Function square

sample/someNone.cpp:6–16

template <typename T>
constexpr auto square(std::optional<T> const &t)
{
  using namespace matchit;
  Id<T> id;
  return match(t)(
      // clang-format off
        pattern | some(id) = id * id,
        pattern | none     = expr(0)
      // clang-format on
  );
}
constexpr auto x = std::make_optional(5);
static_assert(square(x) == 25);