gh-138425: Correctly partially evaluate global generics with undefined params in ref.evaluate(format=Format.FORWARDREF) (#138430)

Co-authored-by: sobolevn <mail@sobolevn.me>

Lib/annotationlib.py

@@ -187,8 +187,11 @@ def evaluate(
             except Exception:
                 if not is_forwardref_format:
                     raise
+
+            # All variables, in scoping order, should be checked before
+            # triggering __missing__ to create a _Stringifier.
             new_locals = _StringifierDict(
-                {**builtins.__dict__, **locals},
+                {**builtins.__dict__, **globals, **locals},
                 globals=globals,
                 owner=owner,
                 is_class=self.__forward_is_class__,

Lib/test/test_annotationlib.py

@@ -1877,6 +1877,32 @@ def test_name_lookup_without_eval(self):
 
         self.assertEqual(exc.exception.name, "doesntexist")
 
+    def test_evaluate_undefined_generic(self):
+        # Test the codepath where have to eval() with undefined variables.
+        class C:
+            x: alias[int, undef]
+
+        generic = get_annotations(C, format=Format.FORWARDREF)["x"].evaluate(
+            format=Format.FORWARDREF,
+            globals={"alias": dict}
+        )
+        self.assertNotIsInstance(generic, ForwardRef)
+        self.assertIs(generic.__origin__, dict)
+        self.assertEqual(len(generic.__args__), 2)
+        self.assertIs(generic.__args__[0], int)
+        self.assertIsInstance(generic.__args__[1], ForwardRef)
+
+        generic = get_annotations(C, format=Format.FORWARDREF)["x"].evaluate(
+            format=Format.FORWARDREF,
+            globals={"alias": Union},
+            locals={"alias": dict}
+        )
+        self.assertNotIsInstance(generic, ForwardRef)
+        self.assertIs(generic.__origin__, dict)
+        self.assertEqual(len(generic.__args__), 2)
+        self.assertIs(generic.__args__[0], int)
+        self.assertIsInstance(generic.__args__[1], ForwardRef)
+
     def test_fwdref_invalid_syntax(self):
         fr = ForwardRef("if")
         with self.assertRaises(SyntaxError):

Misc/NEWS.d/next/Library/2025-09-03-18-26-07.gh-issue-138425.cVE9Ho.rst

@@ -0,0 +1,2 @@
+Fix partial evaluation of :class:`annotationlib.ForwardRef` objects which rely
+on names defined as globals.