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

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