[Boost-maint] [spirit] How to detect Spirit.Qi expressions?

Subject: [Boost-maint] [spirit] How to detect Spirit.Qi expressions?
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-03-30 08:47:33

Hi,

Is there a way to detect at compile time that a certain type is a 
Boost.Spirit.Qi parser? That includes template expressions and grammars. 
Basically, I want to detect if qi::parse(b, e, expr) would be a sensible thing 
to do.

I tried this:

  boost::is_same<
    typename proto::domain_of< ExpressionT >::type,
    spirit::qi::domain
  >

But domain_of returns default_domain for Boost.Spirit.Qi template expressions.