API: Ensure a full mask is returned for masked_invalid

numpy/ma/core.py

@@ -2357,7 +2357,12 @@ def masked_invalid(a, copy=True):
 
     """
     a = np.array(a, copy=False, subok=True)
-    return masked_where(~(np.isfinite(a)), a, copy=copy)
+    res = masked_where(~(np.isfinite(a)), a, copy=copy)
+    # masked_invalid previously never returned nomask as a mask and doing so
+    # threw off matplotlib (gh-22842).  So use shrink=False:
+    if res._mask is nomask:
+        res._mask = make_mask_none(res.shape, res.dtype)
+    return res
 
 ###############################################################################
 #                            Printing options                                 #

numpy/ma/tests/test_core.py

@@ -4519,6 +4519,18 @@ def __array__(self):
         assert_array_equal(arr._data, np.array(Series()))
         assert_array_equal(arr._mask, [False, True, True])
 
+    @pytest.mark.parametrize("copy", [True, False])
+    def test_masked_invalid_full_mask(self, copy):
+        # Matplotlib relied on masked_invalid always returning a full mask
+        # (Also astropy projects, but were ok with it gh-22720 and gh-22842)
+        a = np.ma.array([1, 2, 3, 4])
+        assert a._mask is nomask
+        res = np.ma.masked_invalid(a, copy=copy)
+        assert res.mask is not nomask
+        # mask of a should not be mutated
+        assert a.mask is nomask
+        assert np.may_share_memory(a._data, res._data) != copy
+
     def test_choose(self):
         # Test choose
         choices = [[0, 1, 2, 3], [10, 11, 12, 13],