libgo/go/go/types/typeset.go - gcc - Git at Google

 // Copyright 2021 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package types

 import (
 	"bytes"
 	"fmt"
 	"go/token"
 	"sort"
 )

 // ----------------------------------------------------------------------------
 // API

 // A _TypeSet represents the type set of an interface.
 // Because of existing language restrictions, methods can be "factored out"
 // from the terms. The actual type set is the intersection of the type set
 // implied by the methods and the type set described by the terms and the
 // comparable bit. To test whether a type is included in a type set
 // ("implements" relation), the type must implement all methods _and_ be
 // an element of the type set described by the terms and the comparable bit.
 // If the term list describes the set of all types and comparable is true,
 // only comparable types are meant; in all other cases comparable is false.
 type _TypeSet struct {
 	methods    []*Func  // all methods of the interface; sorted by unique ID
 	terms      termlist // type terms of the type set
 	comparable bool     // invariant: !comparable || terms.isAll()
 }

 // IsEmpty reports whether type set s is the empty set.
 func (s *_TypeSet) IsEmpty() bool { return s.terms.isEmpty() }

 // IsAll reports whether type set s is the set of all types (corresponding to the empty interface).
 func (s *_TypeSet) IsAll() bool { return s.IsMethodSet() && len(s.methods) == 0 }

 // IsMethodSet reports whether the interface t is fully described by its method set.
 func (s *_TypeSet) IsMethodSet() bool { return !s.comparable && s.terms.isAll() }

 // IsComparable reports whether each type in the set is comparable.
 func (s *_TypeSet) IsComparable(seen map[Type]bool) bool {
 	if s.terms.isAll() {
 		return s.comparable
 	}
 	return s.is(func(t *term) bool {
 		return t != nil && comparable(t.typ, false, seen, nil)
 	})
 }

 // NumMethods returns the number of methods available.
 func (s *_TypeSet) NumMethods() int { return len(s.methods) }

 // Method returns the i'th method of type set s for 0 <= i < s.NumMethods().
 // The methods are ordered by their unique ID.
 func (s *_TypeSet) Method(i int) *Func { return s.methods[i] }

 // LookupMethod returns the index of and method with matching package and name, or (-1, nil).
 func (s *_TypeSet) LookupMethod(pkg *Package, name string, foldCase bool) (int, *Func) {
 	return lookupMethod(s.methods, pkg, name, foldCase)
 }

 func (s *_TypeSet) String() string {
 	switch {
 	case s.IsEmpty():
 		return "∅"
 	case s.IsAll():
 		return "𝓤"
 	}

 	hasMethods := len(s.methods) > 0
 	hasTerms := s.hasTerms()

 	var buf bytes.Buffer
 	buf.WriteByte('{')
 	if s.comparable {
 		buf.WriteString("comparable")
 		if hasMethods || hasTerms {
 			buf.WriteString("; ")
 		}
 	}
 	for i, m := range s.methods {
 		if i > 0 {
 			buf.WriteString("; ")
 		}
 		buf.WriteString(m.String())
 	}
 	if hasMethods && hasTerms {
 		buf.WriteString("; ")
 	}
 	if hasTerms {
 		buf.WriteString(s.terms.String())
 	}
 	buf.WriteString("}")
 	return buf.String()
 }

 // ----------------------------------------------------------------------------
 // Implementation

 // hasTerms reports whether the type set has specific type terms.
 func (s *_TypeSet) hasTerms() bool { return !s.terms.isEmpty() && !s.terms.isAll() }

 // subsetOf reports whether s1 ⊆ s2.
 func (s1 *_TypeSet) subsetOf(s2 *_TypeSet) bool { return s1.terms.subsetOf(s2.terms) }

 // TODO(gri) TypeSet.is and TypeSet.underIs should probably also go into termlist.go

