cmd/gc: diagnose '_ = nil' better

Fixes #6004.

R=ken2
CC=golang-dev
https://golang.org/cl/13616044

src/cmd/gc/subr.c

@@ -1411,6 +1411,9 @@ assignconv(Node *n, Type *t, char *context)
 	if(n == N || n->type == T || n->type->broke)
 		return n;
 
+	if(t->etype == TBLANK && n->type->etype == TNIL)
+		yyerror("use of untyped nil");
+
 	old = n;
 	old->diag++;  // silence errors about n; we'll issue one below
 	defaultlit(&n, t);

test/fixedbugs/issue6004.go

+// errorcheck
+
+// Copyright 2013 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.
+
+package main
+
+func main() {
+	_ = nil // ERROR "use of untyped nil"
+	_, _ = nil, 1 // ERROR "use of untyped nil"
+	_, _ = 1, nil // ERROR "use of untyped nil"
+	_ = append(nil, 1, 2, 3) // ERROR "untyped nil"
+}
+