https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
--- Comment #81 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Bernd Edlinger from comment #80) > but they use just: "mutable char data;" No, actually it doesn't, see comment 45. The original testcase that this reprot came from is https://bugzilla.redhat.com/show_bug.cgi?id=1422456 which uses Boost 1.63 and that has an array of char. Jakub used the wrong version of Boost to create the code attached here.