The Effects clause for the default valarray ctor suggests that it is possible to increase the size of an empty valarray object by calling other non-const member functions of the class besides resize(). However, such an interpretation would be contradicted by the requirement on the copy assignment operator (and apparently also that on the computed assignments) that the assigned arrays be the same size. See the reflector discussion starting with c++std-lib-17871.

In addition, Footnote 280 uses some questionable normative language.