source: src/ResolvExpr/Unify.cpp@ 174a11a

//
// 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.cpp --
//
// Author           : Richard C. Bilson
// Created On       : Sun May 17 12:27:10 2015
// Last Modified By : Peter A. Buhr
// Last Modified On : Fri Dec 13 23:43:05 2019
// Update Count     : 46
//

#include "Unify.hpp"

#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/Copy.hpp"
#include "AST/Decl.hpp"
#include "AST/Node.hpp"
#include "AST/Pass.hpp"
#include "AST/Print.hpp"
#include "AST/Type.hpp"
#include "AST/TypeEnvironment.hpp"
#include "Common/Eval.hpp"          // for eval
#include "CommonType.hpp"           // for commonType
#include "FindOpenVars.hpp"         // for findOpenVars
#include "SpecCost.hpp"             // for SpecCost
#include "Tuples/Tuples.hpp"        // for isTtype
#include "Typeops.hpp"              // for flatten, occurs

namespace ast {
        class SymbolTable;
}

// #define DEBUG

namespace ResolvExpr {

bool typesCompatible(
                const ast::Type * first, const ast::Type * second,
                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, newEnv, FirstOpen );

        return unifyExact(newFirst, newSecond, newEnv, need, have, open, noWiden() );
}

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

        ast::Type * newFirst  = shallowCopy( first  );
        ast::Type * newSecond = shallowCopy( second );

        newFirst ->qualifiers = {};
        newSecond->qualifiers = {};
        ast::ptr< ast::Type > t1_(newFirst );
        ast::ptr< ast::Type > t2_(newSecond);

        ast::ptr< ast::Type > subFirst = env.apply(newFirst).node;
        ast::ptr< ast::Type > subSecond = env.apply(newSecond).node;

        return unifyExact(
                subFirst,
                subSecond,
                newEnv, need, have, open, noWiden() );
}

namespace {
        /// 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 : public ast::WithShortCircuiting, public ast::PureVisitor {
                ast::TypeEnvironment & tenv;

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

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

std::vector< ast::ptr< ast::Type > > flattenList(
        const std::vector< ast::ptr< ast::Type > > & src, ast::TypeEnvironment & env
) {
        std::vector< ast::ptr< ast::Type > > dst;
        dst.reserve( src.size() );
        for ( const auto & d : src ) {
                ast::Pass<TtypeExpander> expander( env );
                // TtypeExpander pass is impure (may mutate nodes in place)
                // need to make nodes shared to prevent accidental mutation
                ast::ptr<ast::Type> dc = d->accept(expander);
                auto types = flatten( dc );
                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
                        remove_qualifiers( t, ast::CV::Const | ast::CV::Volatile | ast::CV::Atomic );
                        dst.emplace_back( t );
                }
        }
        return dst;
}

// Unification of Expressions
//
// Boolean outcome (obvious):  Are they basically spelled the same?
// Side effect of binding variables (subtle):  if `sizeof(int)` ===_expr `sizeof(T)` then `int` ===_ty `T`
//
// Context:  if `float[VAREXPR1]` ===_ty `float[VAREXPR2]` then `VAREXPR1` ===_expr `VAREXPR2`
// where the VAREXPR are meant as notational metavariables representing the fact that unification always
// sees distinct ast::VariableExpr objects at these positions

static bool unify( const ast::Expr * e1, const ast::Expr * e2, ast::TypeEnvironment & env,
        ast::AssertionSet & need, ast::AssertionSet & have, const ast::OpenVarSet & open,
        WidenMode widen );

class UnifyExpr final : public ast::WithShortCircuiting {
        const ast::Expr * e2;
        ast::TypeEnvironment & tenv;
        ast::AssertionSet & need;
        ast::AssertionSet & have;
        const ast::OpenVarSet & open;
        WidenMode widen;
public:
        bool result;

private:

        void tryMatchOnStaticValue( const ast::Expr * e1 ) {
                Evaluation r1 = eval(e1);
                Evaluation r2 = eval(e2);

                if ( !r1.hasKnownValue ) return;
                if ( !r2.hasKnownValue ) return;

                if ( r1.knownValue != r2.knownValue ) return;

                visit_children = false;
                result = true;
        }

public:

        void previsit( const ast::Node * ) { assert(false); }

        void previsit( const ast::Expr * e1 ) {
                tryMatchOnStaticValue( e1 );
                visit_children = false;
        }

        void previsit( const ast::CastExpr * e1 ) {
                tryMatchOnStaticValue( e1 );

                if ( result ) {
                        assert( visit_children == false );
                } else {
                        assert( visit_children == true );
                        visit_children = false;

                        auto e2c = dynamic_cast< const ast::CastExpr * >( e2 );
                        if ( !e2c ) return;

                        // inspect casts' target types
                        if ( !unifyExact(
                                e1->result, e2c->result, tenv, need, have, open, widen ) ) return;

                        // inspect casts' inner expressions
                        result = unify( e1->arg, e2c->arg, tenv, need, have, open, widen );
                }
        }

        void previsit( const ast::VariableExpr * e1 ) {
                tryMatchOnStaticValue( e1 );

                if ( result ) {
                        assert( visit_children == false );
                } else {
                        assert( visit_children == true );
                        visit_children = false;

