The single underscore version is actually slightly more supported among
the compilers I've seen so far. Also added the exact list now.
Part of P0183, funded by Yanga and [Anonymous].
Might look uglier, but has the advantage of not generating an empty
segment with the default name… *and* the default padding, which will
really come in handy with the following breakthrough.
Part of P0137, funded by [Anonymous].