cmd/gc: mark broken type declarations as broken.

This fixes a spurious 'invalid recursive type' error, and stops the compiler from emitting errors on uses of the invalid type.

Fixes #3766.

R=golang-dev, dave, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6443100

src/cmd/gc/typecheck.c

@@ -2715,6 +2715,7 @@ typecheckdeftype(Node *n)
 	typecheck(&n->ntype, Etype);
 	if((t = n->ntype->type) == T) {
 		n->diag = 1;
+		n->type = T;
 		goto ret;
 	}
 	if(n->type == T) {

test/fixedbugs/bug451.go

-// run
+// errorcheck
 
-// Copyright 2012 The Go Authors.  All rights reserved.
+// 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 3835: 8g tries to optimize arithmetic involving integer
-// constants, but can run out of registers in the process.
-
 package main
 
-var a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, A, B, C, D, E, F, G int
-
-func foo() int {
-	return a + 1 + b + 2 + c + 3 + d + 4 + e + 5 + f + 6 + g + 7 + h + 8 + i + 9 + j + 10 +
-		k + 1 + l + 2 + m + 3 + n + 4 + o + 5 + p + 6 + q + 7 + r + 8 + s + 9 + t + 10 +
-		u + 1 + v + 2 + w + 3 + x + 4 + y + 5 + z + 6 + A + 7 + B + 8 + C + 9 + D + 10 +
-		E + 1 + F + 2 + G + 3
-}
-
-func bar() int8 {
-	var (
-		W int16
-		X int32
-		Y int32
-		Z int32
-	)
-	return int8(W+int16(X+3)+3) * int8(Y+3+Z*3)
-}
+type T x.T // ERROR "undefined"
 
-func main() {
-	if foo() == 0 {
-		panic("foo")
-	}
-	if bar() == 0 {
-		panic("bar")
-	}
-}
+// bogus "invalid recursive type"