Proposed resolution:

This wording is relative to the FDIS.

In [valarray.assign] update as follows:

valarray<T>& operator=(valarray<T>&& v) noexcept;

3 Effects: *this obtains the value of v. If the length of v is not equal to the length of *this, resizes *this to make the two arrays the same length, as if by calling resize(v.size()), before performing the assignment.The value of v after the assignment is not specified.

4 Complexity: ConstantLinear.