christophe henry wrote:
Thanks Eric for your fast answer! Unfortunately, this doesn't solve my problem. To make a very very long discussion long, here a small example showing the problem from another side: I have an expression wrapper called mp_terminal and I define terminals, but not in the documented way (which I suspect is the problem)
Indeed.
but instead deriving from them (for convenience purposes), for example: struct True : mp_terminal<typename proto::terminal<guard_tag>::type> {...}
<snip>
Then, why do most use cases work like a charm? Only unary operators seem to fail.
It's because (a) True is a Proto type, but hasn't been made so by directly using proto::extends, and (b) Proto's unary operators are being a little too nit-picky about (a). I have fixed (b) on trunk (changelist 54610). In the mean time, you can change your definition of True and False to use this formulation: struct True : proto::extends< mp_terminal<typename proto::terminal<guard_tag>::type> , True , sm_domain > or the simpler and more direct struct True : proto::extends< typename proto::terminal<guard_tag>::type , True , sm_domain > HTH, -- Eric Niebler BoostPro Computing http://www.boostpro.com