On Thu, 4 Jun 2020 at 02:13, Marc Glisse <marc.gli...@inria.fr> wrote: > > On Thu, 4 Jun 2020, Ville Voutilainen wrote: > > > On Thu, 4 Jun 2020 at 01:52, Marc Glisse <marc.gli...@inria.fr> wrote: > >> > >> Hello, > >> > >> is there any drawback to the attached patch? It changes the code generated > >> for > > > > I don't get it. The noexceptness of the defaulted default constructor > > should be a computation > > of the noexceptness of the subobjects, and that should boil down to > > whether optional's storage > > is noexcept-default-constructible. And that thing has a noexcept > > default constructor. Why > > does this patch change anything? > > "noexcept" is a red herring, what matters is defaulted vs user-provided. > In one case, we end up zero-initializing the whole buffer, and not in the > other.
Yes, I just came to that conclusion. This is value-init, so the language manages to zero-init the whole-object, but with the change, it just calls a user-provided constructor. That'll then merely boil down to value-initializing just the _Empty part of the _Storage in _Optional_payload_base. We are in http://eel.is/c++draft/dcl.init#8.1.2. The change takes us to http://eel.is/c++draft/dcl.init#8.1.1.
