cmd/compile: recognize labels even if they have the same name as packages

Another (historic) artifact due to partially resolving symbols too early.

Fixes #13539.

Change-Id: Ie720c491cfa399599454f384b3a9735e75d4e8f1
Reviewed-on: https://go-review.googlesource.com/17600
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -668,10 +668,11 @@ 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, or OTYPE node.
+				// (calling p.name()) and given an ONAME, ONONAME, OTYPE, or OPACK node.
-				if lhs.Op == ONAME || lhs.Op == ONONAME || lhs.Op == OTYPE {
+				switch lhs.Op {
+				case ONAME, ONONAME, OTYPE, OPACK:
 					lhs = newname(lhs.Sym)
-				} else {
+				default:
 					p.syntax_error("expecting semicolon or newline or }")
 					// we already progressed, no need to advance
 				}

test/fixedbugs/issue13539.go

@@ -0,0 +1,20 @@
+// errorcheck
+
+// 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 named like a package is recognized
+// as a label rather than a package and that the package
+// remains unused.
+
+package main
+
+import "math" // ERROR "imported and not used"
+
+func main() {
+math:
+	for {
+		break math
+	}
+}