Add downstream tests for intercrate ambiguity hints.

src/test/compile-fail/coherence-overlap-downstream-inherent.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.
+
+// Tests that we consider `T: Sugar + Fruit` to be ambiguous, even
+// though no impls are found.
+
+struct Sweet<X>(X);
+pub trait Sugar {}
+pub trait Fruit {}
+impl<T:Sugar> Sweet<T> { fn dummy(&self) { } }
+//~^ ERROR E0592
+//~| NOTE duplicate definitions for `dummy`
+impl<T:Fruit> Sweet<T> { fn dummy(&self) { } }
+//~^ NOTE other definition for `dummy`
+
+trait Bar<X> {}
+struct A<T, X>(T, X);
+impl<X, T> A<T, X> where T: Bar<X> { fn f(&self) {} }
+//~^ ERROR E0592
+//~| NOTE duplicate definitions for `f`
+//~| NOTE downstream crates may implement trait `Bar<_>` for type `i32`
+impl<X> A<i32, X> { fn f(&self) {} }
+//~^ NOTE other definition for `f`
+
+fn main() {}

src/test/compile-fail/coherence-overlap-downstream.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.
+
+// Tests that we consider `T: Sugar + Fruit` to be ambiguous, even
+// though no impls are found.
+
+pub trait Sugar {}
+pub trait Fruit {}
+pub trait Sweet {}
+impl<T:Sugar> Sweet for T { }
+//~^ NOTE first implementation here
+impl<T:Fruit> Sweet for T { }
+//~^ ERROR E0119
+//~| NOTE conflicting implementation
+
+pub trait Foo<X> {}
+pub trait Bar<X> {}
+impl<X, T> Foo<X> for T where T: Bar<X> {}
+//~^ NOTE first implementation here
+impl<X> Foo<X> for i32 {}
+//~^ ERROR E0119
+//~| NOTE conflicting implementation for `i32`
+//~| NOTE downstream crates may implement trait `Bar<_>` for type `i32`
+
+fn main() { }