Unlike other containers, a QBitArray's size() is not limited by storage, but, esp. on 32-bit platforms, its size_type: A INT_MAX size() QBitArray only requires 256MiB of storage. So we can't rely on "won't happen in practice" here and need to avoid the potential UB (signed overflow) in the (d.size() * 8 - *d.data()) storage-to-logical-size calculation by using unsigned arithmetic. Use the opportunity to Extract Method adjust_head_and_tail(), centralizing the bit fiddling. Pick-to: 6.6 6.5 6.2 5.15 Change-Id: I485eafdf3ce2087a81c683672ff98a43f97c9968 Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org> Reviewed-by: Thiago Macieira <thiago.macieira@intel.com> (cherry picked from commit 78f8dfc5427457783ceef7d85885cddbec035ebe) Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>