partially revert acba644

This avoids the ICE in `tests/ui/type-alias-impl-trait/reveal_local.rs`.

Author: lqd

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

@@ -2362,17 +2362,18 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             }
 
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
-                if self.infcx.can_define_opaque_ty(def_id) {
-                    unreachable!()
-                } else {
-                    // We can resolve the `impl Trait` to its concrete type,
-                    // which enforces a DAG between the functions requiring
-                    // the auto trait bounds in question.
-                    match self.tcx().type_of_opaque(def_id) {
-                        Ok(ty) => t.rebind(vec![ty.instantiate(self.tcx(), args)]),
-                        Err(_) => {
-                            return Err(SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id));
-                        }
+                // FIXME(new-solver): because we're still using the global cache
+                // when defining opaque types, we can reach here, but shouldn't.
+                // The test `type-alias-impl-trait/reveal_local.rs` is an
+                // example.
+
+                // We can resolve the `impl Trait` to its concrete type,
+                // which enforces a DAG between the functions requiring
+                // the auto trait bounds in question.
+                match self.tcx().type_of_opaque(def_id) {
+                    Ok(ty) => t.rebind(vec![ty.instantiate(self.tcx(), args)]),
+                    Err(_) => {
+                        return Err(SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id));
                     }
                 }
             }