Return type of property setters is not checked

[decorator-factory]

Example code:

class Banana:
    @property
    def size(self) -> int:
        return 42

    @size.setter
    async def size(self, value: int, /) -> None:
        print(f"set value to {value!r}")

b = Banana()
b.size = 42

I expected mypy to complain that the setter function doesn't return None and returns Coroutine[Any, Any, None] instead. Similarly, if the setter is a non-async function returning e.g. str or int, no error is reported.

Environment

• Mypy version used: 1.19.1
• Mypy command-line flags: --strict
• Python version used: 3.14

[TeamSpen210]

Is this actually a typing error? property discards the return value, so any return value is okay. Definitely not a desirable result though. Maybe there should be a diagnostic emitted when coercing any Awaitable-returning callable to one returning object, since code expecting a sync callable won't usually handle async ones.

[decorator-factory]

Various magic methods, such as __del__, __init_subclass__, __setattr__, __delattr__, do ignore the return value and skip checking if it's None (at this point it would be a big backwards incompatible change). However, returning something else is probably a mistake, and mypy already does report an error for __setattr__, __delattr__ and __init_subclass__ returning not-None values.

As a data point, ty does flag this code as an error, but pyright and pyrefly don't

P.S. I wonder why __init__ does check the return value but not __init_subclass__?