If we find a blanket impl for Trait but we're matching on an object

`Trait`, prefer the object. Also give a nice error for attempts to
manually `impl Trait for Trait`, since they will be ineffectual.

Fixes #24015.

Fixes #24051.
Fixes #24037.
Fixes #23853.
Fixes #21942.
cc #21756.

src/test/compile-fail/coherence-impl-trait-for-trait.rs

@@ -0,0 +1,32 @@
+// 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 we give suitable error messages when the user attempts to
+// impl a trait `Trait` for its own object type.
+
+trait Foo { fn dummy(&self) { } }
+trait Bar: Foo { }
+trait Baz: Bar { }
+
+// Subtraits of Baz are not legal:
+impl Foo for Baz { }   //~ ERROR E0371
+impl Bar for Baz { }   //~ ERROR E0371
+impl Baz for Baz { }   //~ ERROR E0371
+
+// But other random traits are:
+trait Other { }
+impl Other for Baz { } // OK, Bar not a subtrait of Baz
+
+// If the trait is not object-safe, we give a more tailored message
+// because we're such schnuckels:
+trait NotObjectSafe { fn eq(&self, other: Self); }
+impl NotObjectSafe for NotObjectSafe { } //~ ERROR E0372
+
+fn main() { }

src/test/run-pass/traits-impl-object-overlap-issue-23853.rs

@@ -0,0 +1,27 @@
+// 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 we are able to compile the case where both a blanket impl
+// and the object type itself supply the required trait obligation.
+// In this case, the blanket impl for `Foo` applies to any type,
+// including `Bar`, but the object type `Bar` also implicitly supplies
+// this context.
+
+trait Foo { fn dummy(&self) { } }
+
+trait Bar: Foo { }
+
+impl<T:?Sized> Foo for T { }
+
+fn want_foo<B:?Sized+Foo>() { }
+
+fn main() {
+    want_foo::<Bar>();
+}