gc: fix typed constant declarations

Fixes #1411.

R=rsc, gri, ejsherry
CC=golang-dev
https://golang.org/cl/4035042

src/cmd/gc/walk.c

@@ -269,9 +269,15 @@ walkdef(Node *n)
 		}
 		t = n->type;
 		if(t != T) {
-			convlit(&e, t);
-			if(!okforconst[t->etype])
+			if(!okforconst[t->etype]) {
 				yyerror("invalid constant type %T", t);
+				goto ret;
+			}
+			if(!isideal(e->type) && !eqtype(t, e->type)) {
+				yyerror("cannot use %+N as type %T in const initializer", e, t);
+				goto ret;
+			}
+			convlit(&e, t);
 		}
 		n->val = e->val;
 		n->type = e->type;

test/fixedbugs/bug318.go

+// errchk $G $D/$F.go
+
+// 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.
+
+// Issue 1411.
+
+package main
+
+const ui uint = 0
+const i int = ui // ERROR "type"