I didn't mean to sound like I didn't believe you about the Qt build with gcc 4.8. I just haven't experienced problems. However I'm not using 4.8 yet and I rarely go back and build older Qt versions. So I absolutely believe you that there were problems. :)

https://bugreports.qt-project.org/browse/QTBUG-32269
http://sourceforge.net/p/icewm/git/ci/395cfbb56a4adaee7028cfcbe6177ec0b94c181d/

g++ -c -include .pch/debug-shared/Qt5Core -m64 -pipe -pthread -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -g -std=c++0x -fvisibility=hidden -fvisibility-inlines-hidden -Wall -W -D_REENTRANT -fPIC -DQT_NO_USING_NAMESPACE -DQT_BUILD_CORE_LIB -DQT_BUILDING_QT -DQT_NO_CAST_TO_ASCII -DQT_ASCII_CAST_WARNINGS -DQT_MOC_COMPAT -DQT_USE_QSTRINGBUILDER -DQT_DEPRECATED_WARNINGS -DQT_DISABLE_DEPRECATED_BEFORE=0x050000 -DELF_INTERPRETER="/lib64/ld-linux-x86-64.so.2" -DQT_USE_ICU -DPCRE_HAVE_CONFIG_H -DHB_EXPORT=Q_CORE_EXPORT -DQT_CORE_LIB -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -I../../mkspecs/linux-g++-64 -I. -I../../include -I../../include/QtCore -I../../include/QtCore/5.0.2 -I../../include/QtCore/5.0.2/QtCore -Iglobal -I../3rdparty/pcre -I../3rdparty/md5 -I../3rdparty/md4 -I.moc/debug-shared -o .obj/debug-shared/qsharedmemory_unix.o kernel/qsharedmemory_unix.cpp
kernel/qtimerinfo_unix.cpp:98:30: error: redeclaration 'constexpr T qAbs(const T&) [with T = timeval]' differs in 'constexpr'
timeval qAbs(const timeval &t)
^
In file included from ../../include/QtCore/qglobal.h:1:0,
from global/qt_pch.h:58:
../../include/QtCore/../../src/corelib/global/qglobal.h:483:27: error: from previous declaration 'constexpr T qAbs(const T&) [with T = timeval]'
Q_DECL_CONSTEXPR inline T qAbs(const T &t) { return t >= 0 ? t : -t; }
^
kernel/qtimerinfo_unix.cpp: In function 'constexpr T qAbs(const T&) [with T = timeval]':
kernel/qtimerinfo_unix.cpp:109:1: error: body of constexpr function 'constexpr T qAbs(const T&) [with T = timeval]' not a return-statement
}
^
In file included from ../../include/QtCore/qglobal.h:1:0,
from global/qt_pch.h:58:
../../include/QtCore/../../src/corelib/global/qglobal.h: In instantiation of 'constexpr T qAbs(const T&) [with T = timeval]':
kernel/qtimerinfo_unix.cpp:147:44: required from here
../../include/QtCore/../../src/corelib/global/qglobal.h:483:55: error: no match for 'operator>=' (operand types are 'const timeval' and 'int')
Q_DECL_CONSTEXPR inline T qAbs(const T &t) { return t >= 0 ? t : -t; }

Older versions compile it fine. 4.8.3 compiles 5.3.1 fine. I just seem to be having a hard time convincing anyone here that problems can actually exist and they can get fixed.

The fact that even in a raw X11 there was the XCB error. That shows there is definitely a problem.

Also, I've never had a problem with gcc/qt. However my version of gcc is 4.7.3 and I rarely build Qt with debug info. Even in release only mode it has info for backtraces so I find I don't need to go deeper than that. That makes it not worth the build time or space required for debug versions of Qt. But release build Qt always builds great with any gcc for me (again havent tried 4.8 though).

There is definitely a problem with your system somewhere. I would lean towards X11. You may want to try to reinstall your X11 system and XCB. That may help.

qt-everywhere-opensource-src-5.0.2/qtbase/src/plugins/platforms/xcb/qxcbwindow.cpp:583

xcb_flush(xcb_connection());

Stepping over that function causes a blank rectangle to appear.
Then I found that place in 5.3.1 (line 710), but that doesn't call xcb_flush(). hmm... I tried adding xcb_flush() back in but that didn't fix the problem.

Then I realised it could be a window manager problem (I use icewm), so I killed that (i.e. no session management, just bare X running), and that made the window appear! And it also gave this error on STDERR:

QXcbConnection: XCB error: 3 (BadWindow), sequence: 170, resource id: 113246225,
major code: 20 (GetProperty), minor code: 0

Interestingly when I restarted icewm the clock window then vanished. Searching for that error finds a few QT bugs, so it seems icewm is just more sensitive about what windows it will display than other WMs (like new gcc versions are more sensitive about what code they'll compile...)

Anyway, try running it from the terminal and see if there are any messages.

Also, if you built Qt yourself, did you make sure you had X11/GUI support? It's kinda hard to exclude support for these things so if you didn't explicitly disable them you should be good on that front.

Which distribution are we talking about? Have you compiled Qt yourself?