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:
Thanks Martin! Please let us know whenever it is in the gcc trunk.
No problem. The patch has been committed into the trunk of GCC 8.
Let me know if there's something else. It may be too late to make
any more changes like this before GCC 8.1 is released but further
refinements to suppress false positives can be made in GCC 8.2.