(Forgot to send to the list; now back to the list again.) Vicente J. Botet Escriba wrote:

OK, I will rollback it and analyze the breaking change.

The important thing is that declval<T>() should preserve the constness/non-constness and the rvalueness/lvalueness of T. Essentially, it's OK to define declval<T>() as simply T. Then, why declval<T>() is defined as add_rvalue_reference<T>::type? This is because * When declval<T>() is defined as T, we cannot use abstract types as T. * When declval<T>() is defined as T&&, we cannot use void as T. The definition with add_rvalue_reference<T>::type also allows to use incomplete types as T. In C++03, it is not possible to construct rvalues of abstract types or incomplete types. So, the correct way to define declval<T>() in C++03 is to just use T. And this is the definition of declval<T>() in the previous version of boost::declval when BOOST_NO_RVALUE_REFERENCES is defined. Summary: Defining declval<T>() as add_lvalue_reference<T>::type is wrong; add_lvalue_reference should be removed! Regards, Michel