semanal: Treat __init_subclass__ as an implicit classmethod

mypy/nodes.py

@@ -439,7 +439,7 @@ class FuncBase(Node):
                  'unanalyzed_type',
                  'info',
                  'is_property',
-                 'is_class',        # Uses "@classmethod"
+                 'is_class',        # Uses "@classmethod" (explicit or implicit)
                  'is_static',       # Uses "@staticmethod"
                  'is_final',        # Uses "@final"
                  '_fullname',

mypy/semanal.py

@@ -611,10 +611,12 @@ def prepare_method_signature(self, func: FuncDef, info: TypeInfo) -> None:
             if not func.arguments:
                 self.fail('Method must have at least one argument', func)
             elif isinstance(functype, CallableType):
+                if func.name() == '__init_subclass__':
+                    func.is_class = True
                 self_type = functype.arg_types[0]
                 if isinstance(self_type, AnyType):
                     leading_type = fill_typevars(info)  # type: Type
-                    if func.is_class or func.name() in ('__new__', '__init_subclass__'):
+                    if func.is_class or func.name() == '__new__':
                         leading_type = self.class_type(leading_type)
                     func.type = replace_implicit_first_type(functype, leading_type)
 

test-data/unit/check-super.test

@@ -252,6 +252,12 @@ def g(b: B) -> None:
     super(B, b).f('42')
     super(B, b).f(42) # E: Argument 1 to "f" of "A" has incompatible type "int"; expected "str"
 
+[case testSuperInInitSubclass]
+class A:
+    def __init_subclass__(cls) -> None:
+        super().__init_subclass__()
+[builtins fixtures/__init_subclass__.pyi]
+
 
 -- Invalid uses of super()
 -- -----------------------

test-data/unit/fixtures/__init_subclass__.pyi

@@ -0,0 +1,11 @@
+# builtins stub with object.__init_subclass__
+
+class object:
+    def __init_subclass__(cls) -> None: pass
+
+class type: pass
+
+class int: pass
+class bool: pass
+class str: pass
+class function: pass