MSVC 1916 does not have a good constexpr 14 support

[apolukhin]

Looks like BOOST_CXX14_CONSTEXPR should not expand into constexpr for _MSC_VER == 1916, because the support does not seem complete. Here are some errors from the Boost.TypeIndex tests:

[00:03:17] compile-c-c++ ..\..\..\bin.v2\libs\type_index\test\type_index_constexpr_test.test\msvc-14.1\debug\cxxstd-14-iso\threading-multi\type_index_constexpr_test.obj
[00:03:17] type_index_constexpr_test.cpp
[00:03:17] type_index_constexpr_test.cpp(97): error C2131: expression did not evaluate to a constant
[00:03:17] C:\boost-local\boost/type_index/ctti_type_index.hpp(185): note: failure was caused by attempting to access a member on an object of dynamic type 'boost::typeindex::type_index_facade<boost::typeindex::ctti_type_index,boost::typeindex::detail::ctti_data>' in which the member is not defined
[00:03:17] C:\boost-local\boost/type_index/ctti_type_index.hpp(185): note: see usage of 'boost::typeindex::ctti_type_index::data_'
[00:03:17] type_index_constexpr_test.cpp(100): error C2131: expression did not evaluate to a constant

Full log is available at https://ci.appveyor.com/project/apolukhin/type-index/builds/21035855/job/9hcff3l2j4r9o70v

Misc: TypeIndex workaround that helped to fix the test https://github.com/boostorg/type_index/commit/c585d7466969387402114650b0387afd428852ce .

[jzmaddock]

Sorry for the delay in getting around to this!

This is a weird one, and I'm not completely convinced msvc is at fault yet. If I change the == operator to:

template <class Derived, class TypeInfo>
BOOST_CXX14_CONSTEXPR inline bool operator == (const type_index_facade<Derived, TypeInfo>& lhs, const type_index_facade<Derived, TypeInfo>& rhs) BOOST_NOEXCEPT {
   constexpr Derived const& d1 = static_cast<Derived const&>(lhs);
   constexpr Derived const& d2 = static_cast<Derived const&>(rhs);
   return d1.equal(d2);
}

Then obviously it can only be used in a constexpr context, but even then both gcc and clang issue (completely inscrutable) errors. But work just fine if I remove the constexpr from d1 and d2.

Confused yours...