExact( combineTypes( list1Begin, list1End, get_type ).get(), t2, env, needAssertions, haveAssertions, openVars, WidenMode( false, false ), indexer );
                        } else return false;
                } else {
                        return true;
                } // if
        }

        void Unify_old::postvisit(TupleType *tupleType) {
                if ( TupleType *otherTuple = dynamic_cast< TupleType* >( type2 ) ) {
                        std::unique_ptr<TupleType> flat1( tupleType->clone() );
                        std::unique_ptr<TupleType> flat2( otherTuple->clone() );
                        std::list<Type *> types1, types2;

                        PassVisitor<TtypeExpander_old> expander( env );
                        flat1->acceptMutator( expander );
                        flat2->acceptMutator( expander );

                        flatten( flat1.get(), back_inserter( types1 ) );
                        flatten( flat2.get(), back_inserter( types2 ) );

                        result = unifyList( types1.begin(), types1.end(), types2.begin(), types2.end(), env, needAssertions, haveAssertions, openVars, indexer );
                } // if
        }

        void Unify_old::postvisit( __attribute__((unused)) VarArgsType *varArgsType ) {
                result = dynamic_cast< VarArgsType* >( type2 );
        }

        void Unify_old::postvisit( __attribute__((unused)) ZeroType *zeroType ) {
                result = dynamic_cast< ZeroType* >( type2 );
        }

        void Unify_old::postvisit( __attribute__((unused)) OneType *oneType ) {
                result = dynamic_cast< OneType* >( type2 );
        }

        Type * extractResultType( FunctionType * function ) {
                if ( function->get_returnVals().size() == 0 ) {
                        return new VoidType( Type::Qualifiers() );
                } else if ( function->get_returnVals().size() == 1 ) {
                        return function->get_returnVals().front()->get_type()->clone();
                } else {
                        std::list< Type * > types;
                        for ( DeclarationWithType * decl : function->get_returnVals() ) {
                                types.push_back( decl->get_type()->clone() );
                        } // for
                        return new TupleType( Type::Qualifiers(), types );
                }
        }

        class Unify_new : public ast::WithShortCircuiting {
                const ast::Type * type2;
                ast::TypeEnvironment & tenv;
                ast::AssertionSet & need;
                ast::AssertionSet & have;
                const ast::OpenVarSet & open;
                WidenMode widen;
                const ast::SymbolTable & symtab;
        public:
                bool result;

                Unify_new( 
                        const ast::Type * type2, ast::TypeEnvironment & env, ast::AssertionSet & need, 
                        ast::AssertionSet & have, const ast::OpenVarSet & open, WidenMode widen, 
                        const ast::SymbolTable & symtab )
                : type2(type2), tenv(env), need(need), have(have), open(open), widen(widen), 
                  symtab(symtab), result(false) {}

                void previsit( const ast::Node * ) { visit_children = false; }
                
                void previsit( const ast::VoidType * ) {
                        visit_children = false;
                        result = dynamic_cast< const ast::VoidType * >( type2 );
                }

                void previsit( const ast::BasicType * basic ) {
                        visit_children = false;
                        if ( auto basic2 = dynamic_cast< const ast::BasicType * >( type2 ) ) {
                                result = basic->kind == basic2->kind;
                        }
                }

                void previsit( const ast::PointerType * pointer ) {
                        visit_children = false;
                        if ( auto pointer2 = dynamic_cast< const ast::PointerType * >( type2 ) ) {
                                result = unifyExact( 
                                        pointer->base, pointer2->base, tenv, need, have, open, 
                                        WidenMode{ false, false }, symtab );
                        }
                }

                void previsit( const ast::ArrayType * array ) {
                        visit_children = false;
                        auto array2 = dynamic_cast< const ast::ArrayType * >( type2 );
                        if ( ! array2 ) return;

