It would be well worth it to improve the error message, however:

```
>>> from functools import singledispatch
>>> @singledispatch
... def func(arg):
...     raise NotImplementedError
... 
>>> @func.register
... def _(arg: list[str]):
...     print('Got a list of strings')
... 
>>> func(1)
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/functools.py", line 830, in dispatch
    impl = dispatch_cache[cls]
  File "/usr/local/lib/python3.9/weakref.py", line 405, in __getitem__
    return self.data[ref(key)]
KeyError: <weakref at 0x7f2a0d9141d0; to 'type' at 0x7f2a0e08b200 (int)>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/functools.py", line 833, in dispatch
    impl = registry[cls]
KeyError: <class 'int'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/functools.py", line 877, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
    return dispatch(args[0].__class__)(*args, **kw)
  File "/usr/local/lib/python3.9/functools.py", line 835, in dispatch
    impl = _find_impl(cls, registry)
  File "/usr/local/lib/python3.9/functools.py", line 782, in _find_impl
    mro = _compose_mro(cls, registry.keys())
  File "/usr/local/lib/python3.9/functools.py", line 743, in _compose_mro
    types = [n for n in types if is_related(n)]
  File "/usr/local/lib/python3.9/functools.py", line 743, in <listcomp>
    types = [n for n in types if is_related(n)]
  File "/usr/local/lib/python3.9/functools.py", line 742, in is_related
    and issubclass(cls, typ))
TypeError: issubclass() argument 2 cannot be a parameterized generic
```