cmd/gc: reject incorrect use of fallthrough.

Fixes #6500.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/14920053

src/cmd/gc/go.y

@@ -557,6 +557,7 @@ caseblock:
 		// This is so that the stmt_list action doesn't look at
 		// the case tokens if the stmt_list is empty.
 		yylast = yychar;
+		$1->xoffset = block;
 	}
 	stmt_list
 	{
@@ -1730,6 +1731,7 @@ non_dcl_stmt:
 	{
 		// will be converted to OFALL
 		$$ = nod(OXFALL, N, N);
+		$$->xoffset = block;
 	}
 |	LBREAK onew_name
 	{

src/cmd/gc/swt.c

@@ -317,7 +317,7 @@ casebody(Node *sw, Node *typeswvar)
 
 		// botch - shouldn't fall thru declaration
 		last = stat->end->n;
-		if(last->op == OXFALL) {
+		if(last->xoffset == n->xoffset && last->op == OXFALL) {
 			if(typeswvar) {
 				setlineno(last);
 				yyerror("cannot fallthrough in type switch");

test/fixedbugs/issue6500.go

@@ -0,0 +1,29 @@
+// errorcheck
+
+// Copyright 2014 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.
+
+// Issue 6500: missing error when fallthrough appears in a block.
+
+package main
+
+func main() {
+	var x int
+	switch x {
+	case 0:
+		{
+			fallthrough // ERROR "fallthrough"
+		}
+	case 1:
+		{
+			switch x {
+			case 2:
+				fallthrough
+			case 3:
+			}
+		}
+		fallthrough
+	default:
+	}
+}