Build failure with Clang 10/libc++ in C++14 mode

I'm trying to build Qt 5 from Git sources with Clang 10/libc++ (downloaded from official LLVM site, Ubuntu 18) in C+14 mode. I got next build errors because qtbase code relies on C+17 features:

qtbase/src/corelib/kernel/qobjectdefs_impl.h:270:18: error: no template named 'void_t' in namespace 'std'; did you mean '__void_t'?
qtbase/src/corelib/kernel/qobjectdefs_impl.h:283:18: error: no member named 'disjunction_v' in namespace 'std'
qtbase/src/corelib/tools/qhashfunctions.h:76:8: warning: constexpr if is a C++17 extension [-Wc++17-extensions]
qtbase/src/corelib/tools/qhash.h:101:35: error: no template named 'is_nothrow_move_assignable_v' in namespace 'std'; did you mean 'is_nothrow_move_assignable'?
qtbase/src/corelib/tools/qhash.h:101:66: error: expected '(' for function-style cast or type construction

etc.

C++17 build looks OK.

If Qt build is switched to C+17, configure should not allow to choose older versions. Otherwise code should check if C+17 is enabled before using its features.

PS

Sorry, there is no tag for dev branch, so I used 6.0