BUG: reindexing empty CategoricalIndex would fail if target had duplicates (pandas-dev#38906)

Author: batterseapower

doc/source/whatsnew/v1.3.0.rst

@@ -192,6 +192,7 @@ Categorical
 - Bug in ``CategoricalIndex.reindex`` failed when ``Index`` passed with elements all in category (:issue:`28690`)
 - Bug where constructing a :class:`Categorical` from an object-dtype array of ``date`` objects did not round-trip correctly with ``astype`` (:issue:`38552`)
 - Bug in constructing a :class:`DataFrame` from an ``ndarray`` and a :class:`CategoricalDtype` (:issue:`38857`)
+- Bug where :class:`DataFrame` axes with an empty :class:`CategoricalIndex` could not be reindexed if the target contained duplicates (:issue:`38906`)
 
 Datetimelike
 ^^^^^^^^^^^^

pandas/core/indexes/base.py

@@ -3745,9 +3745,13 @@ def _reindex_non_unique(self, target):
                 # need to retake to have the same size as the indexer
                 indexer[~check] = -1
 
-                # reset the new indexer to account for the new size
-                new_indexer = np.arange(len(self.take(indexer)))
-                new_indexer[~check] = -1
+                if len(self):
+                    # reset the new indexer to account for the new size
+                    new_indexer = np.arange(len(self.take(indexer)))
+                    new_indexer[~check] = -1
+                else:
+                    # GH#38906
+                    new_indexer = np.arange(0)
 
         if isinstance(self, ABCMultiIndex):
             new_index = type(self).from_tuples(new_labels, names=self.names)

pandas/tests/frame/indexing/test_categorical.py

@@ -385,4 +385,4 @@ def test_loc_indexing_preserves_index_category_dtype(self):
         tm.assert_index_equal(result, expected)
 
         result = df.loc[["a"]].index.levels[0]
-        tm.assert_index_equal(result, expected)
+        tm.assert_index_equal(result, expected)

pandas/tests/indexing/test_categorical.py

@@ -533,3 +533,20 @@ def test_loc_with_non_string_categories(self, idx_values, ordered):
         result.loc[sl, "A"] = ["qux", "qux2"]
         expected = DataFrame({"A": ["qux", "qux2", "baz"]}, index=cat_idx)
         tm.assert_frame_equal(result, expected)
+
+    def test_reindex_empty(self):
+        df = pd.DataFrame(columns=pd.CategoricalIndex([]), index=['K'], dtype='f8')
+
+        # No duplicates
+        cat_idx = pd.CategoricalIndex(['A', 'B'])
+        result = df.reindex(columns=cat_idx)
+        expected = DataFrame(index=['K'], columns=cat_idx, dtype='f8')
+        tm.assert_frame_equal(result, expected)
+
+        # Duplicates
+        # GH#38906
+        cat_idx = pd.CategoricalIndex(['A', 'A'])
+        result = df.reindex(columns=cat_idx)
+        expected = DataFrame(index=['K'], columns=cat_idx, dtype='f8')
+        tm.assert_frame_equal(result, expected)
+