Add test for using existential types in associated types

src/test/ui/impl-trait/existential-associated-type.rs

@@ -0,0 +1,36 @@
+// Copyright 2018 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(existential_type)]
+// compile-pass
+
+trait Bar {}
+struct Dummy;
+impl Bar for Dummy {}
+
+trait Foo {
+    type Assoc: Bar;
+    fn foo() -> Self::Assoc;
+    fn bar() -> Self::Assoc;
+}
+
+existential type Helper: Bar;
+
+impl Foo for i32 {
+    type Assoc = Helper;
+    fn foo() -> Helper {
+        Dummy
+    }
+    fn bar() -> Helper {
+        Dummy
+    }
+}
+
+fn main() {}