fix #55389: type-unstable join

Author: simeonschaub

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

@@ -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