cmd/compile/internal/gc: mark generated wrappers as DUPOK

Interface wrapper functions now get compiled eagerly in some cases.
Consequently, they may be present in multiple translation units.
Mark them as DUPOK, just like closures.

Fixes #19548
Fixes #19550

Change-Id: Ibe74adb5a62dbf6447db37fde22dcbb3479969ef
Reviewed-on: https://go-review.googlesource.com/38156
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/subr.go

@@ -1711,6 +1711,7 @@ func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 	t.Rlist.Set(out)
 
 	fn := nod(ODCLFUNC, nil, nil)
+	fn.Func.SetDupok(true)
 	fn.Func.Nname = newname(newnam)
 	fn.Func.Nname.Name.Defn = fn
 	fn.Func.Nname.Name.Param.Ntype = t

test/fixedbugs/issue19548.dir/a.go

@@ -0,0 +1,26 @@
+// Copyright 2016 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 a
+
+type Mode uint
+
+func (m Mode) String() string { return "mode string" }
+func (m *Mode) Addr() *Mode   { return m }
+
+type Stringer interface {
+	String() string
+}
+
+var global Stringer
+var m Mode
+
+func init() {
+	// force compilation of the (*Mode).String() wrapper
+	global = &m
+}
+
+func String() string {
+	return global.String() + Mode(0).String()
+}

test/fixedbugs/issue19548.dir/b.go

@@ -0,0 +1,24 @@
+// Copyright 2016 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 "./a"
+
+type Value interface {
+	a.Stringer
+	Addr() *a.Mode
+}
+
+var global a.Mode
+
+func f() int {
+	var v Value
+	v = &global
+	return int(v.String()[0])
+}
+
+func main() {
+	f()
+}

test/fixedbugs/issue19548.go

@@ -0,0 +1,9 @@
+// rundir
+
+// Copyright 2017 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.
+
+// Test that interface wrappers can be compiled successfully
+// in multiple translation units.
+package ignore