Function factorial

sample/factorial.cpp:4–14

#include <iostream>

constexpr int32_t factorial(int32_t n)
{
  using namespace matchit;
  assert(n >= 0);
  return match(n)(
      // clang-format off
        pattern | 0 = [] { return 1; },
        pattern | _ = [n] { return n * factorial(n - 1); }
      // clang-format on
  );
}

static_assert(factorial(3) == 6);