Type narrowing on type[T] where T is a union alias of classes

[LoicRiegel]

Summary

Hi! I hope this is not a duplicate:

main.py:

from typing import assert_never

class A: ...
class B: ...

type T = A | B

def f(t: type[T]) -> str:
    if issubclass(t, A):
        return "A"
    if issubclass(t, B):
        return "B"

    assert_never(t)

Ty reports this:

Type `@Todo & ~type[A] & ~type[B]` is not equivalent to `Never` (type-assertion-failure) [Ln 15, Col 5]
Inferred type of argument is `@Todo & ~type[A] & ~type[B]` [Ln 15, Col 18]
info: `Never` and `@Todo & ~type[A] & ~type[B]` are not equivalent types

ty.json:

{
    "environment": {
        "python-version": "3.12"
    },
    "rules": {
        "undefined-reveal": "ignore"
    }
}

Playground

I ran the same thing with mypy and looks like it's supported there.

Note that the equivalent code using an explicit union works with ty:

def f(t: type[A] | type[B]) -> str:
    if issubclass(t, A):
        return "A"
    if issubclass(t, B):
        return "B"

    assert_never(t)

Version

ty 0.0.56 (3078f9f 2026-07-01)

[sharkdp]

Thank you for reporting this.

It looks like a Todo type for intersections in type[..]. I am removing this Todo in my PR here, so this should hopefully soon be supported.

[sharkdp]

Btw, intersections are relevant here, because in the negated case, ~type[A | B] turns into ~type[A] & ~type[B] (de Morgan's law).