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) but instead deriving from them (for convenience purposes), for example: struct True : mp_terminal<typename proto::terminal<guard_tag>::type> {...} These terminals are used with a simple grammar: struct BuildGuards : proto::or_< proto::when< proto::logical_or<BuildGuards,BuildGuards >, GuardOR<BuildGuards(proto::_left),BuildGuards(proto::_right)>() >, proto::when< proto::logical_and<BuildGuards,BuildGuards >, GuardAND<BuildGuards(proto::_left),BuildGuards(proto::_right)>() >, proto::when< proto::logical_not<BuildGuards >, GuardNOT<BuildGuards(proto::_value)>() >, proto::when < proto::terminal<guard_tag>, proto::_()

{}; Then, expressions like: True()|| False() give the expected result (struct GuardOR<struct True,struct False>). Even True()&& (Dummy1() || !(False() || True())) give the expected result: struct GuardAND<struct True,struct GuardOR<struct Dummy1,struct GuardNOT<struct GuardOR<struct False,struct True> > > > But a "!" alone (meaning followed by just a terminal, not the other parts of the BuildGuards grammar) breaks all (as do all unary operators I tried), like !False() or True()|| !False(). !False() => struct GuardNOT<struct mp_terminal<struct boost::proto::exprns_::expr<struct boost::proto::tag::terminal,struct boost::proto::argsns_::term<struct guard_tag>,0> > > Notice that !(False() || True()) works! Now, if I do not derive from mp_terminal but instead proceed as documented, all works as expected: proto::terminal<True>::type True_; ... !False_ => correct result. Sadly, this documented usage is unpractical in my use case. So, supposing that I managed this easy grammar, I imagine that the usage I do of terminals (deriving from them) is not allowed. Then, why do most use cases work like a charm? Only unary operators seem to fail. Attached are 2 test files. V1 doesn't work in all cases while V2 does. I use 1.38. Thanks a lot! Christophe _________________________________________________________________ Windows Live™: Keep your life in sync. Check it out! http://windowslive.com/explore?ocid=TXT_TAGLM_WL_t1_allup_explore_012009