Regression: No matching overload error in 0.780 upwards, while (correctly) not showing a warning in 0.770 and below

[rggjan]

This code:

from typing import List, Tuple, TypeVar

T = TypeVar("T")


def foo(x: T) -> None:
    a: List[Tuple[int, T]] = [(2, x)]

    for t in zip(*a):
        reveal_type(t)
        list(t)

produces the warning:

main.py:10: note: Revealed type is 'builtins.object'
main.py:11: error: No overload variant of "list" matches argument type "object"
main.py:11: note: Possible overload variant:
main.py:11: note:     def [_T] __init__(self, iterable: Iterable[_T]) -> List[_T]
main.py:11: note:     <1 more non-matching overload not shown>

when running mypy test.py on 0.780.

Running the same through mypy 0.770 works fine and doesn't produce a warning.

(Also, running the example works fine of course)

[gvanrossum]

Hm, in 0.770 the revealed type is tuple[Any], which is arguably better indeed, and seems to match what happens at runtime. zip's type is a complex overload -- maybe something changed there since 0.770?

[JelleZijlstra]

I think this is python/typeshed#4226; we got too clever with zip() and had to revert. Does it work for you on current master?

[gvanrossum]

Oh, yeah, this is fixed on master.