#include<type_traits>

class foo {
};

class bar {
public:
  const foo & to_foo() const {
    return f;
  }

  foo & to_foo() {
    return f;
  }
private:
 foo f;
};

template< typename T, typename Enable = void >
class convert {};

template< typename T >
struct convert< T, typename std::enable_if< std::is_member_function_pointer< decltype( &T::to_foo ) >::value >::type > {
  static const foo & call1( const bar & b ) {
    return b.to_foo();
  }

  static foo & call2( bar & b ) {
    return b.to_foo();
  }
};

int main() {
  bar b;
  foo & f = convert<bar>::call2( b );
  return 0;
}