https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101263
TC <rs2740 at gmail dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |rs2740 at gmail dot com --- Comment #7 from TC <rs2740 at gmail dot com> --- (In reply to Barry Revzin from comment #6) > The "real" answer is allowing constexpr placement new, but that obviously > doesn't help you right now. > > But I think the helpful answer is that you can add a constructor to your > storage like storage(init_from_invoke_t, Args&&... args) that initializes > the underlying value from invoke((Args&&)args...), and then > construct_at(&storage, init_from_invoke, [&]() -> decltype(auto) { return > *i; }). > > Something like that? Yes. Something at that level of generality will be needed for the new optional::transform, so it seems the better approach. In my proof-of-concept implementation (which didn't have that concern), I used something tailored to this specific case, along the lines of struct __deref_tag {}; template<class _Tp> struct __cache_wrapper { template<class _Iter> constexpr __cache_wrapper(__deref_tag, const _Iter& __i) : __t(*__i) {} _Tp __t; }; and then stored a __non_propagating_cache<__cache_wrapper<range_value_t<...>>> __cache, so that emplace-deref(i) is __cache.emplace(__deref_tag{}, i);