check cls in is_array_container_type has an mro

Author: alexfikl

arraycontext/container/__init__.py

@@ -173,7 +173,12 @@ def is_array_container_type(cls: type) -> bool:
         function will say that :class:`numpy.ndarray` is an array container
         type, only object arrays *actually are* array containers.
     """
-    return (
+    # NOTE: `is_array_container_type` is used in `dataclass_array_container`
+    # where it looks at field types. The fields can contain typing information
+    # like `typing.Optional[MyType]`, which does not have an `__mro__` and messes
+    # up `singledispatch.dispatch`, so we check for that first
+
+    return hasattr(cls, "__mro__") and (
             cls is ArrayContainer
             or (serialize_container.dispatch(cls)
                 is not serialize_container.__wrapped__))  # type:ignore[attr-defined]