<bit> exists in C++20 and is properly both constexpr and optimized. But in C++17 mode, we don't have constexpr bitops and instead elect to have performance at runtime instead. But somewhere along the line, either when they were added, when C++20 <bit> support was, or in any of the bugfixes for other compilers, the nesting of #ifdef got messed up and we declared that we had constexpr builtins for MSVC in C++17 too. The macro QT_HAS_CONSTEXPR_BUILTINS isn't supposed to be used by anyone else... but we ended up not being able to use it ourselves either. So I'm renaming it to a more precise label. Pick-to: 6.5 Change-Id: I9671dee8ceb64aa9b9cafffd1741b9b4060c9753 Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io> Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>