go/types: permit embedding of non-defined interfaces via alias type names

Embedded interfaces in interfaces must take the form of a (possibly
qualified) type name. Before alias types, a type name always denoted
a defined (formerly "named") type. The introduction of alias types
enabled embedding of non-defined types via alias type names, as in:

type T interface { E }
type E interface { m() }

Both cmd/compile and gccgo accept this kind of code, and the spec does
not prohibit it. There may be code in the wild that makes use of this.

go/types was written under the assumption that embedded interfaces
were always defined types; and that assumption was even reflected in
the go/types API.

This change removes this restriction in the implementation (which
happens to make it simpler), and in the API (by adding additional
functions and deprecating the corresponding older versions).

It also replaces uses of NewInterface and Embedded (old API) by
NewInterface2 and EmbeddedType (new API) in dependent packages
(importers).

The old API remains in place for backward compatibility and is marked
as deprecated.

Fixes #25301.

Change-Id: I272acd498754179efaf0590ca49d3eb4eee4348e
Reviewed-on: https://go-review.googlesource.com/114317Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/internal/gccgoimporter/parser.go

@@ -585,13 +585,13 @@ func (p *parser) parseInterfaceType(pkg *types.Package) types.Type {
 	p.expectKeyword("interface")
 
 	var methods []*types.Func
-	var typs []*types.Named
+	var embeddeds []types.Type
 
 	p.expect('{')
 	for p.tok != '}' && p.tok != scanner.EOF {
 		if p.tok == '?' {
 			p.next()
-			typs = append(typs, p.parseType(pkg).(*types.Named))
+			embeddeds = append(embeddeds, p.parseType(pkg))
 		} else {
 			method := p.parseFunc(pkg)
 			methods = append(methods, method)
@@ -600,7 +600,7 @@ func (p *parser) parseInterfaceType(pkg *types.Package) types.Type {
 	}
 	p.expect('}')
 
-	return types.NewInterface(methods, typs)
+	return types.NewInterface2(methods, embeddeds)
 }
 
 // PointerType = "*" ("any" | Type) .

src/go/internal/gcimporter/bimport.go

@@ -529,13 +529,13 @@ func (p *importer) typ(parent *types.Package, tname *types.Named) types.Type {
 			p.record(nil)
 		}
 
-		var embeddeds []*types.Named
+		var embeddeds []types.Type
 		for n := p.int(); n > 0; n-- {
 			p.pos()
-			embeddeds = append(embeddeds, p.typ(parent, nil).(*types.Named))
+			embeddeds = append(embeddeds, p.typ(parent, nil))
 		}
 
-		t := types.NewInterface(p.methodList(parent, tname), embeddeds)
+		t := types.NewInterface2(p.methodList(parent, tname), embeddeds)
 		p.interfaceList = append(p.interfaceList, t)
 		if p.trackAllTypes {
 			p.typList[n] = t

src/go/internal/gcimporter/gcimporter_test.go

@@ -286,10 +286,12 @@ func verifyInterfaceMethodRecvs(t *testing.T, named *types.Named, level int) {
 		}
 	}
 
-	// check embedded interfaces (they are named, too)
+	// check embedded interfaces (if they are named, too)
 	for i := 0; i < iface.NumEmbeddeds(); i++ {
 		// embedding of interfaces cannot have cycles; recursion will terminate
-		verifyInterfaceMethodRecvs(t, iface.Embedded(i), level+1)
+		if etype, _ := iface.EmbeddedType(i).(*types.Named); etype != nil {
+			verifyInterfaceMethodRecvs(t, etype, level+1)
+		}
 	}
 }
 
@@ -507,6 +509,26 @@ func TestIssue20046(t *testing.T) {
 		t.Fatalf("V.M not found (index = %v, indirect = %v)", index, indirect)
 	}
 }
+func TestIssue25301(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	if f := compile(t, "testdata", "issue25301.go"); f != "" {
+		defer os.Remove(f)
+	}
+
+	importPkg(t, "./testdata/issue25301")
+}
 
 func importPkg(t *testing.T, path string) *types.Package {
 	pkg, err := Import(make(map[string]*types.Package), path, ".", nil)

src/go/internal/gcimporter/iimport.go

@@ -500,10 +500,10 @@ func (r *importReader) doType(base *types.Named) types.Type {
 	case interfaceType:
 		r.currPkg = r.pkg()
 
