Initializing constexpr variable by a lambda

hello11.cpp

#include <iostream>
#include <string>

constexpr auto fx = [] (std::string msg) {
  return msg + "!\n"; };

int main(int argc, char* argv[]) {
  if (argc == 2)
    std::cout << "hello " << fx(argv[1]);
  else
    std::cout << "hello world!" << std::endl;
}