Re: [Boost-bugs] [Boost C++ Libraries] #4540: fold<Sequence>() instantiates result_of::fold<Sequence const>

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #4540: fold<Sequence>() instantiates result_of::fold<Sequence const>
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-08-18 08:58:40

#4540: fold<Sequence>() instantiates result_of::fold<Sequence const>
--------------------------------------+-------------------------------------
  Reporter:  anonymous                |       Owner:  djowel  
      Type:  Bugs                     |      Status:  reopened
 Milestone:  Boost 1.44.0             |   Component:  fusion  
   Version:  Boost Development Trunk  |    Severity:  Problem 
Resolution:                           |    Keywords:          
--------------------------------------+-------------------------------------
Changes (by anonymous):

  * status:  closed => reopened
  * resolution:  wontfix =>

Comment:

 reopening this ticket for now, as I think it can be fixed, even though
 "component" might have to be changed:

 {{{

 BOOST_MPL_HAS_XXX_TRAIT_DEF(type)

 struct invalid;

 template<typename F,typename Args>
 struct result_of_impl{
     typedef typename boost::mpl::eval_if<
         has_type<typename F::template result<Args> >,
         typename F::template result<Args>,
         boost::mpl::identity<invalid>
     >::type type;
 };

 }}}

 with this fix to result_of your code above yields these results:

 {{{

     A a;
     bub(a); //ok
     bub(A()); //error: use of undefined type "invalid"

 }}}

 do you see any problems with this?
 "invalid" could be further extended to give a meaningful error message,
 e.g. undefined_result_type<FArgs>

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/4540#comment:4>
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