-		embeddeds := make([]*types.Named, r.uint64())
+		embeddeds := make([]types.Type, r.uint64())
 		for i := range embeddeds {
 			_ = r.pos()
-			embeddeds[i] = r.typ().(*types.Named)
+			embeddeds[i] = r.typ()
 		}
 
 		methods := make([]*types.Func, r.uint64())
@@ -522,7 +522,7 @@ func (r *importReader) doType(base *types.Named) types.Type {
 			methods[i] = types.NewFunc(mpos, r.currPkg, mname, msig)
 		}
 
-		typ := types.NewInterface(methods, embeddeds)
+		typ := types.NewInterface2(methods, embeddeds)
 		r.p.interfaceList = append(r.p.interfaceList, typ)
 		return typ
 	}

src/go/internal/gcimporter/testdata/issue25301.go

+// Copyright 2018 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 issue25301
+
+type (
+	A = interface {
+		M()
+	}
+	T interface {
+		A
+	}
+	S struct{}
+)
+
+func (S) M() { println("m") }

src/go/types/testdata/issues.src

@@ -248,3 +248,25 @@ func issue25438() {
 	if
 	{ /* ERROR missing condition */ }
 }
+
+// Test that we can embed alias type names in interfaces.
+type issue25301 interface {
+	E
+}
+
+type E = interface {
+	m()
+}
+
+// Test case from issue. Eventually we may disallow this due
+// to the cycle via the alias type name. But for now we make
+// sure this is accepted.
+type issue25301b = interface {
+	m() interface{ issue25301b }
+}
+
+type issue25301c interface {
+	notE // ERROR struct\{\} is not an interface
+}
+
+type notE = struct{}

src/go/types/type.go

@@ -243,7 +243,7 @@ func (s *Signature) Variadic() bool { return s.variadic }
 // An Interface represents an interface type.
 type Interface struct {
 	methods   []*Func // ordered list of explicitly declared methods
-	embeddeds []*Named // ordered list of explicitly embedded types
+	embeddeds []Type  // ordered list of explicitly embedded types
 
 	allMethods []*Func // ordered list of methods declared with or embedded in this interface (TODO(gri): replace with mset)
 }
@@ -256,9 +256,29 @@ var emptyInterface = Interface{allMethods: markComplete}
 var markComplete = make([]*Func, 0)
 
 // NewInterface returns a new (incomplete) interface for the given methods and embedded types.
+// Each embedded type must have an underlying type of interface type.
 // NewInterface takes ownership of the provided methods and may modify their types by setting
 // missing receivers. To compute the method set of the interface, Complete must be called.
+//
+// Deprecated: Use NewInterface2 instead which allows any (even non-defined) interface types
+// to be embedded. This is necessary for interfaces that embed alias type names referring to
+// non-defined (literal) interface types.
 func NewInterface(methods []*Func, embeddeds []*Named) *Interface {
+	var tnames []Type
+	if len(embeddeds) > 0 {
+		tnames := make([]Type, len(embeddeds))
+		for i, t := range embeddeds {
+			tnames[i] = t
+		}
+	}
+	return NewInterface2(methods, tnames)
+}
+
+// NewInterface2 returns a new (incomplete) interface for the given methods and embedded types.
+// Each embedded type must have an underlying type of interface type.
+// NewInterface2 takes ownership of the provided methods and may modify their types by setting
+// missing receivers. To compute the method set of the interface, Complete must be called.
+func NewInterface2(methods []*Func, embeddeds []Type) *Interface {
 	typ := new(Interface)
 
 	if len(methods) == 0 && len(embeddeds) == 0 {
@@ -277,8 +297,13 @@ func NewInterface(methods []*Func, embeddeds []*Named) *Interface {
 	}
 	sort.Sort(byUniqueMethodName(methods))
 
-	if embeddeds != nil {
-		sort.Sort(byUniqueTypeName(embeddeds))
+	if len(embeddeds) > 0 {
+		for _, t := range embeddeds {
+			if !IsInterface(t) {
+				panic("embedded type is not an interface")
+			}
+		}
+		sort.Stable(byUniqueTypeName(embeddeds))
 	}
 
 	typ.methods = methods
@@ -296,9 +321,14 @@ func (t *Interface) ExplicitMethod(i int) *Func { return t.methods[i] }
 // NumEmbeddeds returns the number of embedded types in interface t.
 func (t *Interface) NumEmbeddeds() int { return len(t.embeddeds) }
 
