Same as https://bugs.kde.org/show_bug.cgi?id=371721 , but for plasma-workspace Core issue seems to me that moc does not -D _REDIRECT, so glibc uses an evil #define open open64 which results in moc_iconapplet_FERWDM656PUOUJ.cpp:128:21: error: 'class IconApplet' has no member named 'open64'; did you mean 'open'? IMO this needs to be fixed in Qt, but as everything else works on i686 (except kget, same issue), this is doing something differently. Maybe other components #include different headers that fix it by #undef open? (Feel free to close as WONTFIX if it's too ugly to work around in plasma)
"critical" is used for data loss bugs only.
Copying from the linked bug report #14: >Wait. Something is really broken on CentOS 6.8 though, because according to http://ac-archive.sourceforge.net/largefile/glibc.html the #define open open64 should only happen when __REDIRECT is not set, and it should be set for gcc >= 2.x, see cdefs.h on that page. Can you check if your system headers match what that page says? I'm not pushing the #undef hacks after all, they shouldn't be needed.
Found the issue: moc does not define __GNUC__, so the #if defined __GNUC__ && __GNUC__ >= 2 check does not pass inside glibc. I reported it as https://bugreports.qt.io/browse/QTBUG-57796