                        // to unify, array types must both be VLA or both not VLA and both must have a 
                        // dimension expression or not have a dimension
                        if ( array->isVarLen != array2->isVarLen ) return;
                        if ( ! array->isVarLen && ! array2->isVarLen 
                                        && array->dimension && array2->dimension ) {
                                auto ce1 = array->dimension.as< ast::ConstantExpr >();
                                auto ce2 = array2->dimension.as< ast::ConstantExpr >();

                                // see C11 Reference Manual 6.7.6.2.6
                                // two array types with size specifiers that are integer constant expressions are 
                                // compatible if both size specifiers have the same constant value
                                if ( ce1 && ce2 && ce1->intValue() != ce2->intValue() ) return;
                        }

                        result = unifyExact( 
                                array->base, array2->base, tenv, need, have, open, WidenMode{ false, false }, 
                                symtab );
                }

                void previsit( const ast::ReferenceType * ref ) {
                        visit_children = false;
                        if ( auto ref2 = dynamic_cast< const ast::ReferenceType * >( type2 ) ) {
                                result = unifyExact( 
                                        ref->base, ref2->base, tenv, need, have, open, WidenMode{ false, false }, 
                                        symtab );
                        }
                }

        private:
                /// Replaces ttype variables with their bound types.
                /// If this isn't done when satifying ttype assertions, then argument lists can have 
                /// different size and structure when they should be compatible.
                struct TtypeExpander_new : public ast::WithShortCircuiting {
                        ast::TypeEnvironment & tenv;

                        TtypeExpander_new( ast::TypeEnvironment & env ) : tenv( env ) {}

                        const ast::Type * postmutate( const ast::TypeInstType * typeInst ) {
                                if ( const ast::EqvClass * clz = tenv.lookup( typeInst->name ) ) {
                                        // expand ttype parameter into its actual type
                                        if ( clz->data.kind == ast::TypeVar::Ttype && clz->bound ) {
                                                return clz->bound;
                                        }
                                }
                                return typeInst;
                        }
                };

                /// returns flattened version of `src`
                static std::vector< ast::ptr< ast::DeclWithType > > flattenList(
                        const std::vector< ast::ptr< ast::DeclWithType > > & src, ast::TypeEnvironment & env
                ) {
                        std::vector< ast::ptr< ast::DeclWithType > > dst;
                        dst.reserve( src.size() );
                        for ( const ast::DeclWithType * d : src ) {
                                ast::Pass<TtypeExpander_new> expander{ env };
                                d = d->accept( expander );
                                auto types = flatten( d->get_type() );
                                for ( ast::ptr< ast::Type > & t : types ) {
                                        // outermost const, volatile, _Atomic qualifiers in parameters should not play 
                                        // a role in the unification of function types, since they do not determine 
                                        // whether a function is callable.
                                        // NOTE: **must** consider at least mutex qualifier, since functions can be 
                                        // overloaded on outermost mutex and a mutex function has different 
                                        // requirements than a non-mutex function
                                        t.get_and_mutate()->qualifiers 
                                                -= ast::CV::Const | ast::CV::Volatile | ast::CV::Atomic;
                                        dst.emplace_back( new ast::ObjectDecl{ d->location, "", t } );
                                }
                        }
                        return dst;
                }

                /// Creates a tuple type based on a list of DeclWithType
                template< typename Iter >
                static ast::ptr< ast::Type > tupleFromDecls( Iter crnt, Iter end ) {
                        std::vector< ast::ptr< ast::Type > > types;
                        while ( crnt != end ) {
                                // it is guaranteed that a ttype variable will be bound to a flat tuple, so ensure 
                                // that this results in a flat tuple
                                flatten( (*crnt)->get_type(), types );

                                ++crnt;
                        }

                        return { new ast::TupleType{ std::move(types) } };
                }

                template< typename Iter >
                static bool unifyDeclList( 
                        Iter crnt1, Iter end1, Iter crnt2, Iter end2, ast::TypeEnvironment & env, 
                        ast::AssertionSet & need, ast::AssertionSet & have, const ast::OpenVarSet & open, 
                        const ast::SymbolTable & symtab
                ) {
                        while ( crnt1 != end1 && crnt2 != end2 ) {
                                const ast::Type * t1 = (*crnt1)->get_type();
                                const ast::Type * t2 = (*crnt2)->get_type();
                                bool isTuple1 = Tuples::isTtype( t1 );
                                bool isTuple2 = Tuples::isTtype( t2 );

