cmd/compile: require -lang=go1.14 for overlapping interfaces

Support for overlapping interfaces is a new (proposed) Go language
feature to be supported in Go 1.14, so it shouldn't be supported under
-lang=go1.13 or earlier.

Fixes #34329.

Change-Id: I5fea5716b7d135476980bc40b4f6e8c611b67735
Reviewed-on: https://go-review.googlesource.com/c/go/+/195678
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -34,7 +34,7 @@ func expandiface(t *types.Type) {
 		switch prev := seen[m.Sym]; {
 		case prev == nil:
 			seen[m.Sym] = m
-		case !explicit && types.Identical(m.Type, prev.Type):
+		case langSupported(1, 14) && !explicit && types.Identical(m.Type, prev.Type):
 			return
 		default:
 			yyerrorl(m.Pos, "duplicate method %s", m.Sym.Name)

src/go/types/stdlib_test.go

@@ -181,6 +181,7 @@ func TestStdFixed(t *testing.T) {
 		"issue25507.go",  // go/types does not have constraints on stack size
 		"issue20780.go",  // go/types does not have constraints on stack size
 		"issue31747.go",  // go/types does not have constraints on language level (-lang=go1.12) (see #31793)
+		"issue34329.go",  // go/types does not have constraints on language level (-lang=go1.13) (see #31793)
 	)
 }
 

test/fixedbugs/issue34329.go

+// errorcheck -lang=go1.13
+
+// Copyright 2019 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 p
+
+type I interface { M() }
+
+type _ interface {
+	I
+	I // ERROR "duplicate method M"
+}