It is unclear where and if it is needed and it's very dangerous for the
following reason: Suppose the that you have some class

class C
{
    C(const NullOperator<A>& a): a_(a) {}
    const NullOperator<A>& a_;
};

and make the not so unlikely error to call the constructor of C
with a NullOperator<B>. Then the code will silently compile but the
'a_' is a reference to an invalid temporary after the constructor call.

[[Imported from SVN: r8227]]