Generic TypeGuard inferred as Never when passed as Callable with recursive Union type

[iflan]

Bug Report

I'm trying to make a function called ensure that takes a type guard function and an argument and returns the argument if the type guard function returns true:

def ensure(guard: Callable[[X], TypeGuard[Y]], x: X) -> Y:
    if guard(x):
        return x

    raise TypeError(f'{x} is not the right type')

This works fine for simple Union types, but fails for recursive Union types. Both scenarios work fine in Pylance.

To Reproduce

I created a Gist at:

https://mypy-play.net/?mypy=latest&python=3.12&gist=b48835cb0b0a7a3b5933e10593107dd3

that repros the issue. (It is also a valid pytest test.)

The core of the issue is shown by:

Y = TypeVar('Y')

Foo: TypeAlias = list['Foo'] | str | int

def is_str(o: Foo) -> TypeGuard[str]:
    return isinstance(o, str)

def ensure(guard: Callable[[Foo], TypeGuard[Y]], x: Foo) -> Y:
    if guard(x):
        return x

    raise TypeError(f'{x} is not the right type')

def test_ensure_is_str(s: Foo) -> None:
    # Pylance: list[Foo] | str | int; mypy: Union[builtins.list[...], builtins.str, builtins.int]
    reveal_type(s)
    # mypy: error: Need type annotation for "x"  [var-annotated]
    # mypy: error: Argument 1 to "ensure" has incompatible type "Callable[[Foo], TypeGuard[str]]"; expected "Callable[[Foo], TypeGuard[Never]]"  [arg-type]
    x = ensure(is_str, s)
    reveal_type(x)  # Pylance: str; mypy: Any
    # error: Argument 1 to "ensure" has incompatible type "Callable[[Foo], TypeGuard[str]]"; expected "Callable[[Foo], TypeGuard[Never]]"  [arg-type]
    reveal_type(ensure(is_str, s))  # Pylance: str; mypy: Never
    assert x == s

In this example, mypy can't deduce that x is a str because it expects ensure to take a function returning TypeGuard[Never].

Expected Behavior

mypy should be able to:

• correctly deduce the argument type to ensure
• deduce that x is a str

Actual Behavior

main.py:32: note: Revealed type is "Union[builtins.list[...], builtins.str, builtins.int]"
main.py:35: error: Need type annotation for "x"  [var-annotated]
main.py:35: error: Argument 1 to "ensure" has incompatible type "Callable[[Foo], TypeGuard[str]]"; expected "Callable[[Foo], TypeGuard[Never]]"  [arg-type]
main.py:36: note: Revealed type is "Any"
main.py:38: note: Revealed type is "Never"
main.py:38: error: Argument 1 to "ensure" has incompatible type "Callable[[Foo], TypeGuard[str]]"; expected "Callable[[Foo], TypeGuard[Never]]"  [arg-type]

Your Environment

• Mypy version used: 1.14.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11, 3.12