add PEP-585 support to singledispatch

[deadPix3l]

Proposal: allow more specific dispatching using PEP-585 annotations

@dispatch.register
def _(arg: list[int|str]):
    return "this is a list of ints and strings!"

currently returns:

TypeError: Invalid annotation for 'arg'. list[int|str] is not a class.

This means we have to simplify the annotation at the expense of complicating the function, and less support for mypy:

@dispatch.register
def _(arg: list):
    # list[int|str]
    if all(isinstance(i, (int,str)) for i in arg):
        return "this is a list of ints and strings!"

    # elif <other types>:
        # other logic

    else:
        # list[any]

Personally I find this antitheitcal to singledispatch, which is supposed to solve this exact problem

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://bugs.python.org/issue46191
https://discuss.python.org/t/singledispatch-support-for-pep-585-generic-types/86772

This does not seem to have wide support (yet), but its completely additional to the singledispatch decorator, and the non generic types will still work

Linked PRs

• # gh-131996: Add PEP-585 support to singledispatch types #131995

[sobolevn]

See my comment in the PR and https://discuss.python.org/t/singledispatch-support-for-pep-585-generic-types/86772/2

[deadPix3l]

Very valid reasons this will not be merged have been raised.
It has serious constraints and while it does work for a limited set of containers (list, tuple, set, etc),
it is not nearly good enough to be a core feature of python.
I accept this.

However, if you're reading this, you likely want/need this feature, as i still do, which I will release as a separate package:
https://github.com/deadPix3l/complexdispatch