https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116969
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- tl;dr the attributes are correct, but libstdc++ is cheating. Suppressing the warnings because we want to chat seems appropriate. However, I might need to suppress some more, for the inverse case where __ibm128 is configured as the default and snprintf warns when we use __ieee128. I will check that.