                        auto e2v = dynamic_cast< const ast::VariableExpr * >( e2 );
                        if ( !e2v ) return;

                        assert(e1->var);
                        assert(e2v->var);

                        // conservative: variable exprs match if their declarations are represented by the same C++ AST object
                        result = (e1->var == e2v->var);
                }
        }

        void previsit( const ast::SizeofExpr * e1 ) {
                tryMatchOnStaticValue( e1 );

                if ( result ) {
                        assert( visit_children == false );
                } else {
                        assert( visit_children == true );
                        visit_children = false;

                        auto e2so = dynamic_cast< const ast::SizeofExpr * >( e2 );
                        if ( !e2so ) return;

                        // expression unification calls type unification (mutual recursion)
                        result = unifyExact( e1->type, e2so->type, tenv, need, have, open, widen );
                }
        }

        UnifyExpr( const ast::Expr * e2, ast::TypeEnvironment & env, ast::AssertionSet & need,
                ast::AssertionSet & have, const ast::OpenVarSet & open, WidenMode widen )
        : e2( e2 ), tenv(env), need(need), have(have), open(open), widen(widen), result(false) {}
};

static bool unify( const ast::Expr * e1, const ast::Expr * e2, ast::TypeEnvironment & env,
        ast::AssertionSet & need, ast::AssertionSet & have, const ast::OpenVarSet & open,
        WidenMode widen ) {
        assert( e1 && e2 );
        return ast::Pass<UnifyExpr>::read( e1, e2, env, need, have, open, widen );
}

class Unify final : public ast::WithShortCircuiting {
        const ast::Type * type2;
        ast::TypeEnvironment & tenv;
        ast::AssertionSet & need;
        ast::AssertionSet & have;
        const ast::OpenVarSet & open;
        WidenMode widen;
public:
        static size_t traceId;
        bool result;

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

        void previsit( const ast::Node * ) { visit_children = false; }

        void postvisit( const ast::VoidType * ) {
                result = dynamic_cast< const ast::VoidType * >( type2 );
        }

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

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

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

                if ( array->isVarLen != array2->isVarLen ) return;
                if ( (array->dimension != nullptr) != (array2->dimension != nullptr) ) return;

                if ( array->dimension ) {
                        assert( array2->dimension );
                        // type unification calls expression unification (mutual recursion)
                        if ( !unify(array->dimension, array2->dimension,
                                tenv, need, have, open, widen) ) return;
                }

                result = unifyExact(
                        array->base, array2->base, tenv, need, have, open, noWiden());
        }

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

private:

        template< typename Iter >
        static bool unifyTypeList(
                Iter crnt1, Iter end1, Iter crnt2, Iter end2, ast::TypeEnvironment & env,
                ast::AssertionSet & need, ast::AssertionSet & have, const ast::OpenVarSet & open
        ) {
                while ( crnt1 != end1 && crnt2 != end2 ) {
                        const ast::Type * t1 = *crnt1;
                        const ast::Type * t2 = *crnt2;
                        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, tupleFromTypes( crnt2, end2 ), env, need, have, open,
                                        noWiden() );
                        } else if ( !isTuple1 && isTuple2 ) {
                                // combine remainder of list1, then unify
                                return unifyExact(
                                        tupleFromTypes( crnt1, end1 ), t2, env, need, have, open,
                                        noWiden() );
                        }

                        if ( !unifyExact(
                                t1, t2, env, need, have, open, noWiden() )
                        ) 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;
                        if ( !Tuples::isTtype( t1 ) ) return false;
                        return unifyExact(
                                t1, tupleFromTypes( crnt2, end2 ), env, need, have, open,
                                noWiden() );
                } else if ( crnt2 != end2 ) {
                        // try unifying empty tuple with ttype
                        const ast::Type * t2 = *crnt2;
                        if ( !Tuples::isTtype( t2 ) ) return false;
                        return unifyExact(
                                tupleFromTypes( crnt1, end1 ), t2, env, need, have, open,
                                noWiden() );
                }

                return true;
        }

        static bool unifyTypeList(
                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
        ) {
                return unifyTypeList(
                        list1.begin(), list1.end(), list2.begin(), list2.end(), env, need, have, open);
        }

        static void markAssertionSet( ast::AssertionSet & assns, const ast::VariableExpr * 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::FunctionType * type
        ) {
                for ( auto & assert : type->assertions ) {
                        markAssertionSet( assn1, assert );
                        markAssertionSet( assn2, assert );
                }
        }

public:
        void postvisit( const ast::FunctionType * func ) {
                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 ( !unifyTypeList( params, params2, tenv, need, have, open ) ) return;
                if ( !unifyTypeList(
                        func->returns, func2->returns, tenv, need, have, open ) ) return;

                markAssertions( have, need, func );
                markAssertions( have, need, func2 );

                result = true;
        }

private:
        // Returns: other, cast as XInstType
        // Assigns this->result: whether types are compatible (up to generic parameters)
        template< typename XInstType >
        const XInstType * handleRefType( const XInstType * inst, const ast::Type * other ) {
                // check that the other type is compatible and named the same
                auto otherInst = dynamic_cast< const XInstType * >( other );
                if ( otherInst && inst->name == otherInst->name ) {
                        this->result = otherInst;
                }
                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 XInstType >
        void handleGenericRefType( const XInstType * inst, const ast::Type * other ) {
                // check that other type is compatible and named the same
                const XInstType * otherInst = handleRefType( inst, other );
                if ( !this->result ) 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 = otherInst->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, noWiden() ) ) {
                                result = false;
                                return;
                        }