 // is calls f with the specific type terms of s and reports whether
 // all calls to f returned true. If there are no specific terms, is
 // returns the result of f(nil).
 func (s *_TypeSet) is(f func(*term) bool) bool {
 	if !s.hasTerms() {
 		return f(nil)
 	}
 	for _, t := range s.terms {
 		assert(t.typ != nil)
 		if !f(t) {
 			return false
 		}
 	}
 	return true
 }

 // underIs calls f with the underlying types of the specific type terms
 // of s and reports whether all calls to f returned true. If there are
 // no specific terms, underIs returns the result of f(nil).
 func (s *_TypeSet) underIs(f func(Type) bool) bool {
 	if !s.hasTerms() {
 		return f(nil)
 	}
 	for _, t := range s.terms {
 		assert(t.typ != nil)
 		// x == under(x) for ~x terms
 		u := t.typ
 		if !t.tilde {
 			u = under(u)
 		}
 		if debug {
 			assert(Identical(u, under(u)))
 		}
 		if !f(u) {
 			return false
 		}
 	}
 	return true
 }

 // topTypeSet may be used as type set for the empty interface.
 var topTypeSet = _TypeSet{terms: allTermlist}

 // computeInterfaceTypeSet may be called with check == nil.
 func computeInterfaceTypeSet(check *Checker, pos token.Pos, ityp *Interface) *_TypeSet {
 	if ityp.tset != nil {
 		return ityp.tset
 	}

 	// If the interface is not fully set up yet, the type set will
 	// not be complete, which may lead to errors when using the
 	// type set (e.g. missing method). Don't compute a partial type
 	// set (and don't store it!), so that we still compute the full
 	// type set eventually. Instead, return the top type set and
 	// let any follow-on errors play out.
 	//
 	// TODO(gri) Consider recording when this happens and reporting
 	// it as an error (but only if there were no other errors so to
 	// to not have unnecessary follow-on errors).
 	if !ityp.complete {
 		return &topTypeSet
 	}

 	if check != nil && trace {
 		// Types don't generally have position information.
 		// If we don't have a valid pos provided, try to use
 		// one close enough.
 		if !pos.IsValid() && len(ityp.methods) > 0 {
 			pos = ityp.methods[0].pos
 		}

 		check.trace(pos, "type set for %s", ityp)
 		check.indent++
 		defer func() {
 			check.indent--
 			check.trace(pos, "=> %s ", ityp.typeSet())
 		}()
 	}

 	// An infinitely expanding interface (due to a cycle) is detected
 	// elsewhere (Checker.validType), so here we simply assume we only
 	// have valid interfaces. Mark the interface as complete to avoid
 	// infinite recursion if the validType check occurs later for some
 	// reason.
 	ityp.tset = &_TypeSet{terms: allTermlist} // TODO(gri) is this sufficient?

 	var unionSets map[*Union]*_TypeSet
 	if check != nil {
 		if check.unionTypeSets == nil {
 			check.unionTypeSets = make(map[*Union]*_TypeSet)
 		}
 		unionSets = check.unionTypeSets
 	} else {
 		unionSets = make(map[*Union]*_TypeSet)
 	}

 	// Methods of embedded interfaces are collected unchanged; i.e., the identity
 	// of a method I.m's Func Object of an interface I is the same as that of
 	// the method m in an interface that embeds interface I. On the other hand,
 	// if a method is embedded via multiple overlapping embedded interfaces, we
 	// don't provide a guarantee which "original m" got chosen for the embedding
 	// interface. See also issue #34421.
 	//
 	// If we don't care to provide this identity guarantee anymore, instead of
 	// reusing the original method in embeddings, we can clone the method's Func
 	// Object and give it the position of a corresponding embedded interface. Then
 	// we can get rid of the mpos map below and simply use the cloned method's
 	// position.