                                // assumes here that ttype *must* be last parameter
                                if ( isTuple1 && ! isTuple2 ) {
                                        // combine remainder of list2, then unify
                                        return unifyExact( 
                                                t1, tupleFromDecls( crnt2, end2 ), env, need, have, open, 
                                                WidenMode{ false, false }, symtab );
                                } else if ( ! isTuple1 && isTuple2 ) {
                                        // combine remainder of list1, then unify
                                        return unifyExact( 
                                                tupleFromDecls( crnt1, end1 ), t2, env, need, have, open, 
                                                WidenMode{ false, false }, symtab );
                                }

                                if ( ! unifyExact( 
                                        t1, t2, env, need, have, open, WidenMode{ false, false }, symtab ) 
                                ) return false;

                                ++crnt1; ++crnt2;
                        }

                        // May get to the end of one argument list before the other. This is only okay if the 
                        // other is a ttype
                        if ( crnt1 != end1 ) {
                                // try unifying empty tuple with ttype
                                const ast::Type * t1 = (*crnt1)->get_type();
                                if ( ! Tuples::isTtype( t1 ) ) return false;
                                return unifyExact( 
                                        t1, tupleFromDecls( crnt2, end2 ), env, need, have, open, 
                                        WidenMode{ false, false }, symtab );
                        } else if ( crnt2 != end2 ) {
                                // try unifying empty tuple with ttype
                                const ast::Type * t2 = (*crnt2)->get_type();
                                if ( ! Tuples::isTtype( t2 ) ) return false;
                                return unifyExact( 
                                        tupleFromDecls( crnt1, end1 ), t2, env, need, have, open, 
                                        WidenMode{ false, false }, symtab );
                        }

                        return true;
                }

                static bool unifyDeclList( 
                        const std::vector< ast::ptr< ast::DeclWithType > > & list1, 
                        const std::vector< ast::ptr< ast::DeclWithType > > & list2, 
                        ast::TypeEnvironment & env, ast::AssertionSet & need, ast::AssertionSet & have, 
                        const ast::OpenVarSet & open, const ast::SymbolTable & symtab
                ) {
                        return unifyDeclList( 
                                list1.begin(), list1.end(), list2.begin(), list2.end(), env, need, have, open, 
                                symtab );
                }

                static void markAssertionSet( ast::AssertionSet & assns, const ast::DeclWithType * assn ) {
                        auto i = assns.find( assn );
                        if ( i != assns.end() ) {
                                i->second.isUsed = true;
                        }
                }

                /// mark all assertions in `type` used in both `assn1` and `assn2`
                static void markAssertions( 
                        ast::AssertionSet & assn1, ast::AssertionSet & assn2, 
                        const ast::ParameterizedType * type 
                ) {
                        for ( const auto & tyvar : type->forall ) {
                                for ( const ast::DeclWithType * assert : tyvar->assertions ) {
                                        markAssertionSet( assn1, assert );
                                        markAssertionSet( assn2, assert );
                                }
                        }
                }

        public:
                void previsit( const ast::FunctionType * func ) {
                        visit_children = false;
                        auto func2 = dynamic_cast< const ast::FunctionType * >( type2 );
                        if ( ! func2 ) return;

                        if ( func->isVarArgs != func2->isVarArgs ) return;
                        
                        // Flatten the parameter lists for both functions so that tuple structure does not 
                        // affect unification. Does not actually mutate function parameters.
                        auto params = flattenList( func->params, tenv );
                        auto params2 = flattenList( func2->params, tenv );

                        // sizes don't have to match if ttypes are involved; need to be more precise w.r.t. 
                        // where the ttype is to prevent errors
                        if ( 
                                ( params.size() != params2.size() || func->returns.size() != func2->returns.size() )
                                && ! func->isTtype()
                                && ! func2->isTtype()
                        ) return;

