[Boost-users] [mpl] what is not a placeholder expression

From: JOAQUIN LOPEZ MU?Z (joaquin_at_[hidden])
Date: 2007-07-27 13:38:22

According to the MPL reference, a placeholder expression
X<a1,..,an> must satisfy (among others) the following
condition:

  "All of X's template parameters, including the
  default ones, are types."

According to that the following should then compile
without errors:

  #include <boost/mpl/apply.hpp>
  #include <boost/mpl/assert.hpp>
  #include <boost/type_traits/is_same.hpp>

  using namespace boost;
  using namespace boost::mpl;

  template<typename T,int=0>
  struct foo 
  {
    template<typename Q>
    struct apply{
      typedef Q type;
    };
  };

  typedef apply1<foo<_1>,int>::type t1;

  BOOST_MPL_ASSERT((is_same<t1,int>));

  int main(){}

However, I've tried with GCC 3.4.4 and the static
assertion fails! I'd like to know:

1. Whether I'm right in assuming that the code above
should compile without assertion failures.
2. What the behavior is for other versions of GCC and
compilers: reports most welcome!!
3. Whether this is a known problem specific to GCC
or, more generally, what the status of this issue is.

Thank you,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo