Extend coherence check to understand subtyping.

src/test/compile-fail/coherence-subtyping.rs

@@ -0,0 +1,50 @@
+// 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.
+
+// Test that two distinct impls which match subtypes of one another
+// yield coherence errors (or not) depending on the variance.
+
+trait Contravariant {
+    fn foo(&self) { }
+}
+
+impl Contravariant for for<'a,'b> fn(&'a u8, &'b u8) {
+    //~^ ERROR E0119
+}
+
+impl Contravariant for for<'a> fn(&'a u8, &'a u8) {
+}
+
+///////////////////////////////////////////////////////////////////////////
+
+trait Covariant {
+    fn foo(&self) { }
+}
+
+impl Covariant for for<'a,'b> fn(&'a u8, &'b u8) {
+    //~^ ERROR E0119
+}
+
+impl Covariant for for<'a> fn(&'a u8, &'a u8) {
+}
+
+///////////////////////////////////////////////////////////////////////////
+
+trait Invariant {
+    fn foo(&self) -> Self { }
+}
+
+impl Invariant for for<'a,'b> fn(&'a u8, &'b u8) {
+}
+
+impl Invariant for for<'a> fn(&'a u8, &'a u8) {
+}
+
+fn main() { }