Standardizing argument unpacking behaviors for calls

[mikeshardmind (Michael H)]

@carljm

If we believe the lax handling is necessary in the latter two cases, then it seems to me we should remove the awkward distinction we currently make between tuple[Any, ...] and tuple[int, ...], and just specify variadic tuples as always gradual in their length.

If we think it is necessary that assigning tuple[int, ...] to tuple[int, int] should be an error, then I find it hard to see why it shouldn’t also be an error to unpack it to a fixed number of targets in a call or an assignment.

Neither mypy nor pyright have an issue with unpacking Sequence[T] without knowing the length either. I think the only option that resolves inconsistency here is treat variadic tuples as always gradual with respect to length.

The other option here creates a situation where either tuple[T, ...] shouldn’t be assignable to Sequence[T], or that we have to disallow unpacking sequences too.