如何将强类型enum自动转换为int?

#include <iostream>

struct a
{
  enum LOCAL_A
  {
    A1,
    A2
  };
};
enum class b
{
    B1,
    B2
};

int foo( int input )
{
    return input;
}

int main(void)
{
    std::cout<<foo(a::A1)<<std::endl;
    std::cout<<foo(static_cast<int>(b::B2))<<std::endl;
}
#include <iostream>

struct a
{
  enum LOCAL_A