cmd/compile: don't crash on invalid labeled statement

Fixes #14006.

Change-Id: Ia819073677ad6993c02255e23760ee21598427b4
Reviewed-on: https://go-review.googlesource.com/18736
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -668,9 +668,14 @@ func (p *parser) simple_stmt(labelOk, rangeOk bool) *Node {
 			if labelOk {
 				// If we have a labelname, it was parsed by operand
 				// (calling p.name()) and given an ONAME, ONONAME, OTYPE, OPACK, or OLITERAL node.
+				// We only have a labelname if there is a symbol (was issue 14006).
 				switch lhs.Op {
 				case ONAME, ONONAME, OTYPE, OPACK, OLITERAL:
-					lhs = newname(lhs.Sym)
+					if lhs.Sym != nil {
+						lhs = newname(lhs.Sym)
+						break
+					}
+					fallthrough
 				default:
 					p.syntax_error("expecting semicolon or newline or }")
 					// we already progressed, no need to advance

test/fixedbugs/issue14006.go

@@ -0,0 +1,64 @@
+// errorcheck
+
+// 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.
+
+// Literals that happen to resolve to named constants
+// may be used as label names (see issue 13684). Make
+// sure that other literals don't crash the compiler.
+
+package main
+
+const labelname = 1
+
+func main() {
+	goto labelname
+labelname:
+}
+
+func f() {
+	var x int
+	switch x {
+	case 1:
+		2:	// ERROR "unexpected :"
+	case 2:
+	}
+
+	switch x {
+	case 1:
+		2: ;	// ERROR "unexpected :"
+	case 2:
+	}
+
+	var y string
+	switch y {
+	case "foo":
+		"bar":	// ERROR "unexpected :"
+	case "bar":
+	}
+
+	switch y {
+	case "foo":
+		"bar": ;	// ERROR "unexpected :"
+	case "bar":
+	}
+
+	var z bool
+	switch {
+	case z:
+		labelname:	// ERROR "missing statement after label"
+	case false:
+	}
+
+	switch {
+	case z:
+		labelname:
+	}
+
+	switch {
+	case z:
+		labelname: ;
+	case false:
+	}
+}