Parts of std::vector implementation from https://gcc.gnu.org/onlinedocs/gcc-9.2.0/libstdc++/api/a01699.html

stdArray.h

struct __array_traits
{
  typedef _Tp _Type[_Nm]; //(1)
  typedef __is_swappable<_Tp> _Is_swappable;
  typedef __is_nothrow_swappable<_Tp> _Is_nothrow_swappable;

  static constexpr _Tp&
  _S_ref(const _Type& __t, std::size_t __n) noexcept
  { return const_cast<_Tp&>(__t[__n]); }

  static constexpr _Tp*
  _S_ptr(const _Type& __t) noexcept
  { return const_cast<_Tp*>(__t); }
};

_GLIBCXX17_CONSTEXPR reference
operator[](size_type __n) noexcept
{ return _AT_Type::_S_ref(_M_elems, __n); } //(2)