When I build QT with GCC but explicitly disable the GNU extensions by using -std=c++17  instead of -std=gnu17, the compilation gets aborted by the error pragma in https://github.com/qt/qtbase/blob/v6.8.0/src/corelib/global/qtypes.cpp#L506-L511 . 

QT_SUPPORTS_INT128 is correctly undefined in https://github.com/qt/qtbase/blob/v6.8.0/src/corelib/global/qtypes.h#L72-L74 

This behavior was added in commit https://github.com/qt/qtbase/commit/7805b3c32f88a5405a4a12b402c93cf6cb5dedc4 . But the check "Qt needs to be compiled in a mode that enables INT128 if the compiler supports it in principle." was left in place.

When disabling this additional check the compilation works as expected and the 128-bit types are just not available.

This is for example leading to issues when using QT via conan: https://github.com/conan-io/conan-center-index/issues/27912