common_type.hpp

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// SPDX-License-Identifier: BSL-1.0
// SPDX-FileCopyrightText: Copyright (C) 2019 Tobias Hienzsch
 
#ifndef TETL_TYPE_TRAITS_COMMON_TYPE_HPP
#define TETL_TYPE_TRAITS_COMMON_TYPE_HPP
 
#include <etl/_type_traits/decay.hpp>
#include <etl/_type_traits/declval.hpp>
#include <etl/_type_traits/void_t.hpp>
 
namespace etl {
 
/// \brief Determines the common type among all types `T...`, that is the type
/// all `T...` can be implicitly converted to. If such a type exists, the member
/// type names that type. Otherwise, there is no member type.
///
/// https://en.cppreference.com/w/cpp/types/common_type
template <typename... T>
struct common_type;
 
template <typename T>
struct common_type<T> : common_type<T, T> { };
 
namespace detail {
template <typename T1, typename T2>
using cond_t = decltype(false ? declval<T1>() : declval<T2>());
 
template <typename T1, typename T2, typename = void>
struct common_type_2_impl { };
 
template <typename T1, typename T2>
struct common_type_2_impl<T1, T2, void_t<cond_t<T1, T2>>> {
    using type = decay_t<cond_t<T1, T2>>;
};
 
template <typename AlwaysVoid, typename T1, typename T2, typename... R>
struct common_type_multi_impl { };
 
template <typename T1, typename T2, typename... R>
struct common_type_multi_impl<void_t<typename common_type<T1, T2>::type>, T1, T2, R...>
    : common_type<typename common_type<T1, T2>::type, R...> { };
} // namespace detail
 
template <typename T1, typename T2>
struct common_type<T1, T2> : detail::common_type_2_impl<decay_t<T1>, decay_t<T2>> { };
 
template <typename T1, typename T2, typename... R>
struct common_type<T1, T2, R...> : detail::common_type_multi_impl<void, T1, T2, R...> { };
 
template <typename... T>
using common_type_t = typename common_type<T...>::type;
 
} // namespace etl
 
#endif // TETL_TYPE_TRAITS_COMMON_TYPE_HPP