Fix annotated join with non-concrete eltype iters (#54919)

As raised in
<JuliaLang/StyledStrings.jl#57 (comment)>,
when the eltype of an iterator is non-concrete, `_isannotated` will
return false. To properly check such cases, we need to see if any of the
elements of the iterator are annotated.

This is a bit of an interesting case, since:
- Most of the time it shouldn't be hit, we can reasonably expect most
iterables to infer as producing concrete types
- The eltype of the iterator is (generally) known at compile-time, and
so in any case other than the ambiguous non-concrete one, this check
remains able to be done at compile-time.

With this change, join always preserves annotations. The compromise made
is that iterators with non-concrete eltypes can result in join inferring
a union return type (i.e. type instability with non-concrete iterators),
but that doesn't seem too bad to me (I don't see how we could be
completely type stable without concrete types here).

base/strings/io.jl

@@ -354,12 +354,20 @@ function join(io::IO, iterator, delim="")
 end
 
 function _join_preserve_annotations(iterator, args...)
-    if _isannotated(eltype(iterator)) || any(_isannotated, args)
+    if isconcretetype(eltype(iterator)) && !_isannotated(eltype(iterator)) && !any(_isannotated, args)
+        sprint(join, iterator, args...)
+    else
         io = AnnotatedIOBuffer()
         join(io, iterator, args...)
-        read(seekstart(io), AnnotatedString{String})
-    else
-        sprint(join, iterator, args...)
+        # If we know (from compile time information, or dynamically in the case
+        # 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)
+            read(seekstart(io), AnnotatedString{String})
+        else
+            String(take!(io.io))
+        end
     end
 end
 

test/strings/annotated.jl

@@ -101,6 +101,8 @@ end
                      [(1:4, :label => 5),
                       (5:5, :label => 2),
                       (6:9, :label => 5)])
+    @test join((String(str1), str1), ' ') ==
+        Base.AnnotatedString("test test", [(6:9, :label => 5)])
     @test repeat(str1, 2) == Base.AnnotatedString("testtest", [(1:8, :label => 5)])
     @test repeat(str2, 2) == Base.AnnotatedString("casecase", [(2:3, :label => "oomph"),
                                                        (6:7, :label => "oomph")])