stubtest: don't flag __class_getitem__ missing for PEP 695 generic stubs

PEP 695 generic syntax (class Foo[T]: ...) implicitly provides
__class_getitem__ at runtime. When a stub uses this syntax and the
runtime class defines __class_getitem__ explicitly, stubtest was
incorrectly reporting it as missing from the stub.

Fix: in verify_typeinfo(), discard "__class_getitem__" from the set of
attributes to check when the stub class has PEP 695 type parameters
(ClassDef.type_args is not None).

Fixes: #21253

Author: galsakuri

mypy/stubtest.py

@@ -700,6 +700,10 @@ def verify_typeinfo(
         to_check.discard("__init__")
     if is_runtime_typeddict:
         to_check.discard("__new__")
+    # PEP 695 generic syntax (class Foo[T]: ...) implicitly provides __class_getitem__
+    # at runtime, so don't require an explicit stub definition.
+    if stub.defn.type_args is not None:
+        to_check.discard("__class_getitem__")
 
     for entry in sorted(to_check):
         mangled_entry = entry

mypy/test/teststubtest.py

@@ -1825,6 +1825,19 @@ def test_dunders(self) -> Iterator[Case]:
             runtime="class D:\n  def __class_getitem__(cls, type): ...",
             error=None,
         )
+        if sys.version_info >= (3, 12):
+            # PEP 695 generic syntax implicitly provides __class_getitem__ at runtime
+            yield Case(
+                stub="class D2[T]: ...",
+                runtime="class D2:\n    def __class_getitem__(cls, _): return cls",
+                error=None,
+            )
+            # Non-generic stub still reports missing __class_getitem__
+            yield Case(
+                stub="class D3: ...",
+                runtime="class D3:\n    def __class_getitem__(cls, _): return cls",
+                error="D3.__class_getitem__",
+            )
         yield Case(
             stub="class E:\n  def __getitem__(self, item: object) -> object: ...",
             runtime="class E:\n  def __getitem__(self, item: object, /) -> object: ...",