Mypyc ignores custom __new__ without warning

[nickdrozd]

Mypyc apprently ignores custom __new__ methods without warning. It would be great if it could abide by the custom __new__, but otherwise a warning / error should be raised.

Example code (see #1020 (comment)):

from __future__ import annotations

class Add:
    l: IntLike
    r: IntLike

    def __new__(cls, l: IntLike, r: IntLike) -> IntLike:  # type: ignore[misc]
        print(f'running __new__ with {l} and {r}')

        return (
            l if r == 0 else
            r if l == 0 else
            super().__new__(cls)
        )

    def __init__(self, l: IntLike, r: IntLike):
        self.l = l
        self.r = r

    def __repr__(self) -> str:
        return f'({self.l} + {self.r})'

    def __add__(self, other: IntLike) -> IntLike:
        return Add(self, other)

IntLike = int | Add

Code for running:

print(f'{Add(1, 5)=}')
print(f'{Add(0, 5)=}')
print(f'{Add(1, 0)=}')

Running this interpreted gives:

running __new__ with 1 and 5
Add(1, 5)=(1 + 5)
running __new__ with 0 and 5
Add(0, 5)=5
running __new__ with 1 and 0
Add(1, 0)=1

Running compiled:

Add(1, 5)=(1 + 5)
Add(0, 5)=(0 + 5)
Add(1, 0)=(1 + 0)

The interpreted and compiled behaviors are different, which is bad.