BUG: raise MergeError when both right_on and right_index are specified

Author: lexzlei

pandas/core/reshape/merge.py

@@ -1928,6 +1928,10 @@ def _validate_left_right_on(self, left_on, right_on):
                 )
             if not self.right_index and right_on is None:
                 raise MergeError('Must pass "right_on" OR "right_index".')
+            # if self.right_index and right_on is not None:
+            #     raise MergeError(
+            #         'Can only pass argument "right_on" OR "right_index" not both.'
+            #     )
             n = len(left_on)
             if self.right_index:
                 if len(left_on) != self.right.index.nlevels:

pandas/tests/reshape/merge/test_merge.py

@@ -3149,3 +3149,11 @@ def test_merge_pyarrow_datetime_duplicates():
     )
     expected = expected.convert_dtypes(dtype_backend="pyarrow")
     tm.assert_frame_equal(result, expected)
+
+
+def test_merge_right_on_and_right_index():
+    df1 = pd.DataFrame({"col": [1, 2, 3]})
+    df2 = pd.DataFrame({"col": [2, 3, 4]})
+
+    with pytest.raises(pd.errors.MergeError):
+        df1.merge(df2, left_on="col", right_on="col", right_index=True)