id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
7192	[mpl] std::integral_constant support	Eric Niebler	Aleksey Gurtovoy	"The following reasonable-seeming use of mpl::plus doesn't compile:

{{{
    #include <utility>
    #include <boost/mpl/plus.hpp>

    typedef std::integral_constant<int, 1> one;
    typedef boost::mpl::plus<one, one>::type two;
}}}

That's because mpl only works with its own integral constant wrappers,
not the standard one. That seems unfortunate to me. Is there any
interest in supporting this?

Here's the error (from clang trunk with glibc++):

{{{
    1>  In file included from main.cpp:2:
    1>  In file included from
/home/Eric/boost/org/trunk/boost/mpl/plus.hpp:19:
    1>  In file included from
/home/Eric/boost/org/trunk/boost/mpl/aux_/arithmetic_op.hpp:34:
    1>  In file included from
/home/Eric/boost/org/trunk/boost/mpl/aux_/include_preprocessed.hpp:37:
    1>
/home/Eric/boost/org/trunk/boost/mpl/aux_/preprocessed/gcc/plus.hpp:60:25:
error: no type named 'tag' in 'std::integral_constant<int, 1>'
    1>      typedef typename T::tag type;
    1>              ~~~~~~~~~~~~^~~
    1>
/home/Eric/boost/org/trunk/boost/mpl/aux_/preprocessed/gcc/plus.hpp:111:20:
note: in instantiation of template class
'boost::mpl::plus_tag<std::integral_constant<int, 1> >' requested here
    1>            typename plus_tag<N1>::type
    1>                     ^
}}}
"	Feature Requests	new	Boost 1.52.0	mpl	Boost 1.51.0	Problem