From 4e1759cbc748dbdb672de791e2d58bbd3953849d Mon Sep 17 00:00:00 2001
From: Kristoffer Carlsson <kcarlsson89@gmail.com>
Date: Mon, 23 Oct 2023 14:19:11 +0200
Subject: [PATCH] fix eltype for partition iterators of substrings (#51773)

Fixes https://github.com/JuliaLang/julia/issues/51771

The convert method that asserts in #51771 is arguably still faulty
though.

(cherry picked from commit cf00550f955abd408bf0f478882c57887522f67e)
---
 base/strings/util.jl | 2 ++
 test/iterators.jl    | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/base/strings/util.jl b/base/strings/util.jl
index bd4da03ce1571..1154690ee846b 100644
--- a/base/strings/util.jl
+++ b/base/strings/util.jl
@@ -571,6 +571,8 @@ end
 
 # Specialization for partition(s,n) to return a SubString
 eltype(::Type{PartitionIterator{T}}) where {T<:AbstractString} = SubString{T}
+# SubStrings do not nest
+eltype(::Type{PartitionIterator{T}}) where {T<:SubString} = T
 
 function iterate(itr::PartitionIterator{<:AbstractString}, state = firstindex(itr.c))
     state > ncodeunits(itr.c) && return nothing
diff --git a/test/iterators.jl b/test/iterators.jl
index 59588bdac9684..a60ec32bb9ac0 100644
--- a/test/iterators.jl
+++ b/test/iterators.jl
@@ -1001,3 +1001,7 @@ end
     end
     @test v == ()
 end
+
+@testset "collect partition substring" begin
+    @test collect(Iterators.partition(lstrip("01111", '0'), 2)) == ["11", "11"]
+end