`eltype` could be more accurate for `Iterators.flatten`

[gaurav-arya]

On julia 1.9.0-beta2, I'm observing the following behaviour when using Iterators.flatten to concatenate two iterators of integers:

julia> iter = Iterators.flatten(((3,), [4, 5]))
Base.Iterators.Flatten{Tuple{Tuple{Int64}, Vector{Int64}}}(((3,), [4, 5]))

julia> eltype(iter)
Any

It feels like this could be improved? If all the concatenated iterators have eltype of Int64, then shouldn't iter too?

I'm happy to make a PR for this if I'm not missing something and this indeed could be impoved. Version info:

julia> versioninfo()
Julia Version 1.9.0-beta2
Commit 7daffeecb8c (2022-12-29 07:45 UTC)
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-14.0.6 (ORCJIT, tigerlake)
  Threads: 1 on 8 virtual cores
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 

[jakobnissen]

Maybe something like:

Base.eltype(::Type{Base.Iterators.Flatten{T}}) where T = tuple_typejoin(T)

tuple_typejoin(::Type{Tuple{T}}) where T = eltype(T)
function tuple_typejoin(::Type{T}) where {T <: Tuple}
    E = eltype(Base.tuple_type_head(T))
    E === Any ? Any : typejoin(E, tuple_typejoin(Base.tuple_type_tail(T)))
end

Possibly with some @nospecialize

[andrewjradcliffe]

Eltype is properly inferred, as demonstrated below.

@code_warntype collect(iter)
@code_warntype collect(Iterators.map(identity, iter))
@code_warntype map(identity, iter)

Albeit, I suppose that this is not true of the eltype function itself.

[jakobnissen]

This has been fixed on master