cmd/gc: disallow fallthrough in final case of switch

Small change to cmd/gc to catch a "fallthrough" in the final case of a switch.

R=golang-dev, rsc, mtj
CC=golang-dev
https://golang.org/cl/7841043

src/cmd/gc/swt.c

@@ -322,6 +322,10 @@ casebody(Node *sw, Node *typeswvar)
 				setlineno(last);
 				yyerror("cannot fallthrough in type switch");
 			}
+			if(l->next == nil) {
+				setlineno(last);
+				yyerror("cannot fallthrough final case in switch");
+			}
 			last->op = OFALL;
 		} else
 			stat = list(stat, br);

test/switch.go

@@ -392,15 +392,6 @@ func main() {
 	}
 	assert(count == 2, "fail")
 
-	// fallthrough in final case.
-	count = 0
-	switch i5 {
-	case 5:
-		count++
-		fallthrough
-	}
-	assert(count == 1, "fail")
-
 	i := 0
 	switch x := 5; {
 	case i < x:

test/switch4.go

+// errorcheck
+
+// Copyright 2011 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 erroneous switch statements are detected by the compiler.
+// Does not compile.
+
+package main
+
+type I interface {
+	M()
+}
+
+func bad() {
+
+	i5 := 5
+	switch i5 {
+	case 5:
+		fallthrough // ERROR "cannot fallthrough final case in switch"
+	}
+}
+
+func good() {
+	var i interface{}
+	var s string
+
+	switch i {
+	case s:
+	}
+
+	switch s {
+	case i:
+	}
+}