I'm committing this addition to the release notes as part of my public shaming.
Index: htdocs/gcc-6/changes.html =================================================================== RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/changes.html,v retrieving revision 1.110 retrieving revision 1.111 diff -u -r1.110 -r1.111 --- htdocs/gcc-6/changes.html 26 Oct 2018 09:32:34 -0000 1.110 +++ htdocs/gcc-6/changes.html 31 Oct 2018 13:50:13 -0000 1.111 @@ -46,6 +46,15 @@ for <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71151">PR71151</a> to work.</li> + <li>The GCC 6.5 release has an accidental ABI incompatibility for nested + <code>std::pair</code> objects, for more details see + <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87822">PR 87822</a>. + The bug causes a layout change for pairs where the first member is also + a pair, e.g. <code>std::pair<std::pair<X, Y>, Z></code>. + The GCC 6 release series is closed so the bug in GCC 6.5 will not be fixed + upstream, but there is a patch in the bug report to allow it to be fixed + by anybody packaging GCC 6.5 or installing it themselves.</li> + </ul> <!-- .................................................................. -->