 	var todo []*Func
 	var seen objset
 	var allMethods []*Func
 	mpos := make(map[*Func]token.Pos) // method specification or method embedding position, for good error messages
 	addMethod := func(pos token.Pos, m *Func, explicit bool) {
 		switch other := seen.insert(m); {
 		case other == nil:
 			allMethods = append(allMethods, m)
 			mpos[m] = pos
 		case explicit:
 			if check == nil {
 				panic(fmt.Sprintf("%v: duplicate method %s", m.pos, m.name))
 			}
 			// check != nil
 			check.errorf(atPos(pos), _DuplicateDecl, "duplicate method %s", m.name)
 			check.errorf(atPos(mpos[other.(*Func)]), _DuplicateDecl, "\tother declaration of %s", m.name) // secondary error, \t indented
 		default:
 			// We have a duplicate method name in an embedded (not explicitly declared) method.
 			// Check method signatures after all types are computed (issue #33656).
 			// If we're pre-go1.14 (overlapping embeddings are not permitted), report that
 			// error here as well (even though we could do it eagerly) because it's the same
 			// error message.
 			if check == nil {
 				// check method signatures after all locally embedded interfaces are computed
 				todo = append(todo, m, other.(*Func))
 				break
 			}
 			// check != nil
 			check.later(func() {
 				if !check.allowVersion(m.pkg, 1, 14) || !Identical(m.typ, other.Type()) {
 					check.errorf(atPos(pos), _DuplicateDecl, "duplicate method %s", m.name)
 					check.errorf(atPos(mpos[other.(*Func)]), _DuplicateDecl, "\tother declaration of %s", m.name) // secondary error, \t indented
 				}
 			})
 		}
 	}

 	for _, m := range ityp.methods {
 		addMethod(m.pos, m, true)
 	}

 	// collect embedded elements
 	allTerms := allTermlist
 	allComparable := false
 	for i, typ := range ityp.embeddeds {
 		// The embedding position is nil for imported interfaces
 		// and also for interface copies after substitution (but
 		// in that case we don't need to report errors again).
 		var pos token.Pos // embedding position
 		if ityp.embedPos != nil {
 			pos = (*ityp.embedPos)[i]
 		}
 		var comparable bool
 		var terms termlist
 		switch u := under(typ).(type) {
 		case *Interface:
 			// For now we don't permit type parameters as constraints.
 			assert(!isTypeParam(typ))
 			tset := computeInterfaceTypeSet(check, pos, u)
 			// If typ is local, an error was already reported where typ is specified/defined.
 			if check != nil && check.isImportedConstraint(typ) && !check.allowVersion(check.pkg, 1, 18) {
 				check.errorf(atPos(pos), _UnsupportedFeature, "embedding constraint interface %s requires go1.18 or later", typ)
 				continue
 			}
 			comparable = tset.comparable
 			for _, m := range tset.methods {
 				addMethod(pos, m, false) // use embedding position pos rather than m.pos
 			}
 			terms = tset.terms
 		case *Union:
 			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
 				check.errorf(atPos(pos), _InvalidIfaceEmbed, "embedding interface element %s requires go1.18 or later", u)
 				continue
 			}
 			tset := computeUnionTypeSet(check, unionSets, pos, u)
 			if tset == &invalidTypeSet {
 				continue // ignore invalid unions
 			}
 			assert(!tset.comparable)
 			assert(len(tset.methods) == 0)
 			terms = tset.terms
 		default:
 			if u == Typ[Invalid] {
 				continue
 			}
 			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
 				check.errorf(atPos(pos), _InvalidIfaceEmbed, "embedding non-interface type %s requires go1.18 or later", typ)
 				continue
 			}
 			terms = termlist{{false, typ}}
 		}

 		// The type set of an interface is the intersection of the type sets of all its elements.
 		// Due to language restrictions, only embedded interfaces can add methods, they are handled
 		// separately. Here we only need to intersect the term lists and comparable bits.
 		allTerms, allComparable = intersectTermLists(allTerms, allComparable, terms, comparable)
 	}
 	ityp.embedPos = nil // not needed anymore (errors have been reported)

