Re: [Patch] [add changelog] reduce template instantiation depth in <variant>

[Jonathan Wakely <jwakely at redhat dot com>]

On 12/11/16 14:48 -0600, Barrett Adair wrote:
> On Sat, Nov 12, 2016 at 2:39 PM, Tim Shen <timshen@google.com> wrote:
> > On Sat, Nov 12, 2016 at 4:28 AM, Daniel Krügler
> > <daniel.kruegler@gmail.com> wrote:
> > > 2016-11-12 10:04 GMT+01:00 Barrett Adair <barrettellisadair@gmail.com>:
> > > > > Currently, std::variant exceeds the default ftemplate-depth parameter when instantiated with 297 types. This small patch increases this ceiling to 446 types (from the bottom of the template stack).
> > > >
> > > > Sorry, first patch - I just read the guidelines. I changed the format
> > > > and added a changelog entry. I hope I did it correctly this time.
> > >
> > > Let me add that this patch suggestion causes a change in semantics in
> > > theory, because fold expressions have no short-circuit evaluation,
> >
> > Is it a compiler QoI problem, or part of the language? I'd be
> > surprised if the language somehow prohibits the short circuits from
> > happening.
> >
> > > contrary to __and_. Whether this difference is relevant here is
> > > of-course something the maintainer has to decide.
>
> Since there are no side-effects, and the metafunctions in play cannot
> SFINAE, I don't believe the semantics are actually affected by
> short-circuiting in this case.

I think the only semantic difference would be if one of the types is
an invalid argument to a trait like is_move_constructible, which would
only be possible for an incomplete type, which can't be used with
variant anyway. So I agree the short-circuiting doesn't change
semantics in this case.

An entirely different matter is whether you have a GCC copyright
assignment in place, and if not whether you'd be willing to complete
one. Without that, or a statement to place your changes in the public
domain, we can't accept the patch anyway and the technical merits are
secondary. You can contact me off-list for further details if needed.