Assume a function like:

def get_item(key: str, default: Optional[Item] = None) -> Optional[Item]:
   # implementation goes here
   ...

Now let's assume we want to enhance typing through an @overload by changing it to:

@overload
def get_item(key: str) -> Item:
   ...

def get_item(key: str, default: Optional[Item] = None) -> Optional[Item]:
   # implementation goes here
   ...

mypy will complain with:

error: Single overload definition, multiple required

This check makes sense for a .pyi file where you would expect multiple @overloads on equal footing, but in this case the original declaration might as well serve as an "overload".

A workaround is obviously to add:

@overload
def get_item(key: str, default: Optional[Item]) -> Optional[Item]:
   ...

which adds needless verbosity.