 	// process todo's (this only happens if check == nil)
 	for i := 0; i < len(todo); i += 2 {
 		m := todo[i]
 		other := todo[i+1]
 		if !Identical(m.typ, other.typ) {
 			panic(fmt.Sprintf("%v: duplicate method %s", m.pos, m.name))
 		}
 	}

 	ityp.tset.comparable = allComparable
 	if len(allMethods) != 0 {
 		sortMethods(allMethods)
 		ityp.tset.methods = allMethods
 	}
 	ityp.tset.terms = allTerms

 	return ityp.tset
 }

 // TODO(gri) The intersectTermLists function belongs to the termlist implementation.
 //           The comparable type set may also be best represented as a term (using
 //           a special type).

 // intersectTermLists computes the intersection of two term lists and respective comparable bits.
 // xcomp, ycomp are valid only if xterms.isAll() and yterms.isAll() respectively.
 func intersectTermLists(xterms termlist, xcomp bool, yterms termlist, ycomp bool) (termlist, bool) {
 	terms := xterms.intersect(yterms)
 	// If one of xterms or yterms is marked as comparable,
 	// the result must only include comparable types.
 	comp := xcomp || ycomp
 	if comp && !terms.isAll() {
 		// only keep comparable terms
 		i := 0
 		for _, t := range terms {
 			assert(t.typ != nil)
 			if Comparable(t.typ) {
 				terms[i] = t
 				i++
 			}
 		}
 		terms = terms[:i]
 		if !terms.isAll() {
 			comp = false
 		}
 	}
 	assert(!comp || terms.isAll()) // comparable invariant
 	return terms, comp
 }

 func sortMethods(list []*Func) {
 	sort.Sort(byUniqueMethodName(list))
 }

 func assertSortedMethods(list []*Func) {
 	if !debug {
 		panic("assertSortedMethods called outside debug mode")
 	}
 	if !sort.IsSorted(byUniqueMethodName(list)) {
 		panic("methods not sorted")
 	}
 }

 // byUniqueMethodName method lists can be sorted by their unique method names.
 type byUniqueMethodName []*Func

 func (a byUniqueMethodName) Len() int           { return len(a) }
 func (a byUniqueMethodName) Less(i, j int) bool { return a[i].Id() < a[j].Id() }
 func (a byUniqueMethodName) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

 // invalidTypeSet is a singleton type set to signal an invalid type set
 // due to an error. It's also a valid empty type set, so consumers of
 // type sets may choose to ignore it.
 var invalidTypeSet _TypeSet

 // computeUnionTypeSet may be called with check == nil.
 // The result is &invalidTypeSet if the union overflows.
 func computeUnionTypeSet(check *Checker, unionSets map[*Union]*_TypeSet, pos token.Pos, utyp *Union) *_TypeSet {
 	if tset, _ := unionSets[utyp]; tset != nil {
 		return tset
 	}

 	// avoid infinite recursion (see also computeInterfaceTypeSet)
 	unionSets[utyp] = new(_TypeSet)

 	var allTerms termlist
 	for _, t := range utyp.terms {
 		var terms termlist
 		u := under(t.typ)
 		if ui, _ := u.(*Interface); ui != nil {
 			// For now we don't permit type parameters as constraints.
 			assert(!isTypeParam(t.typ))
 			terms = computeInterfaceTypeSet(check, pos, ui).terms
 		} else if t.typ == Typ[Invalid] {
 			continue
 		} else {
 			if t.tilde && !Identical(t.typ, u) {
 				// There is no underlying type which is t.typ.
 				// The corresponding type set is empty.
 				t = nil // ∅ term
 			}
 			terms = termlist{(*term)(t)}
 		}
 		// The type set of a union expression is the union
 		// of the type sets of each term.
 		allTerms = allTerms.union(terms)
 		if len(allTerms) > maxTermCount {
 			if check != nil {
 				check.errorf(atPos(pos), _InvalidUnion, "cannot handle more than %d union terms (implementation limitation)", maxTermCount)
 			}
 			unionSets[utyp] = &invalidTypeSet
 			return unionSets[utyp]
 		}
 	}
 	unionSets[utyp].terms = allTerms

 	return unionSets[utyp]
 }
	// Copyright 2021 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package types

	import (
	"bytes"
	"fmt"
	"go/token"
	"sort"
	)

