Because of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71962, and because marking constExpr() as __attribute__((no_sanitize_undefined)) doesn't fix a thing, we opted to disable the triggering parts of constExpr() from all GCC builds. This is, of course, unfortunate, since it meaningfully reduces compile-time coverage in the general case for a rather obscure build type most won't ever use. While GCC doesn't give us a predefined macro to check for in the .cpp file, the cmake build system knows whether we use ubsan, so just define a macro of our own. As a drive-by, simplify GCC detection by using Q_CC_GCC_ONLY. Amends de6a004bc5a5b9cd3ecfbb14818bb42fb1ecfd68. Pick-to: 6.7 6.5 6.2 Task-number: QTBUG-101307 Change-Id: I4be5bd103b9d2386b2ac9fd22e0c34f9c63fee04 Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>