Hi Albert,

I remember having been reluctant when we added a parameter to traits::tag for that purpose, but having accepted after struggling to find any other way. I've given it another thought after your email and none of the hacks I've been thinking of can work. So it might indeed be that any traits metafunction should have an extra parameter in order to be usable with enable_if.

Anybody in the BG team has a better idea?

Thanks
Bruno