-// Embedded returns the i'th embedded type of interface t for 0 <= i < t.NumEmbeddeds().
-// The types are ordered by the corresponding TypeName's unique Id.
-func (t *Interface) Embedded(i int) *Named { return t.embeddeds[i] }
+// Embedded returns the i'th embedded defined (*Named) type of interface t for 0 <= i < t.NumEmbeddeds().
+// The result is nil if the i'th embedded type is not a defined type.
+//
+// Deprecated: Use EmbeddedType which is not restricted to defined (*Named) types.
+func (t *Interface) Embedded(i int) *Named { tname, _ := t.embeddeds[i].(*Named); return tname }
+
+// EmbeddedType returns the i'th embedded type of interface t for 0 <= i < t.NumEmbeddeds().
+func (t *Interface) EmbeddedType(i int) Type { return t.embeddeds[i] }
 
 // NumMethods returns the total number of methods of interface t.
 func (t *Interface) NumMethods() int { return len(t.allMethods) }

src/go/types/typestring_test.go

@@ -146,10 +146,10 @@ func TestIncompleteInterfaces(t *testing.T) {
 	}{
 		{new(Interface), "interface{/* incomplete */}"},
 		{new(Interface).Complete(), "interface{}"},
-		{NewInterface(nil, nil), "interface{/* incomplete */}"},
-		{NewInterface(nil, nil).Complete(), "interface{}"},
-		{NewInterface([]*Func{NewFunc(token.NoPos, nil, "m", sig)}, nil), "interface{m() /* incomplete */}"},
-		{NewInterface([]*Func{NewFunc(token.NoPos, nil, "m", sig)}, nil).Complete(), "interface{m()}"},
+		{NewInterface2(nil, nil), "interface{/* incomplete */}"},
+		{NewInterface2(nil, nil).Complete(), "interface{}"},
+		{NewInterface2([]*Func{NewFunc(token.NoPos, nil, "m", sig)}, nil), "interface{m() /* incomplete */}"},
+		{NewInterface2([]*Func{NewFunc(token.NoPos, nil, "m", sig)}, nil).Complete(), "interface{m()}"},
 	} {
 		got := test.typ.String()
 		if got != test.want {

src/go/types/typexpr.go

@@ -511,10 +511,6 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 				if typ == Typ[Invalid] {
 					continue // error reported before
 				}
-				if !isNamed(typ) {
-					check.invalidAST(f.Type.Pos(), "%s is not a named type", f.Type)
-					continue
-				}
 				embed, _ := typ.Underlying().(*Interface)
 				if embed == nil {
 					check.errorf(f.Type.Pos(), "%s is not an interface", typ)
@@ -528,13 +524,12 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 					unreachable()
 				}
 				// collect interface
-				// (at this point we know that typ must be a named, non-basic type)
-				ityp.embeddeds = append(ityp.embeddeds, typ.(*Named))
+				ityp.embeddeds = append(ityp.embeddeds, typ)
 			}
 		}
-		// sort to match NewInterface
+		// sort to match NewInterface/NewInterface2
 		// TODO(gri) we may be able to switch to source order
-		sort.Sort(byUniqueTypeName(ityp.embeddeds))
+		sort.Stable(byUniqueTypeName(ityp.embeddeds))
 	})
 
 	// compute method set
@@ -605,7 +600,7 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 	}
 	check.context = savedContext
 
-	// sort to match NewInterface
+	// sort to match NewInterface/NewInterface2
 	// TODO(gri) we may be able to switch to source order
 	sort.Sort(byUniqueMethodName(ityp.methods))
 
@@ -617,12 +612,19 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 }
 
 // byUniqueTypeName named type lists can be sorted by their unique type names.
-type byUniqueTypeName []*Named
+type byUniqueTypeName []Type
 
 func (a byUniqueTypeName) Len() int           { return len(a) }
-func (a byUniqueTypeName) Less(i, j int) bool { return a[i].obj.Id() < a[j].obj.Id() }
+func (a byUniqueTypeName) Less(i, j int) bool { return sortName(a[i]) < sortName(a[j]) }
 func (a byUniqueTypeName) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
 
+func sortName(t Type) string {
+	if named, _ := t.(*Named); named != nil {
+		return named.obj.Id()
+	}
+	return ""
+}
+
 // byUniqueMethodName method lists can be sorted by their unique method names.
 type byUniqueMethodName []*Func
 

src/go/types/universe.go

@@ -80,7 +80,7 @@ func defPredeclaredTypes() {
 	res := NewVar(token.NoPos, nil, "", Typ[String])
 	sig := &Signature{results: NewTuple(res)}
 	err := NewFunc(token.NoPos, nil, "Error", sig)
-	typ := &Named{underlying: NewInterface([]*Func{err}, nil).Complete()}
+	typ := &Named{underlying: NewInterface2([]*Func{err}, nil).Complete()}
 	sig.recv = NewVar(token.NoPos, nil, "", typ)
 	def(NewTypeName(token.NoPos, nil, "error", typ))
 }