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()

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