fix #55389: type-unstable join (#55395)

(cherry picked from commit 09e5c40)

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))

test/strings/io.jl

@@ -339,3 +339,8 @@ 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