21.4.1 p6 states invalidation of iterators/references is only allowed for — as an argument to any standard library function taking a reference to non-const basic_string as an argument. — Calling non-const member functions, except operator[], at, front, back, begin, rbegin, end, and rend.

The non-const operator[] in a COW string requires making a copy of the string if the ref count > 1 which invalidates references and violates this paragraph.