Z combinator

factorial.C

#include <cstdio>

int main() {
  auto z = [](auto f) {
    auto g = [&f](auto x) {
      return f([&x](auto y) { return x(x)(y); });
    };
    return g(g);
  };
  auto f = [](auto f) {
    return [&f](auto x) { return x ? x * f(x-1) : 1; };
  };
/* can't be compiled.
z.C:6:38: error: function 'operator()<(lambda at z.C:5:14)>' with deduced return type cannot be used before it is defined
      return f([&x](auto y) { return x(x)(y); });
*/
  std::printf("%d\n", z(f)(5)); // expected to print 5!
  return 0;
}