On Saturday, 9 July 2016 at 23:14:38 UTC, ZombineDev wrote:
>
> https://github.com/dlang/druntime/blob/master/src/gc/gcinterface.d
> https://github.com/dlang/druntime/blob/master/src/gc/impl/manual/gc.d
>
> What else do you need to start working on a new GC implementation?

That is actually the only case that I know of that an interface was provided to be implemented by 3rd parties... My reply was about Phobos in general. To repeat again - Phobos should provide the API (interfaces) *and* reference implementations of those.