Fixes generic inference in functions with TypeGuard, refs #11780

[sobolevn]

Closes #11780, closes #11428

[intgr]

Thanks!

I have a similar use case with literals. I think this change helps a little, but my example runs into a different issue: Literal[...] expression is not considered a type in the type system:

from typing import Literal, TypeGuard, TypeVar, get_args

T = TypeVar('T')

def is_literal_value(value: str | int | None, literal_type: type[T]) -> TypeGuard[T]:
    return value in get_args(literal_type)

value: str = input()
if is_literal_value(value, Literal['on', 'off']):
    reveal_type(value)

Expected result: Revealed type is "Literal['on', 'off']"

Output in mypy master 😕

_local/typeguard.py:8: error: Argument 2 to "is_literal_value" has incompatible type "object"; expected "Type[<nothing>]"
_local/typeguard.py:9: note: Revealed type is "T`-1"

Output from this branch

_local/typeguard.py:8: error: Argument 2 to "is_literal_value" has incompatible type "object"; expected "Type[<nothing>]"
_local/typeguard.py:9: note: Revealed type is "<nothing>"

[JelleZijlstra]

Obligatory reference to #9773.

[intgr]

Thanks @JelleZijlstra, I have been searching for discussion on this topic, but had not found that issue.

[amh4r]

Running into this problem, too.

I get these errors:

main.py:23: note: Revealed type is "builtins.list[Any]"
main.py:26: note: Revealed type is "builtins.list[T`-1]"
main.py:27: error: Argument 1 to "do_stuff" has incompatible type "List[T]"; expected "List[Foo]"
Found 1 error in 1 file (checked 1 source file)

When I run this code:

from typing import Any, TypeGuard, Type, TypeVar

def do_stuff(foos: list[Foo]) -> None:
    pass

class Foo:
    pass

T = TypeVar("T")

def is_list_of(value: list, type: Type[T]) -> TypeGuard[list[T]]:
    for item in value:
        if isinstance(item, type) is False:
            return False

    return True

foos: list[Any] = [Foo()]
reveal_type(foos)

if is_list_of(foos, Foo):
    reveal_type(foos)
    do_stuff(foos)

[sobolevn]

I think we can merge this. CC @JukkaL

[hauntsaninja]

This is great!

I think this might change behaviour if you have a function called _ that is type guarded, but that's fine. edit: Github's diff view is out of date

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