diff --git a/pytools/obj_array.py b/pytools/obj_array.py
index 29069647..c81e1185 100644
--- a/pytools/obj_array.py
+++ b/pytools/obj_array.py
@@ -273,7 +273,11 @@ def __matmul__(
                     self: ObjectArray2D[T],
                     other: ObjectArray2D[T],
                 /) -> ObjectArray2D[T]: ...
-
+        @overload
+        def __matmul__(
+                    self: ObjectArray2D[T],
+                    other: np.ndarray[tuple[int, int], np.dtype[Any]],
+                /) -> ObjectArray2D[T]: ...
         @overload
         def __matmul__(
                     self: ObjectArrayND[T],
@@ -281,7 +285,9 @@ def __matmul__(
                 /) -> ObjectArrayND[T] | T: ...
 
         @property
-        def flat(self) -> ObjectArray1D[T]: ...
+        def flat(self) -> Iterator[T]: ...
+
+        def flatten(self) -> ObjectArray1D[T]: ...
 
         @overload
         def tolist(self: ObjectArray0D[T]) -> T: ...
@@ -439,7 +445,7 @@ def sum(
 
 def sum(
             array: ObjectArrayND[T],
-            axis: int | None,
+            axis: int | None = None,
         ) -> ObjectArrayND[T] | T:
     import numpy as np
     return cast("ObjectArrayND[T] | T", np.sum(
@@ -451,7 +457,7 @@ def sum(
 def to_hashable(ary: ObjectArray[ShapeT, T] | Hashable, /) -> Hashable:
     if isinstance(ary, ObjectArray):
         ary = cast("ObjectArray[ShapeT, T]", ary)
-        return tuple(ary.flat.tolist())
+        return tuple(ary.flat)
     return ary