LWG 2939 suggests that the the preconditions of the type traits need reevaluation. This issue focuses specifically on is_assignable and, by extension, its variants:

• is_copy_assignable<T>, equivalent to is_assignable<T&, const T&>,
• is_move_assignable<T>, equivalent to is_assignable<T&, T>,
• is_trivially_assignable<T, U>, equivalent to is_assignable<T, U> && /* magic */,
• is_trivially_copy_assignable<T>, equivalent to is_assignable<T&, const T&> && /* magic */,
• is_trivially_move_assignable<T>, equivalent to is_assignable<T&, T> && /* magic */,
• is_nothrow_assignable<T, U>, equivalent to is_assignable<T, U> && noexcept(declval<T>() = declval<U>()),
• is_nothrow_copy_assignable<T>, equivalent to is_assignable<T&, const T&> && noexcept(declval<T&>() = declval<const T&>()),
• is_nothrow_move_assignable<T>, equivalent to is_assignable<T&, T> && noexcept(declval<T&>() = declval<T>()), and

We note a discrepancy: is_copy_assignable<T> requires T to be a complete type, but the equivalent form is_assignable<T&, const T&> does not. The requirement for is_copy_assignable<T> seems sensible, since there's no way to determine whether or not the assignment declval<T&>() = declval<const T&>() is well-formed when T is incomplete. It seems that the same argument should apply to all of the above "assignable" traits, and that they must require that the referent type is complete when given a reference type parameter to be implementable.