Martin Sebor also asked me about this, he's the one that worked on
the gcc code that introduced the warning. Sorry for not replying earlier.
Maybe you can pass this to him? (maybe open a bug in gcc's bugzilla?)
I've opened bug 84725 to extend attribute nonstring to the other
two character types:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84725
Thanks Martin! Please let us know whenever it is in the gcc trunk.