BUG: Empty CategoricalIndex fails with boolean categories

doc/source/whatsnew/v0.24.0.txt

@@ -616,6 +616,7 @@ Categorical
 ^^^^^^^^^^^
 
 - Bug in :meth:`Categorical.from_codes` where ``NaN`` values in ``codes`` were silently converted to ``0`` (:issue:`21767`). In the future this will raise a ``ValueError``. Also changes the behavior of ``.from_codes([1.1, 2.0])``.
+- Constructing a :class:`pd.CategoricalIndex` with empty values and boolean categories was raising a ``ValueError`` after a change to dtype coercion (:issue:`22702`).
 
 Datetimelike
 ^^^^^^^^^^^^

pandas/core/arrays/categorical.py

@@ -2439,9 +2439,13 @@ def _get_codes_for_values(values, categories):
     """
     utility routine to turn values into codes given the specified categories
     """
-
     from pandas.core.algorithms import _get_data_algo, _hashtables
-    if not is_dtype_equal(values.dtype, categories.dtype):
+    if is_dtype_equal(values.dtype, categories.dtype):
+        # To prevent erroneous dtype coercion in _get_data_algo, retrieve
+        # the underlying numpy array. gh-22702
+        values = getattr(values, 'values', values)
+        categories = getattr(categories, 'values', categories)
+    else:
         values = ensure_object(values)
         categories = ensure_object(categories)
 

pandas/tests/arrays/categorical/test_constructors.py

@@ -42,6 +42,12 @@ def test_constructor_empty(self):
         expected = pd.Int64Index([1, 2, 3])
         tm.assert_index_equal(c.categories, expected)
 
+    def test_constructor_empty_boolean(self):
+        # see gh-22702
+        cat = pd.Categorical([], categories=[True, False])
+        categories = sorted(cat.categories.tolist())
+        assert categories == [False, True]
+
     def test_constructor_tuples(self):
         values = np.array([(1,), (1, 2), (1,), (1, 2)], dtype=object)
         result = Categorical(values)

pandas/tests/indexes/test_category.py

@@ -136,6 +136,12 @@ def test_construction_with_dtype(self):
         result = CategoricalIndex(idx, categories=idx, ordered=True)
         tm.assert_index_equal(result, expected, exact=True)
 
+    def test_construction_empty_with_bool_categories(self):
+        # see gh-22702
+        cat = pd.CategoricalIndex([], categories=[True, False])
+        categories = sorted(cat.categories.tolist())
+        assert categories == [False, True]
+
     def test_construction_with_categorical_dtype(self):
         # construction with CategoricalDtype
         # GH18109