gh-144361: Fix NameError when type parameter default refers to a forward name

Author: tangyuan0821

Lib/typing.py

@@ -1099,7 +1099,12 @@ def _typevartuple_prepare_subst(self, alias, args):
         raise TypeError(f"Too few arguments for {alias};"
                         f" actual {alen}, expected at least {plen-1}")
     if left == alen - right and self.has_default():
-        replacement = _unpack_args(self.__default__)
+        try:
+            default = self.__default__
+        except NameError:
+            default = annotationlib.call_evaluate_function(
+                self.evaluate_default, annotationlib.Format.FORWARDREF)
+        replacement = _unpack_args(default)
     else:
         replacement = args[left: alen - right]
 
@@ -1125,7 +1130,12 @@ def _paramspec_prepare_subst(self, alias, args):
     params = alias.__parameters__
     i = params.index(self)
     if i == len(args) and self.has_default():
-        args = (*args, self.__default__)
+        try:
+            default = self.__default__
+        except NameError:
+            default = annotationlib.call_evaluate_function(
+                self.evaluate_default, annotationlib.Format.FORWARDREF)
+        args = (*args, default)
     if i >= len(args):
         raise TypeError(f"Too few arguments for {alias}")
     # Special case where Z[[int, str, bool]] == Z[int, str, bool] in PEP 612.
@@ -1185,7 +1195,12 @@ def _generic_class_getitem(cls, args):
         for param in parameters:
             prepare = getattr(param, '__typing_prepare_subst__', None)
             if prepare is not None:
-                args = prepare(cls, args)
+                try:
+                    args = prepare(cls, args)
+                except NameError:
+                    default = annotationlib.call_evaluate_function(
+                        param.evaluate_default, annotationlib.Format.FORWARDREF)
+                    args = (*args, default)
         _check_generic_specialization(cls, args)
 
         new_args = []

Misc/NEWS.d/next/Library/2026-06-25-22-30-00.gh-issue-144361.lL9mN2.rst

@@ -0,0 +1,4 @@
+Fix :exc:`NameError` when a type parameter default (PEP 696/PEP 749)
+refers to a name defined later in the module. Now three call sites in
+:mod:`typing` fall back to :func:`annotationlib.call_evaluate_function`
+with :data:`~annotationlib.Format.FORWARDREF` when eager evaluation fails.