test/fixedbugs/bug454.go: add a test for CL 6564052

   Also mention that ignoring second blank identifier of range is required by the spec in the code.

   Fixes #4173.

R=daniel.morsing, remyoudompheng, r
CC=golang-dev
https://golang.org/cl/6594043

src/cmd/gc/range.c

@@ -71,7 +71,11 @@ typecheckrange(Node *n)
 	v2 = N;
 	if(n->list->next)
 		v2 = n->list->next->n;
-	
+
+	// this is not only a optimization but also a requirement in the spec.
+	// "if the second iteration variable is the blank identifier, the range
+	// clause is equivalent to the same clause with only the first variable
+	// present."
 	if(isblank(v2)) {
 		n->list = list1(v1);
 		v2 = N;

test/fixedbugs/bug454.go

+// run
+
+// Copyright 2012 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 4173
+
+package main
+
+func main() {
+	var arr *[10]int
+	s := 0
+	for i, _ := range arr {
+		// used to panic trying to access arr[i]
+		s += i
+	}
+	if s != 45 {
+		println("BUG")
+	}
+}