from functools import singledispatch
import typing as tp

@singledispatch
def f(x: tp.Any) -> None: pass

@f.register
def _f_list(x: list[int]) -> None: pass

if __name__ == '__main__':
    f([1, 2, 3])

$ mypy sp.py
Success: no issues found in 1 source file

# on import no error as well, so Mypy raising an error would be very useful
$ python -c 'import sp'

$ python sp.py
Traceback (most recent call last):
  File "/usr/lib/python3.10/functools.py", line 831, in dispatch
    impl = dispatch_cache[cls]
  File "/usr/lib/python3.10/weakref.py", line 416, in __getitem__
    return self.data[ref(key)]
KeyError: <weakref at 0x7f390d605c60; to 'type' at 0x7f390e07bd00 (list)>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.10/functools.py", line 834, in dispatch
    impl = registry[cls]
KeyError: <class 'list'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/wrobell/projects/rbfly/sp.py", line 11, in <module>
    f([1, 2, 3])
  File "/usr/lib/python3.10/functools.py", line 878, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/usr/lib/python3.10/functools.py", line 836, in dispatch
    impl = _find_impl(cls, registry)
  File "/usr/lib/python3.10/functools.py", line 783, in _find_impl
    mro = _compose_mro(cls, registry.keys())
  File "/usr/lib/python3.10/functools.py", line 744, in _compose_mro
    types = [n for n in types if is_related(n)]
  File "/usr/lib/python3.10/functools.py", line 744, in <listcomp>
    types = [n for n in types if is_related(n)]
  File "/usr/lib/python3.10/functools.py", line 743, in is_related
    and issubclass(cls, typ))
TypeError: issubclass() argument 2 cannot be a parameterized generic