[ithkuil]

Perhaps because they take those features for granted because it uses some GC provided by LLVM ?

(I don't know anything about LLVM support for GC, but I would be surprised if it comes with a GC that doesn't require to do something special in order to make it concurrent)

[azakai]

LLVM GC support has been fairly weak, however Apple has been working on using LLVM in JavaScriptCore (FTL project), which requires that, so it should be getting better. I have no idea if this project is related to that in any way.

[cliffbean]

There is some support for interfacing LLVM with a GC, but LLVM itself does not include a GC.