Proposed resolution:

This wording is relative to N4296.

1. Change [except.nested] as depicted:

   template <class T> [[noreturn]] void throw_with_nested(T&& t);
   

   -6- Let U be remove_reference_t<T>.

   -7- Requires: U shall be CopyConstructible.

   -8- Throws: if U is a non-union class type not derived from nested_exceptionis_class<U>::value && !is_final<U>::value && !is_base_of<nested_exception, U>::value is true, an exception of unspecified type that is publicly derived from both U and nested_exception and constructed from std::forward<T>(t), otherwise std::forward<T>(t).

[ 2015-05, Lenexa ]

STL, MC and JW already do this
MC: move to Ready, bring to motion on Friday
7 in favor, none opposed

When N2559 was voted into the Working Paper, it said "This function template must take special case to handle non-class types, unions and [[final]] classes that cannot be derived from, and [...]". However, its Standardese didn't handle final classes, and this was never revisited. Now that we have is_final, we can achieve this proposal's original intention.

Additionally, we need to handle the case where U is nested_exception itself. is_base_of's wording handles this and ignores cv-qualifiers. (Note that is_class detects "non-union class type".)