                        if ( ! unifyDeclList( params, params2, tenv, need, have, open, symtab ) ) return;
                        if ( ! unifyDeclList( 
                                func->returns, func2->returns, tenv, need, have, open, symtab ) ) return;
                        
                        markAssertions( have, need, func );
                        markAssertions( have, need, func2 );

                        result = true;
                }
        
        private:
                template< typename RefType >
                const RefType * handleRefType( const RefType * inst, const ast::Type * other ) {
                        visit_children = false;
                        // check that the other type is compatible and named the same
                        auto otherInst = dynamic_cast< const RefType * >( other );
                        result = otherInst && inst->name == otherInst->name;
                        return otherInst;
                }

                /// Creates a tuple type based on a list of TypeExpr
                template< typename Iter >
                static const ast::Type * tupleFromExprs( 
                        const ast::TypeExpr * param, Iter & crnt, Iter end, ast::CV::Qualifiers qs
                ) {
                        std::vector< ast::ptr< ast::Type > > types;
                        do {
                                types.emplace_back( param->type );

                                ++crnt;
                                if ( crnt == end ) break;
                                param = strict_dynamic_cast< const ast::TypeExpr * >( crnt->get() );
                        } while(true);

                        return new ast::TupleType{ std::move(types), qs };
                }

                template< typename RefType >
                void handleGenericRefType( const RefType * inst, const ast::Type * other ) {
                        // check that other type is compatible and named the same
                        const RefType * inst2 = handleRefType( inst, other );
                        if ( ! inst2 ) return;
                        
                        // check that parameters of types unify, if any
                        const std::vector< ast::ptr< ast::Expr > > & params = inst->params;
                        const std::vector< ast::ptr< ast::Expr > > & params2 = inst2->params;

                        auto it = params.begin();
                        auto jt = params2.begin();
                        for ( ; it != params.end() && jt != params2.end(); ++it, ++jt ) {
                                auto param = strict_dynamic_cast< const ast::TypeExpr * >( it->get() );
                                auto param2 = strict_dynamic_cast< const ast::TypeExpr * >( jt->get() );

                                ast::ptr< ast::Type > pty = param->type;
                                ast::ptr< ast::Type > pty2 = param2->type;

                                bool isTuple = Tuples::isTtype( pty );
                                bool isTuple2 = Tuples::isTtype( pty2 );

                                if ( isTuple && isTuple2 ) {
                                        ++it; ++jt;  // skip ttype parameters before break
                                } else if ( isTuple ) {
                                        // bundle remaining params into tuple
                                        pty2 = tupleFromExprs( param2, jt, params2.end(), pty->qualifiers );
                                        ++it;  // skip ttype parameter for break
                                } else if ( isTuple2 ) {
                                        // bundle remaining params into tuple
                                        pty = tupleFromExprs( param, it, params.end(), pty2->qualifiers );
                                        ++jt;  // skip ttype parameter for break
                                }

                                if ( ! unifyExact( 
                                                pty, pty2, tenv, need, have, open, WidenMode{ false, false }, symtab ) ) {
                                        result = false;
                                        return;
                                }

                                // ttype parameter should be last
                                if ( isTuple || isTuple2 ) break;
                        }
                        result = it == params.end() && jt == params2.end();
                }

        public:
                void previsit( const ast::StructInstType * aggrType ) {
                        handleGenericRefType( aggrType, type2 );
                }

                void previsit( const ast::UnionInstType * aggrType ) {
                        handleGenericRefType( aggrType, type2 );
                }

                void previsit( const ast::EnumInstType * aggrType ) {
                        handleRefType( aggrType, type2 );
                }

                void previsit( const ast::TraitInstType * aggrType ) {
                        handleRefType( aggrType, type2 );
                }

                void previsit( const ast::TypeInstType * typeInst ) {
                        assert( open.find( typeInst->name ) == open.end() );
                        handleRefType( typeInst, type2 );
                }

        private:
                /// Creates a tuple type based on a list of Type
                static ast::ptr< ast::Type > tupleFromTypes( 
                        const std::vector< ast::ptr< ast::Type > > & tys
                ) {
                        std::vector< ast::ptr< ast::Type > > out;
                        for ( const ast::Type * ty : tys ) {
                                // it is guaranteed that a ttype variable will be bound to a flat tuple, so ensure 
                                // that this results in a flat tuple
                                flatten( ty, out );
                        }

