Add test cases for cyclic specialization graph construction

src/test/compile-fail/coherence-inherited-assoc-ty-cycle-err.rs

@@ -0,0 +1,34 @@
+// Copyright 2017 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.
+
+// Formerly this ICEd with the following message:
+// Tried to project an inherited associated type during coherence checking,
+// which is currently not supported.
+//
+// No we expect to run into a more user-friendly cycle error instead.
+
+#![feature(specialization)]
+
+trait Trait<T> { type Assoc; }
+//~^ unsupported cyclic reference between types/traits detected [E0391]
+
+impl<T> Trait<T> for Vec<T> {
+    type Assoc = ();
+}
+
+impl Trait<u8> for Vec<u8> {}
+
+impl<T> Trait<T> for String {
+    type Assoc = ();
+}
+
+impl Trait<<Vec<u8> as Trait<u8>>::Assoc> for String {}
+
+fn main() {}

src/test/run-pass/specialization/assoc-ty-graph-cycle.rs

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+// Copyright 2017 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.
+
+// Make sure we don't crash with a cycle error during coherence.
+
+#![feature(specialization)]
+
+trait Trait<T> {
+    type Assoc;
+}
+
+impl<T> Trait<T> for Vec<T> {
+    default type Assoc = ();
+}
+
+impl Trait<u8> for Vec<u8> {
+    type Assoc = u8;
+}
+
+impl<T> Trait<T> for String {
+    type Assoc = ();
+}
+
+impl Trait<<Vec<u8> as Trait<u8>>::Assoc> for String {}
+
+fn main() {}