qt-5.15.0: cross-compile with gcc-4.9

[debian]

Hello, @sonicss !
I pass C++ standart at configure step via -c++std parameters.
I was have same error #error "Qt requires C++11 support", I patch source for fix it

$ git diff src/
diff --git a/src/corelib/global/qcompilerdetection.h b/src/corelib/global/qcompilerdetection.h
index ebffe74188..0f5e71b4de 100644
--- a/src/corelib/global/qcompilerdetection.h
+++ b/src/corelib/global/qcompilerdetection.h
@@ -912,7 +912,7 @@
 #         define Q_COMPILER_REF_QUALIFIERS
 #      endif
 #    endif
-#    if Q_CC_GNU >= 500
+#    if Q_CC_GNU >= 409
        /* GCC 4.6 introduces constexpr, but it's bugged (at least) in the whole
         * 4.x series, see e.g. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57694 */
 #      define Q_COMPILER_CONSTEXPR

[debian]

@jsulm , why we are can't use GCC-4.9 if src/corelib/global/qcompilerdetection.h check many versions of GCC from GCC-4.3?

[jsulm]

@debian Which Qt version?

[debian]

@jsulm , now I use Qt-5.12.6 and I try upgrade it to Qt-5.15.0 because it have some important fixes.

[sonicss]

@jsulm Thanks a lot

[sonicss]

@debian Thanks, I will make a try.

[sonicss]

@debian I read the file:
qcompilerdetection.h

#    if Q_CC_GNU >= 500
       /* GCC 4.6 introduces constexpr, but it's bugged (at least) in the whole
        * 4.x series, see e.g. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57694 */
#      define Q_COMPILER_CONSTEXPR
#    endif

I think the new version of qt use constexpr, and the 4.x version of gcc has a bug in it. so can't use the version below 5 of gcc.

[debian]

@sonicss , I think more correct check this at configure stage and why Qt enable different features for different versions of the GCC?
https://github.com/qt/qtbase/blob/5.15.0/src/corelib/global/qcompilerdetection.h#L839
Why minimal version is the GCC-4.3 if I can build Qt only with GCC-5 and above?

[sonicss]

@debian I compared the file (qcompilerdetection.h) in Qt 5.12.8 and Qt5.15.0, the positon of Macro Q_COMPILER_CONSTEXPR is different, so I put this Macro to Q_CC_GNU >= 406, and make the Qt 5.15.0, It still have some errors. But the errors are different.
something like belows:

In file included from ../../include/QtCore/qmetatype.h:1:0,
                 from ../../include/QtCore/../../src/corelib/kernel/qobject.h:54,
                 from ../../include/QtCore/qobject.h:1,
                 from ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:46,
                 from ../../include/QtCore/qcoreapplication.h:1,
                 from global/qt_pch.h:66:
../../include/QtCore/../../src/corelib/kernel/qmetatype.h: In instantiation of 'QtMetaTypePrivate::QSequentialIterableImpl::QSequentialIterableImpl(const T*) [with T = QList<QVariant>]':
../../include/QtCore/../../src/corelib/kernel/qvariant.h:772:139:   required from here
../../include/QtCore/../../src/corelib/kernel/qmetatype.h:1160:135: error: ambiguous class template instantiation for 'struct QtMetaTypePrivate::ContainerCapabilitiesImpl<QList<QVariant>, void>'
       , _iteratorCapabilities(ContainerAPI<T>::IteratorCapabilities | (1 << 4) | (ContainerCapabilitiesImpl<T>::ContainerCapabilities << (4+3)))
                                                                                                                                       ^
In file included from ../../include/QtCore/qmetatype.h:1:0,
                 from ../../include/QtCore/../../src/corelib/kernel/qobject.h:54,
                 from ../../include/QtCore/qobject.h:1,
                 from ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:46,
                 from ../../include/QtCore/qcoreapplication.h:1,
                 from global/qt_pch.h:66:
../../include/QtCore/../../src/corelib/kernel/qmetatype.h:1007:8: error: candidates are: struct QtMetaTypePrivate::ContainerCapabilitiesImpl<Container, decltype (declval<Container>().push_back(declval<typename Container::value_type>()))>
 struct ContainerCapabilitiesImpl<Container, decltype(std::declval<Container>().push_back(std::declval<typename Container::value_type>()))>

[debian]

@sonicss , error is same, error position is not important, possible reason - different number of the build threads.

[debian]

ok, I success build with GCC-5.1 http://releases.linaro.org/components/toolchain/binaries/5.1-2015.08/arm-linux-gnueabihf/