more consistent findall output; fix #45495

[aplavin]

following @nalimilan's suggestion from #45495 discussion

[aplavin]

bump!

[aplavin]

bump

[LilithHafner]

I think the correct behavior would be to return a Vector{eltype(collect(keys(A)))} We currently get that wrong when we can infer a narrower type. This would fix the issue for concrete types but not for non-concrete types.

I won't make a value judgment on whether it is better to patch the common case now and leave a more obscure bug lurking or to wait until we get the behavior right in all cases.

[N5N3]

Another possible solution is defining something like

julia> Base._iterator_upper_bound(itr::Base.Generator) = itr.f(Base._iterator_upper_bound(itr.iter))

julia> Base._iterator_upper_bound(itr::Iterators.Filter) = Base._iterator_upper_bound(itr.itr)

julia> findall(x -> false, (1,2,3))
Int64[]

I'm not sure it's worth making such a change though.

[aplavin]

As I understand from the @LilithHafner comment, this PR is in improvement, even though not perfect. Can this be merged as it is?
I don't grasp the implications of @N5N3 approach, they touch iterators on a deeper level.

[aplavin]

bump...