Add test for combine between two series

pandas/tests/series/methods/test_combine.py

@@ -1,12 +1,13 @@
-from pandas import Series
+from pandas import (
+    NA,
+    Series,
+)
 import pandas._testing as tm
 
 
 class TestCombine:
     def test_combine_scalar(self):
         # GH#21248
-        # Note - combine() with another Series is tested elsewhere because
-        # it is used when testing operators
         ser = Series([i * 10 for i in range(5)])
         result = ser.combine(3, lambda x, y: x + y)
         expected = Series([i * 10 + 3 for i in range(5)])
@@ -15,3 +16,13 @@ def test_combine_scalar(self):
         result = ser.combine(22, lambda x, y: min(x, y))
         expected = Series([min(i * 10, 22) for i in range(5)])
         tm.assert_series_equal(result, expected)
+
+    def test_combine_series(self):
+        # GH#31899
+        # Note - combine() with another Series is also tested elsewhere because
+        # it is used when testing operators
+        s1 = Series([91, NA, 94], dtype="Int8")
+        s2 = Series([91, NA, 11], dtype="Int8")
+        result = s1.combine(s2, lambda x, y: x + y)
+        expected = Series([-74, NA, 105], dtype="Int8")  # dtype should be preserved
+        tm.assert_series_equal(result, expected)