Narrowing causing Mypyc runtime type error

[nickdrozd]

Have code that uses type alias ubiquitously:

type Count = int | Num

where Num is int-like. Num and int interact and coexist. Has been running fine compiled with Mypyc.

e64a84b has introduced some problem. Now getting TypeError: tm.num.Num object expected; got int.

[ilevkivskyi]

@hauntsaninja could you please check this?
@nickdrozd it will be helpful if you can give a full self-consistent repro.

[alansajith]

Now getting TypeError: tm.num.Num object expected; got int.

Root cause analysis
The TypeError: tm.num.Num object expected; got int occurs because Mypyc compiles the
Num class as a strict native class, which enforces nominal type checking at runtime using PyObject_TypeCheck. This check only accepts instances that are exactly of type Num or its subclasses, rejecting int even if it's logically compatible.

We can solve this issue in two ways

1. using a protocol
   Define Num as a Protocol. Mypyc treats Protocols with lenient structural checks that accept compatible types like

2. Disable native compilation
   Use the @mypyc_attr(native_class=False) decorator to compile Num as a regular Python class:

This disables Mypyc's optimizations for Num but allows standard Python duck typing.

[nickdrozd]

@ilevkivskyi I don't have a small reproducer. Here is affected module: https://github.com/nickdrozd/busy-beaver-stuff/blob/main/tm/num.py. Runs fine with previous commit (eabda0e).

[hauntsaninja]

Will take a look! Could you share some setup / repro commands?

[nickdrozd]

test_num.py
num.py

With these two files, run python3 -m mypyc num.py && python3 -m unittest test_num. Should get a bunch of test failures with message TypeError: num.Num object expected; got int.

[hauntsaninja]

Thank you!

Codex minimised the test case a little:

from __future__ import annotations


type Count = int | Num


class Num:
    def __eq__(self, other: object) -> bool:
        return other is self

    def __radd__(self, other: int) -> Count:
        raise NotImplementedError


class Exp(Num):
    exp: Count

    def __init__(self, exp: Count) -> None:
        self.exp = exp

    def __add__(self, other: Count) -> Count:
        if isinstance(other, Exp):
            sexp, oexp = self.exp, other.exp

            if sexp == oexp:
                return Exp(1 + sexp)

        return self


def run() -> Count:
    return Exp(3) + Exp(3)

I see what logic I'm missing (comparison between custom eq expr and custom eq target), will put up a PR soon