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Subject: Re: [Boost-users] [variant] Possible regression from 1.55 to 1.58
From: Shane Turner (shane.turner_at_[hidden])
Date: 2015-08-12 08:28:06

> -----Original Message-----
> From: Boost-users [mailto:boost-users-bounces_at_[hidden]] On Behalf Of
> Mario Lang
> Sent: Tuesday, August 11, 2015 7:54 PM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] [variant] Possible regression from 1.55 to 1.58
> Tim Song <t.canens.cpp_at_[hidden]> writes:
> > On Tue, Aug 11, 2015 at 5:46 AM, Mario Lang <mlang_at_[hidden]> wrote:
> >
> >> The example is a bit silly, since foreach discards the return value
> >> of apply_visitor anyway. So my solution was to change the visitor to
> >> define result_type as void. This works. However, I am wondering why
> >> delayed_visitor does no longer work in this situation? Is it because
> >> I am trying to return a reference?
> >
> > I believe this is caused by a bug in the trait used to detect whether
> > result_type is present. It used to form a pointer to result_type, and
> > pointer to references are ill-formed, so the trait reports that
> > result_type doesn't exist, and then the overloads of apply_visitor do
> > not SFINAE correctly.
> Sounds about right, thanks for the hint!
> > The bug has been fixed in
> >
> > 0539df98223f9
> So I guess this would be fixed with 1.59 then?

It's hard to tell. I took a quick look at GitHub ( , and there doesn't appear to be any tags for the 1.59.0 release candidates.

Assuming that 1.59 the latest release candidate is the tip of the master branch, then I can see that on June 17th the patch is applied.

Shane Turner

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