Proposed resolution:

This wording is relative to N4910.

1. Modify [meta.type.synop], Table 46 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:

   Table 46: Type property predicates [tab:meta.unary.prop]

   Template	Condition	Preconditions
   …
   template<class T, class U> struct reference_constructs_from_temporary;	conjunction_v<is_reference<T>, is_constructible<T, U>> is true, and the initialization T t(VAL<U>); binds t to a temporary object whose lifetime is extended ([class.temporary]).	If T is a reference type, remove_reference_t<T> and UT shall be a complete types, cv void, or an arrays of unknown bound.
   template<class T, class U> struct reference_converts_from_temporary;	conjunction_v<is_reference<T>, is_convertible<U, T>> is true, and the initialization T t = VAL<U>; binds t to a temporary object whose lifetime is extended ([class.temporary]).	If T is a reference type, remove_reference_t<T> and U shall be complete types, cv void, or arrays of unknown bound.

[ 2022-05-17; Reflector poll ]

Set priority to 3 after reflector poll.

std::reference_constructs_from_temporary and std::reference_converts_from_temporary are only useful when T is a reference type, and a reference type is always complete. Whenever T is an incomplete object type, it's clear that these traits inherit from std::false_type, without any further detection which may result in UB. However, when T is X& or X&& where X is an incomplete object type, UB may be needed because of the potentially problematic detection in std::is_constructible or std::is_convertible.

I'm not sure whether the general rule in [meta.rqmts]/5 affects these cases.