Check that traits with default impls have no methods

fixes #23080

src/test/compile-fail/issue-23080.rs

@@ -0,0 +1,29 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(optin_builtin_traits)]
+
+unsafe trait Trait {
+//~^ error: traits with default impls (`e.g. impl Trait for ..`) must have no methods
+    fn method(&self) {
+        println!("Hello");
+    }
+}
+
+unsafe impl Trait for .. {}
+
+fn call_method<T: Trait>(x: T) {
+    x.method();
+}
+
+fn main() {
+    // ICE
+    call_method(());
+}