`IteratorSize` incorrect for some subtypes of `Generator`

[nsajko]

julia> collection_of_iterators = [Iterators.map(identity, 3), Iterators.map(sqrt, 3)];

julia> Base.IteratorSize.(collection_of_iterators)
2-element Vector{Base.HasShape{0}}:
 Base.HasShape{0}()
 Base.HasShape{0}()

julia> Base.IteratorSize(eltype(collection_of_iterators))  # incorrect, hits the fallback method
Base.HasLength()

There are two correct options for the return type here:

• HasShape{0}: precise and accurate
• SizeUnknown: imprecise, but still accurate

The fix should be simple, this is the relevant method, just need to relax the dispatch:

julia/base/generator.jl

Line 101 in 772745b

IteratorSize(::Type{Generator{I,F}}) where {I,F} = IteratorSize(I)

NB: This is perhaps part of a wider issue of HasLength not being a correct fallback, but that's a separate topic.