nothing at all, because X(answer); could be interpreted as the declaration of a variable.
Your answer is hidden in here. If you declare a variable, you invoke its default ctor (if non-POD and all that stuff).
On your edit: To get a temporary, you have a few options:
(X(answer));(X)answer;static_cast<X>(answer)X{answer};(C++11)[]{ return X(answer); }();(C++11, may incur copy)void(), X(answer);X((void(),answer));true ? X(answer) : X();if(X(answer), false){}for(;X(answer), false;);X(+answer);