review comments

src/librustc_typeck/astconv.rs

@@ -1231,6 +1231,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         trait_ref: ty::TraitRef<'tcx>,
     ) -> ty::ExistentialTraitRef<'tcx> {
         if trait_ref.self_ty() != self.tcx().types.trait_object_dummy_self {
+            // FIXME: There appears to be a missing filter on top of `expand_trait_aliases`, which
+            // picks up non-supertraits where clauses - but also, the object safety completely
+            // ignores trait aliases, which could be object safety hazards. We `delay_span_bug`
+            // here to avoid an ICE in stable even when the feature is disabled. (#66420)
             self.tcx().sess.delay_span_bug(DUMMY_SP, &format!(
                 "trait_ref_to_existential called on {:?} with non-dummy Self",
                 trait_ref,

src/test/ui/issues/issue-65673.rs

@@ -1,4 +1,4 @@
-#![feature(trait_alias)]
+#![feature(trait_alias)] // Enabled to reduce stderr output, but can be triggered even if disabled.
 trait Trait {}
 trait WithType {
     type Ctx;