Peter Dimov via Boost
Yes, a compiler may support the constexpr keyword, but have problems with complex uses. This is not a good criterion for banning it Boost-wide, because "complex uses" vary. One library may not be able to support it, another might.
If you insist on not considering msvc-14.0 a C++11 compiler for your libraries, that's your right (and problem.) It has nothing to do with whether it's considered a supported C++11 compiler for the purposes of the announcement.
I don't see how this is helpful to the users of Boost, especially considering the heavy dependencies between individual libraries. As a user I want to know if the set of libraries that I would like to use (plus their transitive dependencies) are compatible with my compiler/version.
(MSVC 14.3 is VS2022, not VS2015.)
Yes, you are right, of course.