compiler: Tweak untyped nil error message.

From-SVN: r205941

gcc/go/gofrontend/expressions.cc

@@ -7310,7 +7310,11 @@ Builtin_call_expression::do_lower(Gogo* gogo, Named_object* function,
 	Type* slice_type = args->front()->type();
 	if (!slice_type->is_slice_type())
 	  {
-	    error_at(args->front()->location(), "argument 1 must be a slice");
+	    if (slice_type->is_nil_type())
+	      error_at(args->front()->location(), "use of untyped nil");
+	    else
+	      error_at(args->front()->location(),
+		       "argument 1 must be a slice");
 	    this->set_is_error();
 	    return this;
 	  }
@@ -8008,7 +8012,10 @@ Builtin_call_expression::do_type()
 	const Expression_list* args = this->args();
 	if (args == NULL || args->empty())
 	  return Type::make_error_type();
-	return args->front()->type();
+	Type *ret = args->front()->type();
+	if (!ret->is_slice_type())
+	  return Type::make_error_type();
+	return ret;
       }
 
     case BUILTIN_REAL: