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partially revert 904a0bd

This preserves the error you currently get on stable for the
old-lub-glb-object.rs test.
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nikomatsakis committed Feb 21, 2019
1 parent 21e9478 commit 33d3598e3b57c0fcfd36f34c346847bcc4edd3d2
@@ -3315,9 +3315,27 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
tcx.mk_existential_predicates(iter)
});
let source_trait = tcx.mk_dynamic(existential_predicates, r_b);

// Require that the traits involved in this upcast are **equal**;
// only the **lifetime bound** is changed.
//
// FIXME: This condition is arguably too strong -- it
// would suffice for the source trait to be a
// *subtype* of the target trait. In particular
// changing from something like `for<'a, 'b> Foo<'a,
// 'b>` to `for<'a> Foo<'a, 'a>` should be
// permitted. And, indeed, in the in commit
// 904a0bde93f0348f69914ee90b1f8b6e4e0d7cbc, this
// condition was loosened. However, when the leak check was added
// back, using subtype here actually guies the coercion code in
// such a way that it accepts `old-lub-glb-object.rs`. This is probably
// a good thing, but I've modified this to `.eq` because I want
// to continue rejecting that test (as we have done for quite some time)
// before we are firmly comfortable with what our behavior
// should be there. -nikomatsakis
let InferOk { obligations, .. } = self.infcx
.at(&obligation.cause, obligation.param_env)
.sup(target, source_trait)
.eq(target, source_trait) // FIXME -- see below
.map_err(|_| Unimplemented)?;
nested.extend(obligations);

@@ -1,9 +1,5 @@
// Test that we give a note when the old LUB/GLB algorithm would have
// succeeded but the new code (which is stricter) gives an error.
//
// compile-pass
//
// TODO -- why does this test pass?

trait Foo<T, U> { }

@@ -13,7 +9,7 @@ fn foo(
) {
let z = match 22 {
0 => x,
_ => y,
_ => y, //~ ERROR match arms have incompatible types
};
}

@@ -0,0 +1,18 @@
error[E0308]: match arms have incompatible types
--> $DIR/old-lub-glb-object.rs:12:14
|
LL | let z = match 22 {
| _____________-
LL | | 0 => x,
| | - this is found to be of type `&dyn for<'a, 'b> Foo<&'a u8, &'b u8>`
LL | | _ => y, //~ ERROR match arms have incompatible types
| | ^ expected bound lifetime parameter 'a, found concrete lifetime
LL | | };
| |_____- `match` arms have incompatible types
|
= note: expected type `&dyn for<'a, 'b> Foo<&'a u8, &'b u8>`
found type `&dyn for<'a> Foo<&'a u8, &'a u8>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

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