Support constraining opaque types while trait upcasting with binders

Author: oli-obk

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

@@ -2594,7 +2594,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                         self.infcx
                             .at(&obligation.cause, obligation.param_env)
                             .eq(
-                                DefineOpaqueTypes::No,
+                                DefineOpaqueTypes::Yes,
                                 upcast_principal.map_bound(|trait_ref| {
                                     ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
                                 }),

tests/ui/traits/trait-upcasting/type-checking-test-opaques.rs

@@ -1,5 +1,7 @@
 #![feature(trait_upcasting, type_alias_impl_trait)]
 
+//@ check-pass
+
 type Tait = impl Sized;
 
 trait Foo<'a>: Bar<'a, 'a, Tait> {}
@@ -15,7 +17,6 @@ fn test_correct2<'a>(x: &dyn Foo<'a>) {
 
 fn test_correct3<'a>(x: &dyn Foo<'a>, _: Tait) {
     let _ = x as &dyn Bar<'_, '_, ()>;
-    //~^ ERROR: non-primitive cast
 }
 
 fn main() {}