select.rs: unsizing coercion should use a subtype

When we coerce `dyn Foo` to `dyn Bar`, that is OK as long as `Foo` is
usable in all contexts where `Bar` is usable (hence using the source
must be a subtype of the target).

This is needed for the universe-based code to handle
`old-lub-glb-object`; that test used to work sort of by accident
before with the old code.

Author: nikomatsakis

src/librustc/traits/select.rs

@@ -3268,10 +3268,10 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                         );
                     tcx.mk_existential_predicates(iter)
                 });
-                let new_trait = tcx.mk_dynamic(existential_predicates, r_b);
+                let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
                 let InferOk { obligations, .. } = self.infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .eq(target, new_trait)
+                    .sup(target, source_trait)
                     .map_err(|_| Unimplemented)?;
                 nested.extend(obligations);
 

src/test/ui/lub-glb/old-lub-glb-object.rs

@@ -7,7 +7,7 @@ fn foo(
     x: &for<'a, 'b> Foo<&'a u8, &'b u8>,
     y: &for<'a> Foo<&'a u8, &'a u8>,
 ) {
-    let z = match 22 { //~ ERROR incompatible types
+    let z = match 22 { //~ ERROR cannot infer
         0 => x,
         _ => y,
     };

src/test/ui/lub-glb/old-lub-glb-object.stderr

@@ -1,17 +1,19 @@
 error[E0308]: match arms have incompatible types
   --> $DIR/old-lub-glb-object.rs:10:13
    |
-LL |       let z = match 22 { //~ ERROR incompatible types
+LL |       let z = match 22 { //~ ERROR cannot infer
    |  _____________^
 LL | |         0 => x,
 LL | |         _ => y,
-   | |              - match arm with an incompatible type
 LL | |     };
-   | |_____^ expected bound lifetime parameter 'a, found concrete lifetime
+   | |_____^
    |
-   = note: expected type `&dyn for<'a, 'b> Foo<&'a u8, &'b u8>`
-              found type `&dyn for<'a> Foo<&'a u8, &'a u8>`
+   = note: first, the lifetime cannot outlive lifetime RePlaceholder(Placeholder { universe: U5, name: BrNamed(crate0:DefIndex(1:11), 'a) })...
+   = note: ...but the lifetime must also be valid for lifetime RePlaceholder(Placeholder { universe: U5, name: BrNamed(crate0:DefIndex(1:12), 'b) })...
+   = note: ...so that the types are compatible:
+           expected dyn for<'a, 'b> Foo<&'a u8, &'b u8>
+              found dyn for<'a> Foo<&'a u8, &'a u8>
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0495`.