Hi Samuel,

Don't misunderstand me, my question was not a criticism in any way ! I
was rather wondering if for some unknown reason to me (compiler
performance ?) a contribution that would require this rather impacting
change from bool to mpl::bool_ would be unwanted...

No problem, it's the way we understood it I think. So as Barend said, it was mostly chosen "by default", although the advantage he mentioned is valid. If you have a good reason to change it, please go ahead and send a PR, and we'll see what it looks like.

Regards
Bruno