	// ----------------------------------------------------------------------------
	// API

	// A _TypeSet represents the type set of an interface.
	// Because of existing language restrictions, methods can be "factored out"
	// from the terms. The actual type set is the intersection of the type set
	// implied by the methods and the type set described by the terms and the
	// comparable bit. To test whether a type is included in a type set
	// ("implements" relation), the type must implement all methods _and_ be
	// an element of the type set described by the terms and the comparable bit.
	// If the term list describes the set of all types and comparable is true,
	// only comparable types are meant; in all other cases comparable is false.
	type _TypeSet struct {
	methods []*Func // all methods of the interface; sorted by unique ID
	terms termlist // type terms of the type set
	comparable bool // invariant: !comparable \|\| terms.isAll()
	}

	// IsEmpty reports whether type set s is the empty set.
	func (s *_TypeSet) IsEmpty() bool { return s.terms.isEmpty() }

	// IsAll reports whether type set s is the set of all types (corresponding to the empty interface).
	func (s *_TypeSet) IsAll() bool { return s.IsMethodSet() && len(s.methods) == 0 }

	// IsMethodSet reports whether the interface t is fully described by its method set.
	func (s *_TypeSet) IsMethodSet() bool { return !s.comparable && s.terms.isAll() }

	// IsComparable reports whether each type in the set is comparable.
	func (s *_TypeSet) IsComparable(seen map[Type]bool) bool {
	if s.terms.isAll() {
	return s.comparable
	}
	return s.is(func(t *term) bool {
	return t != nil && comparable(t.typ, false, seen, nil)
	})
	}

	// NumMethods returns the number of methods available.
	func (s *_TypeSet) NumMethods() int { return len(s.methods) }

	// Method returns the i'th method of type set s for 0 <= i < s.NumMethods().
	// The methods are ordered by their unique ID.
	func (s _TypeSet) Method(i int) Func { return s.methods[i] }

	// LookupMethod returns the index of and method with matching package and name, or (-1, nil).
	func (s _TypeSet) LookupMethod(pkg Package, name string, foldCase bool) (int, *Func) {
	return lookupMethod(s.methods, pkg, name, foldCase)
	}

	func (s *_TypeSet) String() string {
	switch {
	case s.IsEmpty():
	return "∅"
	case s.IsAll():
	return "𝓤"
	}

	hasMethods := len(s.methods) > 0
	hasTerms := s.hasTerms()

	var buf bytes.Buffer
	buf.WriteByte('{')
	if s.comparable {
	buf.WriteString("comparable")
	if hasMethods \|\| hasTerms {
	buf.WriteString("; ")
	}
	}
	for i, m := range s.methods {
	if i > 0 {
	buf.WriteString("; ")
	}
	buf.WriteString(m.String())
	}
	if hasMethods && hasTerms {
	buf.WriteString("; ")
	}
	if hasTerms {
	buf.WriteString(s.terms.String())
	}
	buf.WriteString("}")
	return buf.String()
	}

	// ----------------------------------------------------------------------------
	// Implementation

	// hasTerms reports whether the type set has specific type terms.
	func (s *_TypeSet) hasTerms() bool { return !s.terms.isEmpty() && !s.terms.isAll() }

	// subsetOf reports whether s1 ⊆ s2.
	func (s1 _TypeSet) subsetOf(s2 _TypeSet) bool { return s1.terms.subsetOf(s2.terms) }

	// TODO(gri) TypeSet.is and TypeSet.underIs should probably also go into termlist.go

