[wwwdocs] Document PR 87822 ABI breakage of std::pair in GCC 6.5

[Jonathan Wakely]

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&lt;std::pair&lt;X, Y&gt;, Z&gt;</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>
 
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