BUG: IntervalArray.shift with invalid NA fill_value (#51258)

* BUG: IntervalArray.shift with invalid NA fill_value

* Update doc/source/whatsnew/v2.0.0.rst

---------

Co-authored-by: Matthew Roeschke <[email protected]>

Author: jbrockmendel

doc/source/whatsnew/v2.0.0.rst

@@ -1198,6 +1198,7 @@ Interval
 ^^^^^^^^
 - Bug in :meth:`IntervalIndex.is_overlapping` incorrect output if interval has duplicate left boundaries (:issue:`49581`)
 - Bug in :meth:`Series.infer_objects` failing to infer :class:`IntervalDtype` for an object series of :class:`Interval` objects (:issue:`50090`)
+- Bug in :meth:`Series.shift` with :class:`IntervalDtype` and invalid null ``fill_value`` failing to raise ``TypeError`` (:issue:`51258`)
 -
 
 Indexing

pandas/core/arrays/interval.py

@@ -1051,8 +1051,7 @@ def shift(self, periods: int = 1, fill_value: object = None) -> IntervalArray:
         if not len(self) or periods == 0:
             return self.copy()
 
-        if isna(fill_value):
-            fill_value = self.dtype.na_value
+        self._validate_scalar(fill_value)
 
         # ExtensionArray.shift doesn't work for two reasons
         # 1. IntervalArray.dtype.na_value may not be correct for the dtype.

pandas/tests/arrays/interval/test_interval.py

@@ -95,6 +95,10 @@ def test_shift(self):
         expected = IntervalArray.from_tuples([(np.nan, np.nan), (1.0, 2.0)])
         tm.assert_interval_array_equal(result, expected)
 
+        msg = "can only insert Interval objects and NA into an IntervalArray"
+        with pytest.raises(TypeError, match=msg):
+            a.shift(1, fill_value=pd.NaT)
+
     def test_shift_datetime(self):
         # GH#31502, GH#31504
         a = IntervalArray.from_breaks(date_range("2000", periods=4))
@@ -106,6 +110,10 @@ def test_shift_datetime(self):
         expected = a.take([1, 2, -1], allow_fill=True)
         tm.assert_interval_array_equal(result, expected)
 
+        msg = "can only insert Interval objects and NA into an IntervalArray"
+        with pytest.raises(TypeError, match=msg):
+            a.shift(1, fill_value=np.timedelta64("NaT", "ns"))
+
 
 class TestSetitem:
     def test_set_na(self, left_right_dtypes):