	// is calls f with the specific type terms of s and reports whether
	// all calls to f returned true. If there are no specific terms, is
	// returns the result of f(nil).
	func (s _TypeSet) is(f func(term) bool) bool {
	if !s.hasTerms() {
	return f(nil)
	}
	for _, t := range s.terms {
	assert(t.typ != nil)
	if !f(t) {
	return false
	}
	}
	return true
	}

	// underIs calls f with the underlying types of the specific type terms
	// of s and reports whether all calls to f returned true. If there are
	// no specific terms, underIs returns the result of f(nil).
	func (s *_TypeSet) underIs(f func(Type) bool) bool {
	if !s.hasTerms() {
	return f(nil)
	}
	for _, t := range s.terms {
	assert(t.typ != nil)
	// x == under(x) for ~x terms
	u := t.typ
	if !t.tilde {
	u = under(u)
	}
	if debug {
	assert(Identical(u, under(u)))
	}
	if !f(u) {
	return false
	}
	}
	return true
	}

	// topTypeSet may be used as type set for the empty interface.
	var topTypeSet = _TypeSet{terms: allTermlist}

	// computeInterfaceTypeSet may be called with check == nil.
	func computeInterfaceTypeSet(check Checker, pos token.Pos, ityp Interface) *_TypeSet {
	if ityp.tset != nil {
	return ityp.tset
	}

	// If the interface is not fully set up yet, the type set will
	// not be complete, which may lead to errors when using the
	// type set (e.g. missing method). Don't compute a partial type
	// set (and don't store it!), so that we still compute the full
	// type set eventually. Instead, return the top type set and
	// let any follow-on errors play out.
	//
	// TODO(gri) Consider recording when this happens and reporting
	// it as an error (but only if there were no other errors so to
	// to not have unnecessary follow-on errors).
	if !ityp.complete {
	return &topTypeSet
	}

	if check != nil && trace {
	// Types don't generally have position information.
	// If we don't have a valid pos provided, try to use
	// one close enough.
	if !pos.IsValid() && len(ityp.methods) > 0 {
	pos = ityp.methods[0].pos
	}

	check.trace(pos, "type set for %s", ityp)
	check.indent++
	defer func() {
	check.indent--
	check.trace(pos, "=> %s ", ityp.typeSet())
	}()
	}

	// An infinitely expanding interface (due to a cycle) is detected
	// elsewhere (Checker.validType), so here we simply assume we only
	// have valid interfaces. Mark the interface as complete to avoid
	// infinite recursion if the validType check occurs later for some
	// reason.
	ityp.tset = &_TypeSet{terms: allTermlist} // TODO(gri) is this sufficient?

	var unionSets map[Union]_TypeSet
	if check != nil {
	if check.unionTypeSets == nil {
	check.unionTypeSets = make(map[Union]_TypeSet)
	}
	unionSets = check.unionTypeSets
	} else {
	unionSets = make(map[Union]_TypeSet)
	}

	// Methods of embedded interfaces are collected unchanged; i.e., the identity
	// of a method I.m's Func Object of an interface I is the same as that of
	// the method m in an interface that embeds interface I. On the other hand,
	// if a method is embedded via multiple overlapping embedded interfaces, we
	// don't provide a guarantee which "original m" got chosen for the embedding
	// interface. See also issue #34421.
	//
	// If we don't care to provide this identity guarantee anymore, instead of
	// reusing the original method in embeddings, we can clone the method's Func
	// Object and give it the position of a corresponding embedded interface. Then
	// we can get rid of the mpos map below and simply use the cloned method's
	// position.

