#include <iostream>
// types used to prioritize function overloads
struct Preferred { };
struct NotPreferred { NotPreferred( Preferred ) { } };

template< typename T >
auto doSomething( T &t, Preferred )
    -> decltype( t.doSomething() )
{
    return t.doSomething();
}

template< typename T >
void doSomething( T &t, NotPreferred ) {
    std::cout << "general";
}

struct A { void doSomething() { std::cout << "A, "; } };
int main() {
    A a; int b;
    doSomething( a, Preferred() );
    doSomething( b, Preferred() );
    return 0; // outputs: A, general
}