Allow defining opaque types during trait object upcasting.

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

Author: oli-obk

Diff for: 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(),
                     );

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

@@ -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() {}