gh-130870: Preserve GenericAlias subclasses in typing.get_type_hints()

Author: Viicos

Lib/test/test_typing.py

@@ -7178,6 +7178,22 @@ def func(x: undefined) -> undefined: ...
         self.assertEqual(get_type_hints(func, format=annotationlib.Format.STRING),
                          {'x': 'undefined', 'return': 'undefined'})
 
+    def test_get_type_hints_preserve_generic_alias_subclasses(self):
+        # https://github.com/python/cpython/issues/130870
+        # An real world example of this is `collections.abc.Callable`. When parameterized,
+        # the result is a subclass of `types.GenericAlias`.
+        class MyAlias(GenericAlias):
+            pass
+
+        class MyClass:
+            def __class_getitem__(cls, args):
+                return MyAlias(cls, args)
+
+        # Using a forward reference is important, otherwise it works as expected.
+        def func(x: MyClass['int']): ...
+
+        assert isinstance(get_type_hints(func)['x'], MyAlias)
+
 
 class GetUtilitiesTestCase(TestCase):
     def test_get_origin(self):

Lib/typing.py

@@ -468,25 +468,27 @@ def _eval_type(t, globalns, localns, type_params=_sentinel, *, recursive_guard=f
                 _make_forward_ref(arg) if isinstance(arg, str) else arg
                 for arg in t.__args__
             )
-            is_unpacked = t.__unpacked__
-            if _should_unflatten_callable_args(t, args):
-                t = t.__origin__[(args[:-1], args[-1])]
-            else:
-                t = t.__origin__[args]
-            if is_unpacked:
-                t = Unpack[t]
+        else:
+            args = t.__args__
 
         ev_args = tuple(
             _eval_type(
                 a, globalns, localns, type_params, recursive_guard=recursive_guard,
                 format=format, owner=owner,
             )
-            for a in t.__args__
+            for a in args
         )
-        if ev_args == t.__args__:
+        if ev_args == args:
             return t
         if isinstance(t, GenericAlias):
-            return GenericAlias(t.__origin__, ev_args)
+            is_unpacked = t.__unpacked__
+            if _should_unflatten_callable_args(t, ev_args):
+                t = t.__origin__[(ev_args[:-1], ev_args[-1])]
+            else:
+                t = t.__origin__[ev_args]
+            if is_unpacked:
+                t = Unpack[t]
+            return t
         if isinstance(t, Union):
             return functools.reduce(operator.or_, ev_args)
         else: