Re: [Boost-users] boost::optional with BOOST_FUSION_ADAPT_ADT broken?

Subject: Re: [Boost-users] boost::optional with BOOST_FUSION_ADAPT_ADT broken?
From: Agustín K-ballo Bergé (kaballo86_at_[hidden])
Date: 2014-02-27 09:03:43

On 27/02/2014 10:21 a.m., Andrew wrote:
> There's an issue using get expressions returning optionals with
> BOOST_FUSION_ADAPT_ADT, demonstrated in the simplest case below.
> Optional<T> is trying initialize its internal T object using the
> attribute proxy object returned by fusion, but this can only degrade
> to optional<T> and not T, so the assignment fails.
> K-Ballo, took a quick look at this in #boost and seemed to suggest
> it's the way op= is SFINAEd up for optional.
>

Optional's assignment operator is not SFINAEd at all. If it were, it 
might allow fusion to get away with its implicitly convertible attribute 
proxies.

The fundamental problem is that fusion lies about adapted ADTs, since 
those are sequences of proxies instead of sequences of whatever you 
asked for. Fusion could do better to support its own proxies by adding 
explicit conversions wherever those conversions are expected to happen 
implicitly now. But that's not a general solution, and I don't think 
there is one besides documenting that adapted ADTs are not quite sequences.

Regards,
-- 
Agustín K-ballo Bergé.-
http://talesofcpp.fusionfenix.com