cmd/gc: fix internal crash

TBR=ken2
CC=golang-codereviews
https://golang.org/cl/70200053

src/cmd/gc/typecheck.c

@@ -1358,6 +1358,8 @@ reswitch:
 			goto error;
 		defaultlit(&n->left, T);
 		defaultlit(&n->right, T);
+		if(n->left->type == T || n->right->type == T)
+			goto error;
 
 		// copy([]byte, string)
 		if(isslice(n->left->type) && n->right->type->etype == TSTRING) {

test/fixedbugs/issue7310.go

+// errorcheck
+
+// Copyright 2014 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.
+
+// Internal compiler crash used to stop errors during second copy.
+
+package main
+
+func main() {
+	_ = copy(nil, []int{}) // ERROR "use of untyped nil"
+	_ = copy([]int{}, nil) // ERROR "use of untyped nil"
+	_ = 1+true // ERROR "cannot convert true" "mismatched types int and bool"
+}