gh-90172: add test for functools.singledispatch on Union types with None type (#92174)

Signed-off-by: prwatson <[email protected]>

Co-authored-by: Jelle Zijlstra <[email protected]>

Author: pr-watson

Lib/test/test_functools.py

@@ -2792,6 +2792,49 @@ def _(arg: int | float):
         self.assertEqual(f(1), "types.UnionType")
         self.assertEqual(f(1.0), "types.UnionType")
 
+    def test_union_conflict(self):
+        @functools.singledispatch
+        def f(arg):
+            return "default"
+
+        @f.register
+        def _(arg: typing.Union[str, bytes]):
+            return "typing.Union"
+
+        @f.register
+        def _(arg: int | str):
+            return "types.UnionType"
+
+        self.assertEqual(f([]), "default")
+        self.assertEqual(f(""), "types.UnionType")  # last one wins
+        self.assertEqual(f(b""), "typing.Union")
+        self.assertEqual(f(1), "types.UnionType")
+
+    def test_union_None(self):
+        @functools.singledispatch
+        def typing_union(arg):
+            return "default"
+
+        @typing_union.register
+        def _(arg: typing.Union[str, None]):
+            return "typing.Union"
+
+        self.assertEqual(typing_union(1), "default")
+        self.assertEqual(typing_union(""), "typing.Union")
+        self.assertEqual(typing_union(None), "typing.Union")
+
+        @functools.singledispatch
+        def types_union(arg):
+            return "default"
+
+        @types_union.register
+        def _(arg: int | None):
+            return "types.UnionType"
+
+        self.assertEqual(types_union(""), "default")
+        self.assertEqual(types_union(1), "types.UnionType")
+        self.assertEqual(types_union(None), "types.UnionType")
+
     def test_register_genericalias(self):
         @functools.singledispatch
         def f(arg):