template< typename T >
auto doSomething( T &t ) -> decltype( t.doSomething() )
{ return t.doSomething(); }
template< typename T >
int doSomething( T &t ) { return 0; }

struct A { long doSomething() { return 1; } };
int main() {
    A a;
    doSomething( a );
    return 0;
}