I think the following papers should have had a feature test macro: P0848 <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0848r3.html> Conditionally Trivial Special Member Functions
P1330 <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1330r0.pdf> Changing the active member of a union inside constexpr I need a union in order to implement the std::variant changes in P2231 <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2231r1.html> 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 <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0784r7.html>?) 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
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