Bind self correctly when mapping class methods from supertype (#7474)

Fixes #2511

Author: ilevkivskyi

mypy/checker.py

@@ -1530,7 +1530,11 @@ def bind_and_map_method(self, sym: SymbolTableNode, typ: FunctionLike,
         """
         if (isinstance(sym.node, (FuncDef, OverloadedFuncDef, Decorator))
                 and not is_static(sym.node)):
-            bound = bind_self(typ, self.scope.active_self_type())
+            if isinstance(sym.node, Decorator):
+                is_class_method = sym.node.func.is_class
+            else:
+                is_class_method = sym.node.is_class
+            bound = bind_self(typ, self.scope.active_self_type(), is_class_method)
         else:
             bound = typ
         return cast(FunctionLike, map_type_from_supertype(bound, sub_info, super_info))

test-data/unit/check-generics.test

@@ -2062,3 +2062,52 @@ class A:
 x, y = A()
 reveal_type(x)  # N: Revealed type is 'Any'
 reveal_type(y)  # N: Revealed type is 'Any'
+
+[case testSubclassingGenericSelfClassMethod]
+from typing import TypeVar, Type
+
+AT = TypeVar('AT', bound='A')
+
+class A:
+    @classmethod
+    def from_config(cls: Type[AT]) -> AT:
+        ...
+
+class B(A):
+    @classmethod
+    def from_config(cls: Type[B]) -> B:
+        return B()
+[builtins fixtures/classmethod.pyi]
+
+[case testSubclassingGenericSelfClassMethodOptional]
+# flags: --strict-optional
+from typing import TypeVar, Type, Optional
+
+AT = TypeVar('AT', bound='A')
+
+class A:
+    @classmethod
+    def from_config(cls: Type[AT]) -> Optional[AT]:
+        return None
+
+class B(A):
+    @classmethod
+    def from_config(cls: Type[B]) -> Optional[B]:
+        return B()
+[builtins fixtures/classmethod.pyi]
+
+[case testSubclassingGenericSelfClassMethodNonAnnotated]
+from typing import TypeVar, Type
+
+AT = TypeVar('AT', bound='A')
+
+class A:
+    @classmethod
+    def from_config(cls: Type[AT]) -> AT:
+        ...
+
+class B(A):
+    @classmethod
+    def from_config(cls) -> B:
+        return B()
+[builtins fixtures/classmethod.pyi]