I think the following papers should have had a feature test macro:

P0848 Conditionally Trivial Special Member Functions

P1330 Changing the active member of a union inside constexpr

I need a union in order to implement the std::variant changes in P2231 and a conditionally trivial destructor makes it easier. In order to define the __cpp_lib_constexpr_variant macro correctly I need to know if the union usage is actually going to work, but there's no macro for P1330, so I can't tell.

A macro for constexpr destructors (P0784R7?) might have been nice too, because if the compiler doesn't support it then you get an error for even trying to declare a destructor constexpr. Without a feature test macro there's no reliable way to try to use the feature.

If all the compilers I need to support already implemented those features, I wouldn't care, but they don't.

https://godbolt.org/z/qG6d7TxhE