                        return { new ast::TupleType{ std::move(out) } };
                }

                static bool unifyList( 
                        const std::vector< ast::ptr< ast::Type > > & list1, 
                        const std::vector< ast::ptr< ast::Type > > & list2, ast::TypeEnvironment & env, 
                        ast::AssertionSet & need, ast::AssertionSet & have, const ast::OpenVarSet & open, 
                        const ast::SymbolTable & symtab
                ) {
                        auto crnt1 = list1.begin();
                        auto crnt2 = list2.begin();
                        while ( crnt1 != list1.end() && crnt2 != list2.end() ) {
                                const ast::Type * t1 = *crnt1;
                                const ast::Type * t2 = *crnt2;
                                bool isTuple1 = Tuples::isTtype( t1 );
                                bool isTuple2 = Tuples::isTtype( t2 );

                                // assumes ttype must be last parameter
                                if ( isTuple1 && ! isTuple2 ) {
                                        // combine entirety of list2, then unify
                                        return unifyExact( 
                                                t1, tupleFromTypes( list2 ), env, need, have, open, 
                                                WidenMode{ false, false }, symtab );
                                } else if ( ! isTuple1 && isTuple2 ) {
                                        // combine entirety of list1, then unify
                                        return unifyExact(
                                                tupleFromTypes( list1 ), t2, env, need, have, open, 
                                                WidenMode{ false, false }, symtab );
                                }

                                if ( ! unifyExact( 
                                        t1, t2, env, need, have, open, WidenMode{ false, false }, symtab ) 
                                ) return false;

                                ++crnt1; ++crnt2;
                        }

                        if ( crnt1 != list1.end() ) {
                                // try unifying empty tuple type with ttype
                                const ast::Type * t1 = *crnt1;
                                if ( ! Tuples::isTtype( t1 ) ) return false;
                                // xxx - this doesn't generate an empty tuple, contrary to comment; both ported 
                                // from Rob's code
                                return unifyExact( 
                                                t1, tupleFromTypes( list2 ), env, need, have, open, 
                                                WidenMode{ false, false }, symtab );
                        } else if ( crnt2 != list2.end() ) {
                                // try unifying empty tuple with ttype
                                const ast::Type * t2 = *crnt2;
                                if ( ! Tuples::isTtype( t2 ) ) return false;
                                // xxx - this doesn't generate an empty tuple, contrary to comment; both ported 
                                // from Rob's code
                                return unifyExact(
                                                tupleFromTypes( list1 ), t2, env, need, have, open, 
                                                WidenMode{ false, false }, symtab );
                        }

                        return true;
                }

        public:
                void previsit( const ast::TupleType * tuple ) {
                        visit_children = false;
                        auto tuple2 = dynamic_cast< const ast::TupleType * >( type2 );
                        if ( ! tuple2 ) return;

                        ast::Pass<TtypeExpander_new> expander{ tenv };
                        const ast::Type * flat = tuple->accept( expander );
                        const ast::Type * flat2 = tuple2->accept( expander );

                        auto types = flatten( flat );
                        auto types2 = flatten( flat2 );

