Backport type_params fix from CPython (#646)

JelleZijlstra · web-flow · commit 7ee9e05fd484 · 2025-08-18T08:02:02.000+02:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -4,7 +4,8 @@
   [`annotationlib.type_repr`](https://docs.python.org/3.14/library/annotationlib.html#annotationlib.type_repr),
   introduced in Python 3.14 (CPython PR [#124551](https://github.com/python/cpython/pull/124551),
   originally by Jelle Zijlstra). Patch by Semyon Moroz.
-
+- Fix behavior of type params in `typing_extensions.evaluate_forward_ref`. Backport of
+  CPython PR [#137227](https://github.com/python/cpython/pull/137227) by Jelle Zijlstra.
 
 # Release 4.14.1 (July 4, 2025)
 
diff --git a/src/test_typing_extensions.py b/src/test_typing_extensions.py
@@ -9194,10 +9194,9 @@ class Gen[Tx]:
         not_Tx = TypeVar("Tx")  # different TypeVar with same name
         self.assertIs(evaluate_forward_ref(typing.ForwardRef("Tx"), type_params=(not_Tx,), owner=Gen), not_Tx)
 
-        # globals can take higher precedence
-        if _FORWARD_REF_HAS_CLASS:
-            self.assertIs(evaluate_forward_ref(typing.ForwardRef("Tx", is_class=True), owner=Gen, globals={"Tx": str}), str)
-            self.assertIs(evaluate_forward_ref(typing.ForwardRef("Tx", is_class=True), owner=Gen, type_params=(not_Tx,), globals={"Tx": str}), str)
+        # globals do not take higher precedence
+        self.assertIs(evaluate_forward_ref(typing.ForwardRef("Tx", is_class=True), owner=Gen, globals={"Tx": str}), Tx)
+        self.assertIs(evaluate_forward_ref(typing.ForwardRef("Tx", is_class=True), owner=Gen, type_params=(not_Tx,), globals={"Tx": str}), not_Tx)
 
         with self.assertRaises(NameError):
             evaluate_forward_ref(typing.ForwardRef("alias"), type_params=Gen.__type_params__)
diff --git a/src/typing_extensions.py b/src/typing_extensions.py
@@ -4065,23 +4065,13 @@ def _eval_with_owner(
             # as a way of emulating annotation scopes when calling `eval()`
             type_params = getattr(owner, "__type_params__", None)
 
-        # type parameters require some special handling,
-        # as they exist in their own scope
-        # but `eval()` does not have a dedicated parameter for that scope.
-        # For classes, names in type parameter scopes should override
-        # names in the global scope (which here are called `localns`!),
-        # but should in turn be overridden by names in the class scope
-        # (which here are called `globalns`!)
+        # Type parameters exist in their own scope, which is logically
+        # between the locals and the globals. We simulate this by adding
+        # them to the globals.
         if type_params is not None:
             globals = dict(globals)
-            locals = dict(locals)
             for param in type_params:
-                param_name = param.__name__
-                if (
-                    _FORWARD_REF_HAS_CLASS and not forward_ref.__forward_is_class__
-                ) or param_name not in globals:
-                    globals[param_name] = param
-                    locals.pop(param_name, None)
+                globals[param.__name__] = param
 
         arg = forward_ref.__forward_arg__
         if arg.isidentifier() and not keyword.iskeyword(arg):
