https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101263
--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> --- (In reply to Jonathan Wakely from comment #4) > (In reply to Patrick Palka from comment #1) > > We might first need to implement P2231 (for constexpr optional) before this > > function can be properly constexpr. > > Implemented in r12-4389 Yay, thanks! I thought this would be enough to straightforwardly make emplace-deref constexpr, but I ran into an unrelated wrinkle. During constexpr evaluation we can't use placement new to initialize the contained value directly from *__i and instead have to use the equivalent of construct_at(&value, *__i), but the latter incurs an extra move due to the temporary materialization of *__i, which is contrary to the effects of emplace-deref ([range.nonprop.cache]/1.6): Calls reset(). Then initializes the contained value as if direct-non-list-initializing an object of type T with the argument *i. Am I missing something or is it not possible to perform this direct-init in the constexpr case?
