`Iterators.partition` does not work on substrings

[chrstphrbrns]

julia> lstrip("01111", '0')
"1111"

julia> collect(Iterators.partition("1111", 2))
2-element Vector{SubString{String}}:
 "11"
 "11"

julia> collect(Iterators.partition(lstrip("01111", '0'), 2))
ERROR: TypeError: in typeassert, expected SubString{SubString{String}}, got a value of type SubString{String}
Stacktrace:
 [1] convert(::Type{SubString{SubString{String}}}, s::SubString{String})
   @ Base ./strings/substring.jl:57
 [2] setindex!(A::Vector{SubString{SubString{String}}}, x::SubString{String}, i1::Int64)
   @ Base ./array.jl:1019
 [3] copyto!(dest::Vector{SubString{SubString{String}}}, src::Base.Iterators.PartitionIterator{SubString{String}})
   @ Base ./abstractarray.jl:941
 [4] _collect(cont::UnitRange{Int64}, itr::Base.Iterators.PartitionIterator{SubString{String}}, ::Base.HasEltype, isz::Base.HasLength)
   @ Base ./array.jl:763
 [5] collect(itr::Base.Iterators.PartitionIterator{SubString{String}})
   @ Base ./array.jl:757
 [6] top-level scope
   @ REPL[76]:1

julia> collect(Iterators.partition(string(lstrip("01111", '0')), 2))
2-element Vector{SubString{String}}:
 "11"
 "11"

[KristofferC]

#51773 should fix this. The issue is that eltype here lied about what element that iterating it actually produce:

julia> itr = Iterators.partition(lstrip("01111", '0'), 2)
Base.Iterators.PartitionIterator{SubString{String}}("1111", 2)

julia> eltype(itr)
SubString{SubString{String}}

julia> foreach(x -> @show(typeof(x)), itr)
typeof(x) = SubString{String}
typeof(x) = SubString{String}