Class constexpr_write_ptr

cpp/simdutf.h:1427–1455

 * given in the constructor, which is of another type TargetType
 */
template <typename SrcType, typename TargetType> struct constexpr_write_ptr {
  constexpr explicit constexpr_write_ptr(TargetType *raw) : p(raw) {}

  constexpr constexpr_write_ptr_proxy<SrcType, TargetType> operator*() const {
    return constexpr_write_ptr_proxy<SrcType, TargetType>{p};
  }

  constexpr constexpr_write_ptr_proxy<SrcType, TargetType>
  operator[](std::ptrdiff_t n) const {
    return constexpr_write_ptr_proxy<SrcType, TargetType>{p + n};
  }

  constexpr constexpr_write_ptr &operator++() {
    ++p;
    return *this;
  }

  constexpr constexpr_write_ptr operator++(int) {
    constexpr_write_ptr old = *this;
    ++p;
    return old;
  }

  constexpr std::ptrdiff_t operator-(const constexpr_write_ptr &other) const {
    return p - other.p;
  }

  TargetType *p;
};

template <typename SrcType, typename TargetType>
constexpr auto constexpr_cast_writeptr(TargetType *raw) {