Allow defining opaque types during trait object upcasting.

No stable code is affected, as this requires the `trait_upcasting` feature gate.

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2631,7 +2631,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                     nested.extend(
                         self.infcx
                             .at(&obligation.cause, obligation.param_env)
-                            .eq(DefineOpaqueTypes::No, source_projection, target_projection)
+                            .eq(DefineOpaqueTypes::Yes, source_projection, target_projection)
                             .map_err(|_| SelectionError::Unimplemented)?
                             .into_obligations(),
                     );

tests/ui/traits/trait-upcasting/upcast-defining-opaque.current.stderr

@@ -1,17 +0,0 @@
-error[E0308]: mismatched types
-  --> $DIR/upcast-defining-opaque.rs:21:5
-   |
-LL | type Foo = impl Sized;
-   |            ---------- the found opaque type
-LL |
-LL | fn upcast(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
-   |                                        ----------------------- expected `&dyn Super<Assoc = i32>` because of return type
-LL |     x
-   |     ^ expected trait `Super`, found trait `Sub`
-   |
-   = note: expected reference `&dyn Super<Assoc = i32>`
-              found reference `&dyn Sub<Assoc = Foo>`
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/trait-upcasting/upcast-defining-opaque.rs

@@ -1,7 +1,7 @@
 //@ revisions: current next
 //@[next] compile-flags: -Znext-solver
 //@ ignore-compare-mode-next-solver (explicit revisions)
-//@[next] check-pass
+//@check-pass
 
 #![feature(trait_upcasting, type_alias_impl_trait)]
 
@@ -18,7 +18,7 @@ impl<T: ?Sized> Super for T {
 type Foo = impl Sized;
 
 fn upcast(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
-    x //[current]~ mismatched types
+    x
 }
 
 fn main() {}