Proposed resolution:

This wording is relative to N4606.

1. In [string.view.template], add constexpr to copy:

   constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;
   

2. In [string.view.ops], add constexpr to copy:

   constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;