1 files changed, 298 insertions, 0 deletions
diff --git a/src/go/types/typeset.go b/src/go/types/typeset.go
new file mode 100644
index 0000000000..836f93047a
--- /dev/null
+++ b/src/go/types/typeset.go
@@ -0,0 +1,298 @@
+// 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.
+type _TypeSet struct {
+	comparable bool // if set, the interface is or embeds comparable
+	// TODO(gri) consider using a set for the methods for faster lookup
+	methods []*Func // all methods of the interface; sorted by unique ID
+	types   Type    // typically a *Union; nil means no type restrictions
+}
+
+// IsTop reports whether type set s is the top type set (corresponding to the empty interface).
+func (s *_TypeSet) IsTop() bool { return !s.comparable && len(s.methods) == 0 && s.types == nil }
+
+// IsMethodSet reports whether the type set s is described by a single set of methods.
+func (s *_TypeSet) IsMethodSet() bool { return !s.comparable && s.types == nil }
+
+// IsComparable reports whether each type in the set is comparable.
+// TODO(gri) this is not correct - there may be s.types values containing non-comparable types
+func (s *_TypeSet) IsComparable() bool {
+	if s.types == nil {
+		return s.comparable
+	}
+	tcomparable := s.underIs(func(u Type) bool {
+		return Comparable(u)
+	})
+	if !s.comparable {
+		return tcomparable
+	}
+	return s.comparable && tcomparable
+}
+
+// TODO(gri) IsTypeSet is not a great name. Find a better one.
+
+// IsTypeSet reports whether the type set s is represented by a finite set of underlying types.
+func (s *_TypeSet) IsTypeSet() bool {
+	return !s.comparable && len(s.methods) == 0
+}
+
+// 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) (int, *Func) {
+	// TODO(gri) s.methods is sorted - consider binary search
+	return lookupMethod(s.methods, pkg, name)
+}
+
+func (s *_TypeSet) String() string {
+	if s.IsTop() {
+		return "⊤"
+	}
+
+	var buf bytes.Buffer
+	buf.WriteByte('{')
+	if s.comparable {
+		buf.WriteString(" comparable")
+		if len(s.methods) > 0 || s.types != nil {
+			buf.WriteByte(';')
+		}
+	}
+	for i, m := range s.methods {
+		if i > 0 {
+			buf.WriteByte(';')
+		}
+		buf.WriteByte(' ')
+		buf.WriteString(m.String())
+	}
+	if len(s.methods) > 0 && s.types != nil {
+		buf.WriteByte(';')
+	}
+	if s.types != nil {
+		buf.WriteByte(' ')
+		writeType(&buf, s.types, nil, nil)
+	}
+
+	buf.WriteString(" }") // there was a least one method or type
+	return buf.String()
+}
+
+// ----------------------------------------------------------------------------
+// Implementation
+
+// underIs reports whether f returned true for the underlying types of the
+// enumerable types in the type set s. If the type set comprises all types
+// f is called once with the top type; if the type set is empty, the result
+// is false.
+func (s *_TypeSet) underIs(f func(Type) bool) bool {
+	switch t := s.types.(type) {
+	case nil:
+		return f(theTop)
+	default:
+		return f(t)
+	case *Union:
+		return t.underIs(f)
+	}
+}
+
+// topTypeSet may be used as type set for the empty interface.
+var topTypeSet _TypeSet
+
+// computeTypeSet may be called with check == nil.
+func computeTypeSet(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 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 = new(_TypeSet) // TODO(gri) is this sufficient?
+
+	// 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 methods []*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:
+			methods = append(methods, 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
+	var allTypes Type
+	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 types Type
+		switch t := under(typ).(type) {
+		case *Interface:
+			tset := computeTypeSet(check, pos, t)
+			if tset.comparable {
+				ityp.tset.comparable = true
+			}
+			for _, m := range tset.methods {
+				addMethod(pos, m, false) // use embedding position pos rather than m.pos
+
+			}
+			types = tset.types
+		case *Union:
+			// TODO(gri) combine with default case once we have
+			//           converted all tests to new notation and we
+			//           can report an error when we don't have an
+			//           interface before go1.18.
+			types = typ
+		case *TypeParam:
+			// Embedding stand-alone type parameters is not permitted for now.
+			// This case is handled during union parsing.
+			unreachable()
+		default:
+			if typ == Typ[Invalid] {
+				continue
+			}
+			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
+				check.errorf(atPos(pos), _InvalidIfaceEmbed, "%s is not an interface", typ)
+				continue
+			}
+			types = typ
+		}
+		allTypes = intersect(allTypes, types)
+	}
+	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))
+		}
+	}
+
+	if methods != nil {
+		sort.Sort(byUniqueMethodName(methods))
+		ityp.tset.methods = methods
+	}
+	ityp.tset.types = allTypes
+
+	return ityp.tset
+}
+
+func sortMethods(list []*Func) {
+	sort.Sort(byUniqueMethodName(list))
+}
+
+func assertSortedMethods(list []*Func) {
+	if !debug {
+		panic("internal error: assertSortedMethods called outside debug mode")
+	}
+	if !sort.IsSorted(byUniqueMethodName(list)) {
+		panic("internal error: 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] }