Add a test for issue 12773 (rapidsai#14529)

Closes rapidsai#12773.

Authors:
   - Vyas Ramasubramani (https://github.com/vyasr)
   - Lawrence Mitchell (https://github.com/wence-)

Approvers:
   - GALI PREM SAGAR (https://github.com/galipremsagar)
   - Lawrence Mitchell (https://github.com/wence-)

python/dask_cudf/dask_cudf/tests/test_join.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2019-2022, NVIDIA CORPORATION.
+# Copyright (c) 2019-2023, NVIDIA CORPORATION.
 
 from functools import partial
 
@@ -359,3 +359,22 @@ def test_single_partition():
     m2 = dleft.merge(right, how="inner")
     assert len(m2.dask) < len(dleft.dask) * 3
     assert len(m2) == 100
+
+
+def test_issue_12773():
+    # https://github.com/rapidsai/cudf/issues/12773
+    df1 = cudf.DataFrame({"a": ["a", "b"], "b": [1, 2]})
+    df2 = cudf.DataFrame({"a": ["a", "c"], "b": [2, 3]})
+
+    dleft = dd.from_pandas(df1, npartitions=2).set_index("a")
+    dright = dd.from_pandas(df2, npartitions=2).set_index("a")
+
+    expected = df1.set_index("a").merge(
+        df2.set_index("a"), left_index=True, right_index=True, how="left"
+    )
+    result = dleft.merge(dright, left_index=True, right_index=True, how="left")
+    dd.assert_eq(
+        result.compute().to_pandas(),
+        expected.to_pandas(),
+        check_index=False,
+    )