My favourite is:

    struct Dummy {
        Dummy(int) {}
    };

    struct World {
        World(const Dummy &) {}
        void hello() {}
    };

    int main() {
        int value = 42;
        World world(Dummy(value)); // fails to compile
        // World world(Dummy(42)); // works
        world.hello();
        return 0;
    }