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boost/mp11/detail/mp_defer.hpp

#ifndef BOOST_MP11_DETAIL_MP_DEFER_HPP_INCLUDED
#define BOOST_MP11_DETAIL_MP_DEFER_HPP_INCLUDED

// Copyright 2015-2020, 2023 Peter Dimov
// Distributed under the Boost Software License, Version 1.0.
// https://www.boost.org/LICENSE_1_0.txt

#include <boost/mp11/integral.hpp>
#include <boost/mp11/detail/config.hpp>

namespace boost
{
namespace mp11
{

// mp_if, mp_if_c
namespace detail
{

template<bool C, class T, class... E> struct mp_if_c_impl
{
};

template<class T, class... E> struct mp_if_c_impl<true, T, E...>
{
    using type = T;
};

template<class T, class E> struct mp_if_c_impl<false, T, E>
{
    using type = E;
};

} // namespace detail

template<bool C, class T, class... E> using mp_if_c = typename detail::mp_if_c_impl<C, T, E...>::type;
template<class C, class T, class... E> using mp_if = typename detail::mp_if_c_impl<static_cast<bool>(C::value), T, E...>::type;

// mp_valid

#if BOOST_MP11_WORKAROUND( BOOST_MP11_INTEL, != 0 ) // tested at 1800

// contributed by Roland Schulz in https://github.com/boostorg/mp11/issues/17

namespace detail
{

template<class...> using void_t = void;

template<class, template<class...> class F, class... T>
struct mp_valid_impl: mp_false {};

template<template<class...> class F, class... T>
struct mp_valid_impl<void_t<F<T...>>, F, T...>: mp_true {};

} // namespace detail

template<template<class...> class F, class... T> using mp_valid = typename detail::mp_valid_impl<void, F, T...>;

#else

// implementation by Bruno Dutra (by the name is_evaluable)
namespace detail
{

template<template<class...> class F, class... T> struct mp_valid_impl
{
    template<template<class...> class G, class = G<T...>> static mp_true check(int);
    template<template<class...> class> static mp_false check(...);

    using type = decltype(check<F>(0));
};

} // namespace detail

template<template<class...> class F, class... T> using mp_valid = typename detail::mp_valid_impl<F, T...>::type;

#endif

template<class Q, class... T> using mp_valid_q = mp_valid<Q::template fn, T...>;

// mp_defer
namespace detail
{

template<template<class...> class F, class... T> struct mp_defer_impl
{
    using type = F<T...>;
};

struct mp_no_type
{
};

#if BOOST_MP11_WORKAROUND( BOOST_MP11_CUDA, >= 9000000 && BOOST_MP11_CUDA < 10000000 )

template<template<class...> class F, class... T> struct mp_defer_cuda_workaround
{
    using type = mp_if<mp_valid<F, T...>, detail::mp_defer_impl<F, T...>, detail::mp_no_type>;
};

#endif

} // namespace detail

#if BOOST_MP11_WORKAROUND( BOOST_MP11_CUDA, >= 9000000 && BOOST_MP11_CUDA < 10000000 )

template<template<class...> class F, class... T> using mp_defer = typename detail::mp_defer_cuda_workaround< F, T...>::type;

#else

template<template<class...> class F, class... T> using mp_defer = mp_if<mp_valid<F, T...>, detail::mp_defer_impl<F, T...>, detail::mp_no_type>;

#endif

} // namespace mp11
} // namespace boost

#endif // #ifndef BOOST_MP11_DETAIL_MP_DEFER_HPP_INCLUDED