gcc/testsuite/g++.dg/cpp2a/construct_at.h

// A minimal conforming implementation of std::construct_at/destroy_at,
// used by some C++20 constexpr tests to avoid including all of <memory>.

namespace std
{
  typedef __SIZE_TYPE__ size_t;

  template <typename T>
  struct allocator
  {
    constexpr allocator () noexcept {}

    constexpr T *allocate (size_t n)
    { return static_cast<T *> (::operator new (n * sizeof(T))); }

    constexpr void
    deallocate (T *p, size_t n)
    { ::operator delete (p); }
  };

  template <typename T, typename U = T &&>
  U __declval (int);
  template <typename T>
  T __declval (long);
  template <typename T>
  auto declval () noexcept -> decltype (__declval<T> (0));

  template <typename T>
  struct remove_reference
  { typedef T type; };
  template <typename T>
  struct remove_reference<T &>
  { typedef T type; };
  template <typename T>
  struct remove_reference<T &&>
  { typedef T type; };

  template <typename T>
  constexpr T &&
  forward (typename std::remove_reference<T>::type &t) noexcept
  { return static_cast<T&&> (t); }

  template<typename T>
  constexpr T &&
  forward (typename std::remove_reference<T>::type &&t) noexcept
  { return static_cast<T&&> (t); }

  template <typename T, typename... A>
  constexpr auto
  construct_at (T *l, A &&... a)
  noexcept (noexcept (::new ((void *) 0) T (std::declval<A> ()...)))
  -> decltype (::new ((void *) 0) T (std::declval<A> ()...))
  { return ::new ((void *) l) T (std::forward<A> (a)...); }

  template <typename T>
  constexpr inline void
  destroy_at (T *l)
  { l->~T (); }
}

inline void *operator new (std::size_t, void *p) noexcept
{ return p; }