Disallow assignment of protocol or abstract classes to Callable

[hauntsaninja]

https://discuss.python.org/t/protocol-classes-should-not-match-callable-proto/75475

[hauntsaninja]

Prefect crash minimised is:

from typing import Protocol

class CP(Protocol):
    def __call__(self, parameters: str) -> str: ...

class NotSet: ...

class Task:
    def with_opt(self, trn: CP | type[NotSet] | None):
        if trn is not NotSet:
            reveal_type(trn)

[hauntsaninja]

CPython is a true positive, you get a TypeError with:

from libclinic.cli import *
parse_file("x.py", limited_capi=True)

cloud-init is also arguably a true positive, since they're testing that it raises TypeError

The diagnostics could be much better though.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

CPython (Argument Clinic) (https://github.com/python/cpython)
+ Tools/clinic/libclinic/cli.py:46: error: Incompatible types in assignment (expression has type "type[PythonLanguage]", target has type "Callable[[str], Language]")  [assignment]

pytest (https://github.com/pytest-dev/pytest)
+ testing/test_monkeypatch.py:418: error: Unused "type: ignore" comment  [unused-ignore]

cloud-init (https://github.com/canonical/cloud-init)
+ tests/unittests/reporting/test_reporting.py:181: error: Argument 3 to "assertRaisesRegex" of "TestCase" has incompatible type "type[ReportingHandler]"; expected "Callable[..., object]"  [arg-type]