Client code on MSVC *must* pass /Zc:__cplusplus when using Qt.
Otherwise, this makes Qt code that relies on feature-testing
macros a mess. For instance, in QTBUG-91117, we trip on this code:
// C++ version guard is necessary: you may have the header,
// but including it in pre-C++20 will cause an hard error
#if __has_include(<bit>) && __cplusplus > 201703L
#include <bit>
#endif
#if defined(__cpp_lib_bitops)
// use some <bit> functionality
#endif
The #define __cpp_lib_bitops should've come from the preceding include
directive, but there's another possibility: that it comes from
<version> (or some other similar header) included transitively,
when compiling in C++20 mode, and *without* a bumped __cplusplus.
Yes, that's an actual possibility on MSVC.
Then, since we did not include <bit> ourselves due to the __cplusplus
version check, using the functionality will cause a compile error.
We're not going to fix *every* post C++-17 feature detection macro
because of MSVC and feature-test shenanigans. It's time to require
compilers to tell us the truth about what they support.
Fixes: QTBUG-91117
Change-Id: I9d74f9d8b74b5ac35dce3528e7a2006746a00676
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
647c0e80ed