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

[Jonathan Wakely <jwakely at redhat dot com>]

On 12/11/16 12:39 -0800, Tim Shen 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.

It's a language feature.

In a fold expression every member of the pack expansion gets
instantiated. If one of them produces an invalid instantiation you get
an error.

Using std::conjunction (or our non-standard std::__and_) instantiation
stops as soon as the result is known (i.e. if one of the conditions is
false).