                        result = unifyList( types, types2, tenv, need, have, open, symtab );
                }

                void previsit( const ast::VarArgsType * ) {
                        visit_children = false;
                        result = dynamic_cast< const ast::VarArgsType * >( type2 );
                }

                void previsit( const ast::ZeroType * ) {
                        visit_children = false;
                        result = dynamic_cast< const ast::ZeroType * >( type2 );
                }

                void previsit( const ast::OneType * ) {
                        visit_children = false;
                        result = dynamic_cast< const ast::OneType * >( type2 );
                }       

          private:
                template< typename RefType > void handleRefType( RefType *inst, Type *other );
                template< typename RefType > void handleGenericRefType( RefType *inst, Type *other );
        };

        bool unifyExact( 
                        const ast::Type * type1, const ast::Type * type2, ast::TypeEnvironment & env, 
                        ast::AssertionSet & need, ast::AssertionSet & have, const ast::OpenVarSet & open, 
                        WidenMode widen, const ast::SymbolTable & symtab
        ) {
                if ( type1->qualifiers != type2->qualifiers ) return false;

                auto var1 = dynamic_cast< const ast::TypeInstType * >( type1 );
                auto var2 = dynamic_cast< const ast::TypeInstType * >( type2 );
                ast::OpenVarSet::const_iterator 
                        entry1 = var1 ? open.find( var1->name ) : open.end(), 
                        entry2 = var2 ? open.find( var2->name ) : open.end();
                bool isopen1 = entry1 != open.end();
                bool isopen2 = entry2 != open.end();

                if ( isopen1 && isopen2 ) {
                        if ( entry1->second.kind != entry2->second.kind ) return false;
                        return env.bindVarToVar( 
                                var1, var2, ast::TypeDecl::Data{ entry1->second, entry2->second }, need, have, 
                                open, widen, symtab );
                } else if ( isopen1 ) {
                        return env.bindVar( var1, type2, entry1->second, need, have, open, widen, symtab );
                } else if ( isopen2 ) {
                        return env.bindVar( var2, type1, entry2->second, need, have, open, widen, symtab );
                } else {
                        ast::Pass<Unify_new> comparator{ type2, env, need, have, open, widen, symtab };
                        type1->accept( comparator );
                        return comparator.pass.result;
                }
        }

        bool unifyInexact( 
                        ast::ptr<ast::Type> & type1, ast::ptr<ast::Type> & type2, ast::TypeEnvironment & env, 
                        ast::AssertionSet & need, ast::AssertionSet & have, const ast::OpenVarSet & open, 
                        WidenMode widen, const ast::SymbolTable & symtab, ast::ptr<ast::Type> & common 
        ) {
                ast::CV::Qualifiers q1 = type1->qualifiers, q2 = type2->qualifiers;
                
                // force t1 and t2 to be cloned if their qualifiers must be stripped, so that type1 and 
                // type2 are left unchanged; calling convention forces type{1,2}->strong_ref >= 1
                ast::ptr<ast::Type> t1{ type1 }, t2{ type2 };
                clear_qualifiers( t1 );
                clear_qualifiers( t2 );
                
                if ( unifyExact( t1, t2, env, need, have, open, widen, symtab ) ) {
                        t1 = nullptr; t2 = nullptr; // release t1, t2 to avoid spurious clones

                        // if exact unification on unqualified types, try to merge qualifiers
                        if ( q1 == q2 || ( ( q1 > q2 || widen.first ) && ( q2 > q1 || widen.second ) ) ) {
                                common.set_and_mutate( type1 )->qualifiers = q1 | q2;
                                return true;
                        } else {
                                return false;
                        }

                } else if (( common = commonType( t1, t2, widen, symtab, env, open ) )) {
                        t1 = nullptr; t2 = nullptr; // release t1, t2 to avoid spurious clones

                        // no exact unification, but common type
                        common.get_and_mutate()->qualifiers = q1 | q2;
                        return true;
                } else {
                        return false;
                }
        }

        ast::ptr<ast::Type> extractResultType( const ast::FunctionType * func ) {
                if ( func->returns.empty() ) return new ast::VoidType{};
                if ( func->returns.size() == 1 ) return func->returns[0]->get_type();

                std::vector<ast::ptr<ast::Type>> tys;
                for ( const ast::DeclWithType * decl : func->returns ) {
                        tys.emplace_back( decl->get_type() );
                }
                return new ast::TupleType{ std::move(tys) };
        }
} // namespace ResolvExpr

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