[Bug c++/108993] Value initialization does not occur for derived class , for gcc versions > 5

["panigstein at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108993

--- Comment #7 from Pablo Anigstein <panigstein at hotmail dot com> ---
(In reply to Jonathan Wakely from comment #5)
> (In reply to Jonathan Wakely from comment #3)
> > (In reply to Pablo Anigstein from comment #2)
> > > (In reply to Andrew Pinski from comment #1)
> > > > Hmm,
> > > >  I noticed that since GCC 7 with -std=c++17, the b.x is not initialized at
> > > > all. So the question I have is there a difference between C++ standards here?
> > 
> > Derived is an aggregate in C++17, so b{} does aggregate init, not value init.
> 
> And that means its Base subobject is copy-initialized from {} which means we
> get a value-initialized object, so it's correct that b.x is not initialized
> in C++17 (which is what is shown in your godbolt link, because you didn't
> specify any -std option to override the -std=gnu++17 default).
> 
> With -std=c++14 it looks like b.x is always set to zero, if I'm reading the
> assembly output correctly (but I'm probably not).

Here is an updated example: https://godbolt.org/z/YePjhxKE4. Note that now
Derived is not an aggregate for any standard version due to the private member.
Still zero-initialization does not happen for the Base sub-object for the case
where Base has a user-provided default constructor when compiling with -O1 and
above.