[Boost-users] Using boost::enable_if with template copy constructors

From: Tim Robertson (timr_at_[hidden])
Date: 2006-09-05 19:45:31

Hi,

Can anyone explain why the following won't compile (using g++ 3.3.2 
under Linux):

#include <boost/utility/enable_if.hpp>
#include <boost/type_traits/is_convertible.hpp>

using namespace std;

struct Foo
{
   Foo() {}

   template <typename T>
   Foo(T const & src,
       typename boost::enable_if<
       boost::is_convertible<T,Foo>, void >::type * dummy = 0)
  {
    int i = 1;  // filler.
  }

   template <typename T>
   Foo(T const & src,
       typename boost::disable_if<
       boost::is_convertible<T,Foo>, void >::type * dummy = 0)
  {
    int i = 2;  // ditto.
  }

};

int main()
{
   Foo f;
   Foo f2(f);

   return 0;
};

When I attempt to compile this code with g++ 3.3.2, I get the following 
error messages:

/boost/boost/type_traits/is_convertible.hpp: In instantiation of 
`boost::disable_if<boost::is_convertible<Foo, Foo>, void>':
/boost/boost/type_traits/is_convertible.hpp:128:   instantiated from 
`boost::detail::is_convertible_basic_impl<Foo&, Foo>'
/boost/boost/type_traits/is_convertible.hpp:228:   instantiated from 
`boost::detail::is_convertible_impl<Foo, Foo>'
test.cpp:36:   instantiated from 
`boost::detail::is_convertible_impl_dispatch<Foo, Foo>'
test.cpp:36:   instantiated from `boost::is_convertible<Foo, Foo>'
test.cpp:36:   instantiated from 
`boost::enable_if<boost::is_convertible<Foo, Foo>, void>'
test.cpp:36:   instantiated from here
/boost/boost/type_traits/is_convertible.hpp:128: error: `value'
    is not a member of type `boost::is_convertible<Foo, Foo>'
/boost/boost/type_traits/is_convertible.hpp: In instantiation of 
`boost::detail::is_convertible_basic_impl<Foo&, Foo>':
/boost/boost/type_traits/is_convertible.hpp:228:   instantiated from 
`boost::detail::is_convertible_impl<Foo, Foo>'
test.cpp:36:   instantiated from 
`boost::detail::is_convertible_impl_dispatch<Foo, Foo>'
test.cpp:36:   instantiated from `boost::is_convertible<Foo, Foo>'
test.cpp:36:   instantiated from 
`boost::enable_if<boost::is_convertible<Foo, Foo>, void>'
test.cpp:36:   instantiated from here
/boost/boost/type_traits/is_convertible.hpp:128: error:
    initializing argument 1 of `static yes_type
    boost::detail::checker<T>::_m_check(T, int) [with T = Foo]'

If I use boost::is_same<> in place of boost::is_convertible<>, 
everything is fine.  Is there something that I'm missing, or is this a bug?

Thanks in advance,
Tim