From 09e5c40173297365cf09a09aec876d0ba6493958 Mon Sep 17 00:00:00 2001
From: Simeon David Schaub <simeon@schaub.rocks>
Date: Tue, 6 Aug 2024 19:03:54 +0200
Subject: [PATCH] fix #55389: type-unstable `join` (#55395)

---
 base/strings/io.jl | 5 +++--
 test/strings/io.jl | 5 +++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/base/strings/io.jl b/base/strings/io.jl
index c88b7ac939c8d..acbd945c8e137 100644
--- a/base/strings/io.jl
+++ b/base/strings/io.jl
@@ -354,7 +354,8 @@ function join(io::IO, iterator, delim="")
 end
 
 function _join_preserve_annotations(iterator, args...)
-    if isconcretetype(eltype(iterator)) && !_isannotated(eltype(iterator)) && !any(_isannotated, args)
+    et = @default_eltype(iterator)
+    if isconcretetype(et) && !_isannotated(et) && !any(_isannotated, args)
         sprint(join, iterator, args...)
     else
         io = AnnotatedIOBuffer()
@@ -363,7 +364,7 @@ function _join_preserve_annotations(iterator, args...)
         # of iterators with a non-concrete eltype), that the result is annotated
         # in nature, we extract an `AnnotatedString`, otherwise we just extract
         # a plain `String` from `io`.
-        if isconcretetype(eltype(iterator)) || !isempty(io.annotations)
+        if isconcretetype(et) || !isempty(io.annotations)
             read(seekstart(io), AnnotatedString{String})
         else
             String(take!(io.io))
diff --git a/test/strings/io.jl b/test/strings/io.jl
index 79e11d8cf5184..209844580b3cd 100644
--- a/test/strings/io.jl
+++ b/test/strings/io.jl
@@ -344,3 +344,8 @@ end
 @testset "`string` return types" begin
     @test all(T -> T <: AbstractString, Base.return_types(string))
 end
+
+@testset "type stable `join` (#55389)" begin
+    itr = ("foo" for _ in 1:100)
+    @test Base.return_types(join, (typeof(itr),))[] == String
+end