Type narrowing fails when narrowing Optional NewType

Bug Report

from typing import Union, NewType

Int = NewType("Int", int)
Float = NewType("Float", float)

def raw_types(arg: Union[int, float, None]) -> None:
    if isinstance(arg, int):
        reveal_type(arg)  # Revealed type is "builtins.int"
    elif isinstance(arg, float):
        reveal_type(arg)  # Revealed type is "builtins.float"
    else:
        reveal_type(arg)  # Revealed type is "None"

def new_types(arg: Union[Int, Float, None]) -> None:
    if isinstance(arg, int):
        reveal_type(arg)  # Revealed type is "__main__.Int"
    elif isinstance(arg, float):
        reveal_type(arg)  # Revealed type is "Union[__main__.Int, __main__.Float]" (NARROWING FAILED)
    else:
        reveal_type(arg)  # Revealed type is "None"

def new_types_not_optional(arg: Union[Int, Float]) -> None:
    if isinstance(arg, int):
        reveal_type(arg)  # Revealed type is "__main__.Int"
    elif isinstance(arg, float):
        reveal_type(arg)  # Revealed type is "__main__.Float"
    else:
        reveal_type(arg)

The bug is fairly straightforward. When narrowing a Union of NewType types with isinstance, the narrowing fails if one of the possible types of the variable is None.

To Reproduce

https://mypy-play.net/?mypy=0.931&python=3.10&gist=557b54a098f8ed366fa62ff380ec12c8

Expected Behavior

All revealed types after isinstance(arg, float) would be builtins.float or __main__.Float.

Actual Behavior

The revealed type in the buggy case is still Union[__main__.Int, __main__.Float].

It is not just about Optional. Any extra type reveals narrowing issue:

def new_types(arg: Union[Int, Float, str]) -> None:
    if isinstance(arg, int):
        reveal_type(arg)  # Revealed type is "__main__.Int"
    elif isinstance(arg, float):
        reveal_type(arg)  # Revealed type is "Union[__main__.Int, __main__.Float]" (NARROWING FAILED)
    else:
        reveal_type(arg)  # Revealed type is "builtins.str"

This happens because Int is treated as a subtype of float here:

mypy/mypy/meet.py

Line 530 in 17850b3

elif is_subtype(self.s, t):

I don't think that this is correct 🤔

Funny enough, this does type-check:

from typing import NewType

UserId = NewType('UserId', int)
some_id: int = UserId(1)
other_id: float = UserId(2)

This bug appears to have been fixed. The latest version of mypy (1.5) does not exhibit the incorrect behavior indicated in the bug report.

Fixed in #13781

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Type narrowing fails when narrowing Optional NewType #11995

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Type narrowing fails when narrowing Optional NewType #11995

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sobolevn commented on Jan 15, 2022

sobolevn commented on Jan 15, 2022

sobolevn commented on Jan 15, 2022

erictraut commented on Aug 12, 2023

hauntsaninja commented on Aug 13, 2023

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