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?).

Python 3.6.3 と mypy 0.650 を使って minimal.py の例で報告を再現し、CustomProperty デコレータの場合と builtin property の場合を比較します。mypy による property サブクラスとデコレータ setter の処理を追跡します。完了の条件は、報告された duplicate-name または missing-attribute エラーなしに、カスタムクラスが builtin property と同じように型チェックされることです。