cmd/gc: mark wrapper methods for unnamed types as DUPOK.

Unnamed types like structs with embedded fields can have methods.
These methods are generated on-the-fly by the compiler and
it may happen for identical types in different packages.
The linker must accept these multiple definitions.

Fixes #4590.

R=golang-dev, rsc
CC=golang-dev, remy
https://golang.org/cl/7030051

src/cmd/gc/subr.c

@@ -2521,6 +2521,9 @@ genwrapper(Type *rcvr, Type *method, Sym *newnam, int iface)
 
 	funcbody(fn);
 	curfn = fn;
+	// wrappers where T is anonymous (struct{ NamedType }) can be duplicated.
+	if(rcvr->etype == TSTRUCT || isptr[rcvr->etype] && rcvr->type->etype == TSTRUCT)
+		fn->dupok = 1;
 	typecheck(&fn, Etop);
 	typechecklist(fn->nbody, Etop);
 	curfn = nil;

test/fixedbugs/issue4590.dir/pkg1.go

+// Copyright 2012 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 pkg1
+
+type A interface {
+	Write() error
+}
+
+type B interface {
+	Hello()
+	world()
+}
+
+type C struct{}
+
+func (c C) Write() error { return nil }
+
+var T = struct{ A }{nil}
+var U = struct{ B }{nil}
+var V A = struct{ *C }{nil}
+var W = interface {
+	Write() error
+	Hello()
+}(nil)

test/fixedbugs/issue4590.dir/pkg2.go

+// Copyright 2012 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 pkg2
+
+import "./pkg1"
+
+var T = struct{ pkg1.A }{nil}
+var U = struct{ pkg1.B }{nil}
+var V pkg1.A = struct{ *pkg1.C }{nil}
+var W = interface {
+	Write() error
+	Hello()
+}(nil)

test/fixedbugs/issue4590.dir/prog.go

+// Copyright 2012 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 main
+
+import (
+	"./pkg1"
+	"./pkg2"
+)
+
+func main() {
+	if pkg1.T != pkg2.T {
+		panic("pkg1.T != pkg2.T")
+	}
+	if pkg1.U != pkg2.U {
+		panic("pkg1.U != pkg2.U")
+	}
+	if pkg1.V != pkg2.V {
+		panic("pkg1.V != pkg2.V")
+	}
+	if pkg1.W != pkg2.W {
+		panic("pkg1.W != pkg2.W")
+	}
+}

test/fixedbugs/issue4590.go

+// rundir
+
+// Copyright 2012 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.
+
+// Issue 4590: linker fails on multiple imports of
+// an anonymous struct with methods.
+
+package ignored