Type narrowing of `target: type[T]` using insinstance seems to fool mypy

[juliancoffee]

Bug Report
Well, I was writing some sort of converter from unstructured data to a dataclass, so I had a function that uses f[T](obj, target_type: type[T]) -> T and while in the end I stick to typing.cast for that, I also tried several solutions, they are below.

One of them intuitively should(?) work (return_checktype), but mypy refuses to follow them, and one of them is obviously wrong in hindsight (return_checkval), but mypy is happy with it.

To Reproduce

from typing import reveal_type


def return_checkval[T](target_type: type[T]) -> T:
    # runtime check that target_type is int
    if isinstance(target_type, int):
        reveal_type(target_type)
        return 42  # and they lived happily ever after

    raise ValueError("Unsupported type")


x = return_checkval(int)
print(x)


def return_checktype[T](target_type: type[T]) -> T:
    # runtime check that target_type is int
    if target_type is int:
        reveal_type(target_type)
        return 42  # errors here

    raise ValueError("Unsupported type")


# example test
x = return_checktype(int)
print(x)

Expected Behavior

At the very least, it should have two reveal_type outputs, and it should reject the first function completely. That's what (based)pyright.
Ideally, it should also allow the second function, but (based)pyright also rejects it :(

Actual Behavior

t.py:20: note: Revealed type is "type[T`-1]"
t.py:21: error: Incompatible return value type (got "int", expected "T")  [return-value]
Found 1 error in 1 file (checked 1 source file)

Traceback (most recent call last):
  File "/private/tmp/test/t.py", line 13, in <module>
    x = return_checkval(int)
  File "/private/tmp/test/t.py", line 10, in return_checkval
    raise ValueError("Unsupported type")
ValueError: Unsupported type

Your Environment

mypy version 1.19.1 (compiled yes)
python version .. honestly have no idea which one it uses, but python --version is 3.14
just run it mypy . from the folder in /tmp

[hauntsaninja]

Thanks! Note mypy will complain about return_checkval if you use --warn-unreachable

Mypy thinks the branch is unreachable because the isinstance will always be False (ignoring multiple inheritance, which type checkers often have to do to stay sane). Mypy then (somewhat controversially) does not report type errors in unreachable code.

[juliancoffee]

Oh, that explains it, thanks 😅

And I assume return_checktype behaviour is somewhat expected, or is there some issue to track the progress of it?
In which case, this issue could probably be closed.

[hauntsaninja]

Yes, it is a kind of known deficiency. I am currently in the midst of rewriting a lot of mypy's type narrowing logic (#20564), so I will maybe look at it, but will be tricky to do fully generally

As with most things mypy there is probably some duplicate issue, but feel free to keep the issue open... We have some nice tech for figuring out which PRs affect which issues