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// Copyright (C) 2022 johannst
#include <type_traits>
template<typename T, template<typename> class Checker, typename = void>
struct is_valid : std::false_type {};
template<typename T, template<typename> class Checker>
struct is_valid<T, Checker, std::void_t<Checker<T>>> : std::true_type {};
template<typename T, template<typename> class Checker>
static constexpr bool is_valid_v = is_valid<T, Checker>::value;
// -----------------------------------------------------------------------------
template<typename T, typename R, template<typename> class Checker, typename = void>
struct is_valid_with_ret : std::false_type {};
template<typename T, typename R, template<typename> class Checker>
struct is_valid_with_ret<T, R, Checker, std::void_t<Checker<T>>> : std::is_same<R, Checker<T>> {};
template<typename T, typename R, template<typename> class Checker>
static constexpr bool is_valid_with_ret_v = is_valid_with_ret<T, R, Checker>::value;
// -----------------------------------------------------------------------------
template<typename T>
struct is_entry {
template<typename TT>
using init = decltype(std::declval<TT>().init());
template<typename TT>
using tag = decltype(std::declval<TT>().tag());
template<typename TT>
using val = decltype(std::declval<TT>().val());
static constexpr bool value = is_valid_v<T, init> &&
is_valid_with_ret_v<T, int, tag> &&
is_valid_with_ret_v<T, typename T::Type, val>;
};
template<typename T>
static constexpr bool is_entry_v = is_entry<T>::value;
template<typename E>
struct Entry {
using Type = E;
void init();
int tag() const;
E val() const;
};
int main() {
static_assert(is_entry_v<Entry<bool>>, "");
}
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