BUG: merge cross with Series (#54087)

Author: jbrockmendel

pandas/core/reshape/merge.py

@@ -145,10 +145,12 @@ def merge(
     indicator: str | bool = False,
     validate: str | None = None,
 ) -> DataFrame:
+    left_df = _validate_operand(left)
+    right_df = _validate_operand(right)
     if how == "cross":
         return _cross_merge(
-            left,
-            right,
+            left_df,
+            right_df,
             on=on,
             left_on=left_on,
             right_on=right_on,
@@ -162,8 +164,8 @@ def merge(
         )
     else:
         op = _MergeOperation(
-            left,
-            right,
+            left_df,
+            right_df,
             how=how,
             on=on,
             left_on=left_on,
@@ -179,8 +181,8 @@ def merge(
 
 
 def _cross_merge(
-    left: DataFrame | Series,
-    right: DataFrame | Series,
+    left: DataFrame,
+    right: DataFrame,
     on: IndexLabel | None = None,
     left_on: IndexLabel | None = None,
     right_on: IndexLabel | None = None,

pandas/tests/reshape/merge/test_merge_cross.py

@@ -1,6 +1,9 @@
 import pytest
 
-from pandas import DataFrame
+from pandas import (
+    DataFrame,
+    Series,
+)
 import pandas._testing as tm
 from pandas.core.reshape.merge import (
     MergeError,
@@ -96,3 +99,13 @@ def test_join_cross_error_reporting():
     )
     with pytest.raises(MergeError, match=msg):
         left.join(right, how="cross", on="a")
+
+
+def test_merge_cross_series():
+    # GH#54055
+    ls = Series([1, 2, 3, 4], index=[1, 2, 3, 4], name="left")
+    rs = Series([3, 4, 5, 6], index=[3, 4, 5, 6], name="right")
+    res = merge(ls, rs, how="cross")
+
+    expected = merge(ls.to_frame(), rs.to_frame(), how="cross")
+    tm.assert_frame_equal(res, expected)