Great!

But I prefer a little bit more generic solution:

class MyRestInterfaceClient(T) : RestInterfaceClient!T
{
    override Json request(...) const
    {
        ...     
    }
}

interface IGitlabApi
{
    @path("projects/:p/merge_requests/:mr/notes")
    void postCommentForMergeRequest(uint _p, uint _mr, string body_);
}

...

auto new api = MyRestInterfaceClient!IGitlabApi(host);