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

The proposal (wg21.link/p3019) makes a bunch of references to the small-buffer optimization, like

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)