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);