About the error message in this posted code:

> class T {
>   T();
>   ~T();
> }
>
> class X {
>   X();
>   ~X();
>   f(T& t);
> }
>
> int main() {
>   X x;
>   x.f(T());  // Compiler complained on this line
> }

The 'temporary' (it is an UNNAMED) T()-object is NOT const!
Consider:
 class T {
 public:
  T();
  ~T();
  void nonConstFct() {}
 };

 int main() {
  T().nonConstFct();  // Allowed!!!
 }

And so the compiler should have no problem with this code!
I compiled it without any problem!

Bye.
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