Treat __class_getitem__ as an implicit classmethod like __init_subclass__ (#7647)

Fixes #7319.

Author: AFanaei

mypy/semanal.py

@@ -616,7 +616,7 @@ def prepare_method_signature(self, func: FuncDef, info: TypeInfo) -> None:
         # Only non-static methods are special.
         functype = func.type
         if not func.is_static:
-            if func.name() == '__init_subclass__':
+            if func.name() in ['__init_subclass__', '__class_getitem__']:
                 func.is_class = True
             if not func.arguments:
                 self.fail('Method must have at least one argument', func)

test-data/unit/check-super.test

@@ -258,6 +258,17 @@ class A:
         super().__init_subclass__()
 [builtins fixtures/__init_subclass__.pyi]
 
+[case testSuperClassGetItem]
+from typing import TypeVar, Type, Any
+
+T = TypeVar("T", bound="B")
+
+class A:
+    def __class_getitem__(cls, item) -> None: pass
+
+class B(A):
+    def __class_getitem__(cls: Type[T], item: Any) -> None:
+        super(B, cls).__class_getitem__(item)
 
 -- Invalid uses of super()
 -- -----------------------