Make hidden type registration opt-in, so that each site can be reviewed on its own and we have the right defaults for trait solvers

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

@@ -821,6 +821,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         self.infcx
             .at(&obligation.cause, obligation.param_env)
+            // needed for tests/ui/type-alias-impl-trait/assoc-projection-ice.rs
+            .define_opaque_types(true)
             .sup(obligation_trait_ref, expected_trait_ref)
             .map(|InferOk { mut obligations, .. }| {
                 obligations.extend(nested);