/**

* An instance of the C++11 `decltype` operator. For example:

* ```

* int a;

* decltype(1+a) b;

* ```

* Here `expr` is `1+a`.

*

* Sometimes an additional pair of parentheses around the expression

* would change the semantics of this decltype, e.g.

* ```

* struct A { double x; };

* const A* a = new A();

* decltype( a->x ); // type is double

* decltype((a->x)); // type is const double&

* ```

* (Please consult the C++11 standard for more details).

* `parentheses_would_change_meaning` is `true` iff that is the case.

*/

#keyset[id, expr]

int id: @decltype,