cast gives cryptic error: Invalid type "self"

[trickeydan]

• Are you reporting a bug, or opening a feature request?
  Bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import *

Integer = NewType("Integer", int)

class Foo:

    _bar: Integer

    def __init__(self, val: int) -> None:
        self._val = val
        self._bar = cast(self._val, self._bar)

• What is the actual behavior/output?
  type.py:11: error: Invalid type "self"
• What is the behavior/output you expect?
  An error message that explains that you cannot cast to an instance of a type.
• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?
  mypy 0.701
python 3.7.3
• What are the mypy flags you are using? (For example --strict-optional)
  mypy.ini is as follows.

[mypy]
mypy_path = stubs

warn_unused_ignores = True
warn_return_any = True

strict_optional = True
no_implicit_optional = True

disallow_any_unimported = True
#disallow_any_expr = True
#disallow_any_decorated = True
disallow_any_explicit = True
disallow_subclassing_any = True
disallow_any_generics = True

disallow_untyped_calls = True
disallow_untyped_defs = True
disallow_incomplete_defs = True
disallow_untyped_decorators = True

check_untyped_defs = True

[gvanrossum]

I wonder if using ‘self.something’ in other places that need a type give the same error.

[ilevkivskyi]

This is a duplicate of a more general issue #4030. I prefer to track this in "centralized" way, so closing this one.