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

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

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

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

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

        void postvisit( const ast::TypeInstType * typeInst ) {
                // assert( open.find( *typeInst ) == open.end() );
                auto otherInst = dynamic_cast< const ast::TypeInstType * >( type2 );
                if ( otherInst && typeInst->name == otherInst->name ) {
                        this->result = otherInst;
                }
        }

private:
        /// Creates a tuple type based on a list of Type
        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
        ) {
                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,
                                        noWiden() );
                        } else if ( !isTuple1 && isTuple2 ) {
                                // combine entirety of list1, then unify
                                return unifyExact(
                                        tupleFromTypes( list1 ), t2, env, need, have, open,
                                        noWiden() );
                        }

                        if ( !unifyExact(
                                t1, t2, env, need, have, open, noWiden() )
                        ) 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,
                                        noWiden() );
                } 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,
                                        noWiden() );
                }

                return true;
        }

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

                ast::Pass<TtypeExpander> 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 );
        }

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

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

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

// size_t Unify::traceId = Stats::Heap::new_stacktrace_id("Unify");

bool unify(
                const ast::ptr<ast::Type> & type1, const ast::ptr<ast::Type> & type2,
                ast::TypeEnvironment & env, ast::AssertionSet & need, ast::AssertionSet & have,
                ast::OpenVarSet & open
) {
        ast::ptr<ast::Type> common;
        return unify( type1, type2, env, need, have, open, common );
}

bool unify(
                const ast::ptr<ast::Type> & type1, const ast::ptr<ast::Type> & type2,
                ast::TypeEnvironment & env, ast::AssertionSet & need, ast::AssertionSet & have,
                ast::OpenVarSet & open, ast::ptr<ast::Type> & common
) {
        ast::OpenVarSet closed;
        // findOpenVars( type1, open, closed, need, have, FirstClosed );
        findOpenVars( type2, open, closed, need, have, env, FirstOpen );
        return unifyInexact(
                type1, type2, env, need, have, open, WidenMode{ true, true }, common );
}

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
) {
        if ( type1->qualifiers != type2->qualifiers ) return false;

        auto var1 = dynamic_cast< const ast::TypeInstType * >( type1 );
        auto var2 = dynamic_cast< const ast::TypeInstType * >( type2 );
        bool isopen1 = var1 && env.lookup(*var1);
        bool isopen2 = var2 && env.lookup(*var2);

        if ( isopen1 && isopen2 ) {
                if ( var1->base->kind != var2->base->kind ) return false;
                return env.bindVarToVar(
                        var1, var2, ast::TypeData{ var1->base->kind, var1->base->sized||var2->base->sized }, need, have,
                        open, widen );
        } else if ( isopen1 ) {
                return env.bindVar( var1, type2, ast::TypeData{var1->base}, need, have, open, widen );
        } else if ( isopen2 ) {
                return env.bindVar( var2, type1, ast::TypeData{var2->base}, need, have, open, widen );
        } else {
                return ast::Pass<Unify>::read(
                        type1, type2, env, need, have, open, widen );
        }
}

bool unifyInexact(
                const ast::ptr<ast::Type> & type1, const ast::ptr<ast::Type> & type2,
                ast::TypeEnvironment & env, ast::AssertionSet & need, ast::AssertionSet & have,
                const ast::OpenVarSet & open, WidenMode widen,
                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::Type * t1 = shallowCopy(type1.get());
        ast::Type * t2 = shallowCopy(type2.get());
        t1->qualifiers = {};
        t2->qualifiers = {};
        ast::ptr< ast::Type > t1_(t1);
        ast::ptr< ast::Type > t2_(t2);

        if ( unifyExact( t1, t2, env, need, have, open, widen ) ) {
                // if exact unification on unqualified types, try to merge qualifiers
                if ( q1 == q2 || ( ( q1 > q2 || widen.first ) && ( q2 > q1 || widen.second ) ) ) {
                        t1->qualifiers = q1 | q2;
                        common = t1;
                        return true;
                } else {
                        return false;
                }
        } else if (( common = commonType( t1, t2, env, need, have, open, widen ))) {
                // no exact unification, but common type
                auto c = shallowCopy(common.get());
                c->qualifiers = q1 | q2;
                common = c;
                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];

        std::vector<ast::ptr<ast::Type>> tys;
        for ( const auto & decl : func->returns ) {
                tys.emplace_back( decl );
        }
        return new ast::TupleType( std::move(tys) );
}

} // namespace ResolvExpr

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