Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

boost/spirit/home/x3/support/utility/sfinae.hpp

/*=============================================================================
    Copyright (c) 2001-2014 Joel de Guzman
    Copyright (c) 2013 Agustin Berge

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
==============================================================================*/
#if !defined(BOOST_SPIRIT_X3_SFINAE_MAY_20_2013_0840AM)
#define BOOST_SPIRIT_X3_SFINAE_MAY_20_2013_0840AM

namespace boost { namespace spirit { namespace x3
{
    template <typename Expr, typename T = void>
    struct disable_if_substitution_failure
    {
        typedef T type;
    };
    
    template <typename Expr, typename T>
    struct lazy_disable_if_substitution_failure
    {
        typedef typename T::type type;
    };
}}}

#endif