Even aside from deliberate backwards-compatibility breaks in the standard, compilers sometimes break compatibility. Both MSVC and GCC 11 have changed their head

Even aside from deliberate backwards-compatibility breaks in the standard, compilers sometimes break compatibility. Both MSVC and GCC 11 have changed their header file transitive includes within the past few years, causing projects (like doctest and Qt5) to stop compiling because they forgot to include headers, which built fine in the past but not anymore. IDK if it's "very common", but it's definitely happening in the wild. MSVC: https://github.com/onqtam/doctest/issues/183 https://github.com/onqtam/doctest/issues/183 GCC: - https://invent.kde.org/qt/qt/qtbase/-/commit/8252ef5fc6d043004ddf7085e1c1fe1bf2ca39f7 https://invent.kde.org/qt/qt/qtbase/-/commit/8252ef5fc6d0430... - https://invent.kde.org/qt/qt/qtbase/-/commit/cb2da673f53815a5cfe15f50df49b98032429f9e https://invent.kde.org/qt/qt/qtbase/-/commit/cb2da673f53815a...