Mypy fails on `Union[float, int] + Union[float, int]`

[rowillia]

def add_value(a: Union[float, int], b: Union[float, int]) -> Union[float, int]:
    return a + b

Results in:

error: Unsupported operand types for + ("Union[float, int]" and "int")

Interestingly both

def add_value(a: Union[float, int], b: float) -> Union[float, int]:
    return a + b

and

def add_value(a: int, b: Union[float, int]) -> Union[float, int]:
    return a + b

work fine

[gvanrossum]

That's definitely problematic. I suspect it's because we don't try all combinations of all unions involved in an expression. We should probably do better, although overuse of this could cause performance regressions.

[elazarg]

Now I believe that this is another incarnation of overload python/typing#253 and therefore #1941 and the others mentioned there. I cannot base this claim though.

[wsanchez]

Regarding @gvanrossum's comment: "overuse of this could cause performance regressions"…

Uh… code that's wrong isn't especially useful no matter how fast it is.

If it's too slow for whatever criteria, then it would be good to make this case a warning rather than an error, as false positives break CI builds that are trying to use mypy for code quality checks, and this check is clearly incomplete/broken.

[wsanchez]

(or give up with a warning if the matrix is too large)