	var todo []*Func
	var seen objset
	var allMethods []*Func
	mpos := make(map[*Func]token.Pos) // method specification or method embedding position, for good error messages
	addMethod := func(pos token.Pos, m *Func, explicit bool) {
	switch other := seen.insert(m); {
	case other == nil:
	allMethods = append(allMethods, m)
	mpos[m] = pos
	case explicit:
	if check == nil {
	panic(fmt.Sprintf("%v: duplicate method %s", m.pos, m.name))
	}
	// check != nil
	check.errorf(atPos(pos), _DuplicateDecl, "duplicate method %s", m.name)
	check.errorf(atPos(mpos[other.(*Func)]), _DuplicateDecl, "\tother declaration of %s", m.name) // secondary error, \t indented
	default:
	// We have a duplicate method name in an embedded (not explicitly declared) method.
	// Check method signatures after all types are computed (issue #33656).
	// If we're pre-go1.14 (overlapping embeddings are not permitted), report that
	// error here as well (even though we could do it eagerly) because it's the same
	// error message.
	if check == nil {
	// check method signatures after all locally embedded interfaces are computed
	todo = append(todo, m, other.(*Func))
	break
	}
	// check != nil
	check.later(func() {
	if !check.allowVersion(m.pkg, 1, 14) \|\| !Identical(m.typ, other.Type()) {
	check.errorf(atPos(pos), _DuplicateDecl, "duplicate method %s", m.name)
	check.errorf(atPos(mpos[other.(*Func)]), _DuplicateDecl, "\tother declaration of %s", m.name) // secondary error, \t indented
	}
	})
	}
	}

	for _, m := range ityp.methods {
	addMethod(m.pos, m, true)
	}

	// collect embedded elements
	allTerms := allTermlist
	allComparable := false
	for i, typ := range ityp.embeddeds {
	// The embedding position is nil for imported interfaces
	// and also for interface copies after substitution (but
	// in that case we don't need to report errors again).
	var pos token.Pos // embedding position
	if ityp.embedPos != nil {
	pos = (*ityp.embedPos)[i]
	}
	var comparable bool
	var terms termlist
	switch u := under(typ).(type) {
	case *Interface:
	// For now we don't permit type parameters as constraints.
	assert(!isTypeParam(typ))
	tset := computeInterfaceTypeSet(check, pos, u)
	// If typ is local, an error was already reported where typ is specified/defined.
	if check != nil && check.isImportedConstraint(typ) && !check.allowVersion(check.pkg, 1, 18) {
	check.errorf(atPos(pos), _UnsupportedFeature, "embedding constraint interface %s requires go1.18 or later", typ)
	continue
	}
	comparable = tset.comparable
	for _, m := range tset.methods {
	addMethod(pos, m, false) // use embedding position pos rather than m.pos
	}
	terms = tset.terms
	case *Union:
	if check != nil && !check.allowVersion(check.pkg, 1, 18) {
	check.errorf(atPos(pos), _InvalidIfaceEmbed, "embedding interface element %s requires go1.18 or later", u)
	continue
	}
	tset := computeUnionTypeSet(check, unionSets, pos, u)
	if tset == &invalidTypeSet {
	continue // ignore invalid unions
	}
	assert(!tset.comparable)
	assert(len(tset.methods) == 0)
	terms = tset.terms
	default:
	if u == Typ[Invalid] {
	continue
	}
	if check != nil && !check.allowVersion(check.pkg, 1, 18) {
	check.errorf(atPos(pos), _InvalidIfaceEmbed, "embedding non-interface type %s requires go1.18 or later", typ)
	continue
	}
	terms = termlist{{false, typ}}
	}

	// The type set of an interface is the intersection of the type sets of all its elements.
	// Due to language restrictions, only embedded interfaces can add methods, they are handled
	// separately. Here we only need to intersect the term lists and comparable bits.
	allTerms, allComparable = intersectTermLists(allTerms, allComparable, terms, comparable)
	}
	ityp.embedPos = nil // not needed anymore (errors have been reported)

	// process todo's (this only happens if check == nil)
	for i := 0; i < len(todo); i += 2 {
	m := todo[i]
	other := todo[i+1]
	if !Identical(m.typ, other.typ) {
	panic(fmt.Sprintf("%v: duplicate method %s", m.pos, m.name))
	}
	}

	ityp.tset.comparable = allComparable
	if len(allMethods) != 0 {
	sortMethods(allMethods)
	ityp.tset.methods = allMethods
	}
	ityp.tset.terms = allTerms

	return ityp.tset
	}

	// TODO(gri) The intersectTermLists function belongs to the termlist implementation.
	// The comparable type set may also be best represented as a term (using
	// a special type).

