Re: [boost] [optional] Changes in Boost.Optional

Subject: Re: [boost] [optional] Changes in Boost.Optional
From: Dean Michael Berris (mikhailberis_at_[hidden])
Date: 2014-09-02 09:39:20

On Tue Sep 02 2014 at 9:39:24 PM Andrzej Krzemienski <akrzemi1_at_[hidden]>
wrote:

> 2014-09-02 11:23 GMT+02:00 Dean Michael Berris <mikhailberis_at_[hidden]>:
> > Right. My hunch is that 1.56 now has suppression for the cases where
> > references would have been deduced (as discussed in
> >
> > http://www.boost.org/doc/libs/1_56_0/libs/optional/doc/html/
> boost_optional/dependencies_and_portability/optional_
> reference_binding.html
> > ).
> > Maybe the suppression applies even for the case when the conversion
> > operator may have been useful for copy initialization.
> >
>
> However, it works fine with GCC. It looks like VC++ compiler has a bug in
> overload resolution in copy initialization. It should never consider
> explicit constructors.
>
>
Agreed that it works with GCC. I tested with Clang on OS X too and it
doesn't display this problem.

I think the specialization of the behavior for VC++ 2010 that came in
1.56.0 may be disabling the copy constructor inadvertently because the RHS
resolves to a `const boost::optional<T>&` -- it may just be the case that
the suppression to address the bug is too aggressive, and is causing a
once-valid non-explicit constructor to be considered explicit? I can't be
sure until I get to play around with it, or look deeply at the code to try
and understand it.

I won't be surprised if this is a compiler bug though. I would still like
to know though whether there's a work-around, or whether I'm going to have
to say in my upcoming release of cpp-netlib that we can't support Boost
1.56.0 with MSVC 2010. That announcement would be unfortunate. :(