Concatenate implementation generates runtime exception if parameterized by ...

[erictraut]

The following code should work without a runtime exception.

from typing import Any, Callable
from typing_extensions import ParamSpec, Concatenate


P = ParamSpec("P")
MyAlias = Callable[Concatenate[int, P], Any]

x: MyAlias[...]

It runs fine on Python 3.10 if using typing.Concatenate but fails on Python 3.9 if using typing_extensions.Concatenate. The runtime exception is:

TypeError: Parameters to generic types must be types. Got Ellipsis.

[Fidget-Spinner]

This seems like a bug in Python's typing module itself.

# In 3.9
typing.Callable[T, int][...]

TypeError: Parameters to generic types must be types. Got Ellipsis.

The problem is that Callable uses the default _GenericAlias's substitution behavior pre 3.10. Which means its checks are too restrictive. We had to intentionally loosen the checks in 3.10 to get around this.

I don't think we can fix this from typing_extensions unless we monkey patch typing._GenericAlias.__getitem__ (or typing._type_check). I don't feel too good doing that though.

[Fidget-Spinner]

Also, we're sadly past the stage for bugfixes in 3.9. I suggest that we add this to the known limitations in the README and (unfortunately) close as can't fix.

[JelleZijlstra]

This is now documented as a limitation at https://typing-extensions.readthedocs.io/en/latest/#Concatenate. If there is a reasonable way to fix it, I'd accept a PR.