`singledispatch.register` doesn't typecheck registration of generic functions

To Reproduce

Run mypy (no options) on this file:

⚠️ See below for actual MWE. This is preserved here to preserve the flow of discussion.

from typing import TypeVar
from functools import singledispatch

T = TypeVar("T")


@singledispatch
def f(x: T) -> T:
    return x


class A:
    pass


@f.register(A)
def _f_a(x: A) -> A:
    return x

Expected Behavior

No complaint. There's no bound on T, so I feel that _f_a should be acceptable.

Actual Behavior

mypybug.py:15: error: Dispatch type "A" must be subtype of fallback function first argument "T"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.961
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.10
• Operating system and version: Debian Linux

Start by running mypy without options on the reproduced mypybug.py example and compare the diagnostic from the singledispatch.register call with the expected behavior. Trace the type-checking entry point for generic fallback functions and registration dispatch types; done means the example produces no complaint while incompatible registrations remain checked.