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.