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Chapter18/cib/libs/boost/mp11/detail/mp_defer.hpp

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+#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