Mypy does not detect overriding __new__ with a different number of arguments

[RadixSeven]

On the code below, mypy 0.620 (executed as mypy foo.py) under python 3.6.3 says:

foo.py:17: error: Too few arguments for "FR"

despite the fact that the code is fine. I expect no warnings.

from typing import Type, NamedTuple


_FR = NamedTuple('_FR', [
    ('forward', str),
    ('reverse', str)
])


class FR(_FR):
    def __new__(cls: Type['FR'], fw: str) -> 'FR':
        # The ignore on the next line works around a bug
        # reported as fixed https://github.com/python/typeshed/issues/2091
        #
        # NOTE: it is still not fixed with 
        # mypy-0.630+dev.b2d61741623f2a0573f2b1bdb8391ceed862cf69
        return super().__new__(cls, fw, fw[::-1])  # type: ignore


print(FR('stuff'))

I also tried executing with latest from git (mypy-0.630+dev.b2d61741623f2a0573f2b1bdb8391ceed862cf69) and the error persists.

[JelleZijlstra]

This is #2852 and #1279.