Backport test case from #python/cpython/96358 (#71)

Author: sobolevn

src/test_typing_extensions.py

@@ -488,6 +488,9 @@ def some_other_func(): pass
         other_overload = some_other_func
         def some_other_func(): pass
         self.assertEqual(list(get_overloads(some_other_func)), [other_overload])
+        # Unrelated function still has no overloads:
+        def not_overloaded(): pass
+        self.assertEqual(list(get_overloads(not_overloaded)), [])
 
         # Make sure that after we clear all overloads, the registry is
         # completely empty.