Subtype predicates only exist on inference types, so we can allow them to register opaque types within them.

Author: oli-obk

compiler/rustc_infer/src/infer/mod.rs

@@ -945,14 +945,20 @@ impl<'tcx> InferCtxt<'tcx> {
             (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
                 return Err((a_vid, b_vid));
             }
+            (ty::Alias(ty::Opaque, ..), ty::Infer(ty::TyVar(_)))
+            | (ty::Infer(ty::TyVar(_)), ty::Alias(ty::Opaque, ..)) => {}
+            (ty::Alias(ty::Opaque, ..), _) | (_, ty::Alias(ty::Opaque, ..)) => span_bug!(
+                cause.span(),
+                "opaque types got hidden types registered from within subtype predicate: {r_a:?} vs {r_b:?}"
+            ),
             _ => {}
         }
 
         self.enter_forall(predicate, |ty::SubtypePredicate { a_is_expected, a, b }| {
             if a_is_expected {
-                Ok(self.at(cause, param_env).sub(DefineOpaqueTypes::No, a, b))
+                Ok(self.at(cause, param_env).sub(DefineOpaqueTypes::Yes, a, b))
             } else {
-                Ok(self.at(cause, param_env).sup(DefineOpaqueTypes::No, b, a))
+                Ok(self.at(cause, param_env).sup(DefineOpaqueTypes::Yes, b, a))
             }
         })
     }