Class inheriting from property doesn't typecheck properly when used as a decorator

I have a custom class inheriting from property. When I go to use this as a decorator though mypy seems to miss that it's a property at all. This is with Python 3.6.3 and mypy 0.650.

Example:

class CustomProperty(property):
    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        super().__init__(fget, fset, fdel, doc)

class A:
    def __init__(self) -> None:
        self._val = 1

    @CustomProperty
    def val(self) -> int:
        return self._val

    @val.setter
    def val(self, v: int) -> None:
        self._val = v


def test_a() -> None:
    print(A.val)

Running mypy on this gives the following output:
mypy minimal2.py

minimal.py:14: error: Name 'val' already defined on line 10
minimal.py:14: error: Name 'val' is not defined
minimal.py:20: error: "Type[A]" has no attribute "val"

Contrast this with the same code with the builtin property which typechecks fine:

class B:
    def __init__(self) -> None:
        self._val = 1

    @property
    def val(self) -> int:
        return self._val

    @val.setter
    def val(self, v: int) -> None:
        self._val = v

def test_b() -> None:
    print(B.val)

It's possible that this is related to #1529 where @ilevkivskyi suggested that:

The original example now passes mypy, not sure however how useful the subclass can be (property is heavily special-cased in mypy), but at least the original bug is fixed, so closing this.

Is subclassing property frowned upon (or just not supported in mypy?).

Reproduce el informe con el ejemplo minimal.py usando Python 3.6.3 y mypy 0.650; después, compara el caso del decorador CustomProperty con el caso de la propiedad builtin. Rastrea cómo mypy gestiona las subclases de property y los setters de los decoradores; el trabajo estará terminado cuando la clase personalizada pase la comprobación de tipos igual que la propiedad builtin, sin los errores duplicate-name o missing-attribute indicados.