	// intersectTermLists computes the intersection of two term lists and respective comparable bits.
	// xcomp, ycomp are valid only if xterms.isAll() and yterms.isAll() respectively.
	func intersectTermLists(xterms termlist, xcomp bool, yterms termlist, ycomp bool) (termlist, bool) {
	terms := xterms.intersect(yterms)
	// If one of xterms or yterms is marked as comparable,
	// the result must only include comparable types.
	comp := xcomp \|\| ycomp
	if comp && !terms.isAll() {
	// only keep comparable terms
	i := 0
	for _, t := range terms {
	assert(t.typ != nil)
	if Comparable(t.typ) {
	terms[i] = t
	i++
	}
	}
	terms = terms[:i]
	if !terms.isAll() {
	comp = false
	}
	}
	assert(!comp \|\| terms.isAll()) // comparable invariant
	return terms, comp
	}

	func sortMethods(list []*Func) {
	sort.Sort(byUniqueMethodName(list))
	}

	func assertSortedMethods(list []*Func) {
	if !debug {
	panic("assertSortedMethods called outside debug mode")
	}
	if !sort.IsSorted(byUniqueMethodName(list)) {
	panic("methods not sorted")
	}
	}

	// byUniqueMethodName method lists can be sorted by their unique method names.
	type byUniqueMethodName []*Func

	func (a byUniqueMethodName) Len() int { return len(a) }
	func (a byUniqueMethodName) Less(i, j int) bool { return a[i].Id() < a[j].Id() }
	func (a byUniqueMethodName) Swap(i, j int) { a[i], a[j] = a[j], a[i] }

	// invalidTypeSet is a singleton type set to signal an invalid type set
	// due to an error. It's also a valid empty type set, so consumers of
	// type sets may choose to ignore it.
	var invalidTypeSet _TypeSet

	// computeUnionTypeSet may be called with check == nil.
	// The result is &invalidTypeSet if the union overflows.
	func computeUnionTypeSet(check Checker, unionSets map[Union]_TypeSet, pos token.Pos, utyp Union) *_TypeSet {
	if tset, _ := unionSets[utyp]; tset != nil {
	return tset
	}

	// avoid infinite recursion (see also computeInterfaceTypeSet)
	unionSets[utyp] = new(_TypeSet)

	var allTerms termlist
	for _, t := range utyp.terms {
	var terms termlist
	u := under(t.typ)
	if ui, _ := u.(*Interface); ui != nil {
	// For now we don't permit type parameters as constraints.
	assert(!isTypeParam(t.typ))
	terms = computeInterfaceTypeSet(check, pos, ui).terms
	} else if t.typ == Typ[Invalid] {
	continue
	} else {
	if t.tilde && !Identical(t.typ, u) {
	// There is no underlying type which is t.typ.
	// The corresponding type set is empty.
	t = nil // ∅ term
	}
	terms = termlist{(*term)(t)}
	}
	// The type set of a union expression is the union
	// of the type sets of each term.
	allTerms = allTerms.union(terms)
	if len(allTerms) > maxTermCount {
	if check != nil {
	check.errorf(atPos(pos), _InvalidUnion, "cannot handle more than %d union terms (implementation limitation)", maxTermCount)
	}
	unionSets[utyp] = &invalidTypeSet
	return unionSets[utyp]
	}
	}
	unionSets[utyp].terms = allTerms

	return unionSets[utyp]
	}