is_convertible.hpp

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// SPDX-License-Identifier: BSL-1.0
// SPDX-FileCopyrightText: Copyright (C) 2019 Tobias Hienzsch
 
#ifndef TETL_TYPE_TRAITS_IS_CONVERTIBLE_HPP
#define TETL_TYPE_TRAITS_IS_CONVERTIBLE_HPP
 
#include <etl/_type_traits/bool_constant.hpp>
#include <etl/_type_traits/declval.hpp>
#include <etl/_type_traits/is_void.hpp>
 
namespace etl {
 
namespace detail {
template <typename>
using true_type_for = etl::true_type;
 
template <typename T>
auto test_returnable(int) -> true_type_for<T()>;
template <typename>
auto test_returnable(...) -> etl::false_type;
 
template <typename From, typename To>
auto test_nonvoid_convertible(int) -> true_type_for<decltype(etl::declval<void (&)(To)>()(etl::declval<From>()))>;
template <typename, typename>
auto test_nonvoid_convertible(...) -> etl::false_type;
 
} // namespace detail
 
/// \brief If the imaginary function definition `To test() { return
/// etl::declval<From>(); }` is well-formed, (that is, either
/// `etl::declval<From>()` can be converted to To using implicit conversions, or
/// both From and To are possibly cv-qualified void), provides the member
/// constant value equal to true. Otherwise value is false. For the purposes of
/// this check, the use of `etl::declval` in the return statement is not
/// considered an odr-use. Access checks are performed as if from a context
/// unrelated to either type. Only the validity of the immediate context of the
/// expression in the return statement (including conversions to the return
/// type) is considered.
template <typename From, typename To>
struct is_convertible
    : bool_constant<
          (decltype(detail::test_returnable<To>(0))::value
           && decltype(detail::test_nonvoid_convertible<From, To>(0))::value)
          || (is_void_v<From> && is_void_v<To>)
      > { };
 
template <typename From, typename To>
inline constexpr bool is_convertible_v = is_convertible<From, To>::value;
 
} // namespace etl
 
#endif // TETL_TYPE_TRAITS_IS_CONVERTIBLE_HPP