cmd/compile: const name and label name may match

Fixes #13684.

Change-Id: I3977119b6eb1d6b7dc2ea1e7d6656a8f0d421bc1
Reviewed-on: https://go-review.googlesource.com/18060
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

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

@@ -667,9 +667,9 @@ func (p *parser) simple_stmt(labelOk, rangeOk bool) *Node {
 			// labelname ':' stmt
 			if labelOk {
 				// If we have a labelname, it was parsed by operand
-				// (calling p.name()) and given an ONAME, ONONAME, OTYPE, or OPACK node.
+				// (calling p.name()) and given an ONAME, ONONAME, OTYPE, OPACK, or OLITERAL node.
 				switch lhs.Op {
-				case ONAME, ONONAME, OTYPE, OPACK:
+				case ONAME, ONONAME, OTYPE, OPACK, OLITERAL:
 					lhs = newname(lhs.Sym)
 				default:
 					p.syntax_error("expecting semicolon or newline or }")

test/fixedbugs/issue13684.go

+// run
+
+// Copyright 2015 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.
+
+// Verify that a label name matching a constant name
+// is permitted.
+
+package main
+
+const labelname = 1
+
+func main() {
+	goto labelname
+labelname:
+}