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Does anybody know if there is a conforming implementation of C++26's std::polymorphic that stores small object in situ? I'm trying to avoid writing one. #cpp

I don't think it can be conforming and do short buffer optimization. Spec is constexpr, and this is still not supported.

Only typy able to do it in constexpr is the string type; std::function is not constexpr mainly because of the SBO. There is no mechanism how to store unknown type somewhere without allocation in constexpr (you need reinterpret_cast)

In consteval you could always allocate and ignore the inner buffer.