#if 0
set -xe
g++     -std=c++20 -fsyntax-only cond_trivial_destr.cc
clang++ -std=c++20 -fsyntax-only cond_trivial_destr.cc
exit 0
#endif

#include <type_traits>

// Conditional trivial destructible.
//
// Since destructors can't have template parameters, std::enable_if can't be
// used to selectively enable or disable the destructor via SFINAE.
//
// If a template class wants to be trivially destructible depending on the
// template parameter, one has to work around it with some template
// specialization (pre cpp20). The implementation in the namespace v1 gives an
// example for that.
//
// Starting with cpp20 we can leverage concepts to enable / disable different
// destructor implementations. The implementations in the namespace v2 gives an
// example for that.

// -- PRE CPP20 ----------------------------------------------------------------

namespace v1 {
template <typename T, bool = std::is_trivially_destructible_v<T>>
struct storage {
  ~storage() {
    if (has_val) {
      val.~T();
      has_val = false;
    }
  }

  union {
    T val;
  };
  bool has_val = false;
};

template <typename T>
struct storage<T, true> {
  union {
    T val;
  };
};

template <typename T>
struct option : private storage<T> {};
}  // namespace v1

// -- SINCE CPP20 --------------------------------------------------------------

namespace v2 {
#if __cplusplus >= 202002L
template <typename T>
struct option {
  ~option() = default;
  ~option()
    requires(!std::is_trivially_destructible_v<T>)
  {
    val.~T();
  }

private:
  union {
    T val;
  };
  bool has_value = false;
};
#endif
}  // namespace v2

// -- TESTEE -------------------------------------------------------------------

struct trivial {};
static_assert(std::is_trivially_destructible_v<trivial>, "");

struct non_trivial {
  ~non_trivial() {
  }
};
static_assert(!std::is_trivially_destructible_v<non_trivial>, "");

// -- TEST ---------------------------------------------------------------------

static_assert(std::is_trivially_destructible_v<v1::option<trivial>>, "");
static_assert(!std::is_trivially_destructible_v<v1::option<non_trivial>>, "");

#if __cplusplus >= 202002L
static_assert(std::is_trivially_destructible_v<v2::option<trivial>>, "");
static_assert(!std::is_trivially_destructible_v<v2::option<non_trivial>>, "");
#endif