can't compose @future_safe coroutines that specify picky exceptions

まず、returns[compatible-mypy]==0.25.0 と mypy を使って提供された mre.py の例を実行し、次に future_safe と FutureResult.bind の型付けの挙動を調べます。2 つのコルーチンが型エラーなしに合成され、それぞれの ConnectionError と ZeroDivisionError のケースが分離されたままになれば完了です。

Given coroutines coro1, coro2:

@future_safe(exceptions=(ConnectionError,))
async def coro1(c: int | None) -> int:
    if c is None:
        raise ConnectionError("not connected")
    await asyncio.sleep(0)  # emulate I/O
    return c


@future_safe(exceptions=(ZeroDivisionError,))
async def coro2(n: int) -> float:
    await asyncio.sleep(0)  # emulate I/O
    return 1 / n

I want to combine them via bind: coro1(c).bind(coro2). It results in type error:

$ uvx --with 'returns[compatible-mypy]==0.25.0' mypy mre.py; ./mre.py
mre.py:30: error: Argument 1 to "bind" of "FutureResult" has incompatible type "Callable[[int], FutureResult[float, ZeroDivisionError]]"; expected "Callable[[int], KindN[FutureResult[Any, Any], float, ConnectionError, Any]]"  [arg-type]

where mre.py:

#!/usr/bin/env -S uv run --script
# /// script
# requires-python = ">=3.11"
# dependencies = [
#     "returns >=0.25.0",
# ]
# ///
from typing import assert_never
from returns.result import Success, Failure
from returns.io import IOSuccess, IOFailure
from returns.future import future_safe


@future_safe(exceptions=(ConnectionError,))
async def coro1(c: int | None) -> int:
    if c is None:
        raise ConnectionError("not connected")
    await asyncio.sleep(0)  # emulate I/O
    return c


@future_safe(exceptions=(ZeroDivisionError,))
async def coro2(n: int) -> float:
    await asyncio.sleep(0)  # emulate I/O
    return 1 / n


async def run() -> None:
    for c in [2, 0, None]:
        match await coro1(c).bind(coro2):
            case IOSuccess(Success(r)):
                assert r == 1 / 2, r
            case IOFailure(Failure(ZeroDivisionError(args=(msg,)))):
                assert msg == "division by zero", msg
            case IOFailure(Failure(ConnectionError(args=(msg,)))):
                assert msg == "not connected", msg
            case _ as unreachable:
                assert_never(unreachable)  # type: ignore[arg-type]


if __name__ == "__main__":
    import asyncio

    asyncio.run(run())

If picky exceptions are removed:

@future_safe
async def coro1(c: int | None) -> int: ...

@future_safe
async def coro2(n: int) -> float: ...

I get the desired behavior that coroutines are composed without type errors but these declarations catch too much.