Support mypy_extensions.FlexibleAlias

[adamtheturtle]

Describe the Bug

I expect no error with:

from typing import TypeVar
from mypy_extensions import FlexibleAlias

T = TypeVar("T")
Alias = FlexibleAlias[T, T]

def example(arg: Alias[int]) -> None:
    pass

example(arg=1)

mypy / pyright / ty give no error

With pyrefly I get:

Argument `Literal[1]` is not assignable to parameter `arg` with type `FlexibleAlias[int, int]` in function `example` [[bad-argument-type](https://pyrefly.org/en/docs/error-kinds/#bad-argument-type)]

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeSIAxAAQDqMUAxrgLYzUAuunAFuwAqkATjDBRS1AMoYcBADS8IcakuqpqxVAHMYAHXQ0A7nxHVSuAK7VGGaoaEQOMBXB6pTM6rHfo1eCxycpMQQ6FoqvoIcPLjohPqUCdQAEjAeHtaWxLDUYDCoHBYiyrhgispoWHj4iEkAtNQA4jCBIqhQdRwQbEHE7Ix8jADWoeF5MDiow-XUAKpw7CIAbvlQAPocwTAAFACUnNyhcH2MgaF5QiKYvfAzMPh9DmzogYaOPLzsAJIAIgCiuXyhWKvAKdnYcAsxGIuCEgW2MEIWkIagCuGYLGyLWc1C0uE6%2BMwolCjggsQUMRWQgULUYuxmtAcTjUWGoQgskVI0VimSJ1EwEBEp3EETMliE1GwQlwhgWQn0%2BjA0pYvVGKkxsMCABUtgA1dyK5XUFjBUhre5OdBwMlW9UwuHUABisHwEGwsAAglAIKg4Ar0FrqABeag6vr6oTbXQgLXR%2BnoL0%2B5Qh533N2e72%2BgDaWoUWoAuv6iWV7qhMbBtu4tIhqIns6EOPn9nUAHzUABysRgtV81D7mjgfvQ%2BlL5Z2VaDAEZdiA5CAAtA4CRyIgqHMut7NrkOacbUP9MXcrCWAU1ugLCxsGltjUIhxm224BwhD2%2B32RMDfGBo%2B2L1eX9QwD4AAvtG%2BiziAZAiGIpCEFwLBQBQNCCNBIqVLI%2BCZOgkBaEUBQ2vEBhSDA7A8BwHDEHAiAAPTUVBojiIQsJaNRMDoNRmDonA1HMNhEC4UI%2BGxNRR4SqgSyoNAqDuv0sQ4XhXQ8rgxCKVahFkNy6B1FS1o8iG0YAMyEJOABMYHoCAwFzlMXQrI60AwBQ6HVMullAA

(Only applicable for extension issues) IDE Information

No response

[yangdanny97]

Generally speaking, I wouldn't expect type system extensions specifically for mypy to work in every type checker.

Pyright seems to understand that Alias[int] is int.

Ty doesn't understand the alias, it gives up and infers Any so there's no error but also no type checking.

Maybe it's possible to support for Pyrefly - could you clarify what the use case for this type is? I don't see any docs on it

[adamtheturtle]

I build and maintain a plugin for mypy. I use pyrefly in my dev workflow / IDE. I added this ignore to avoid this error. This issue is a narrowed down version of the issue I had.