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: 2010-08-17 22:53:07

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

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

Comment:

 Replying to [comment:2 anonymous]:
 > I see.
 > but I guess there is another solution, because the following works:
 >
 > {{{
 >
 > struct A{};
 >
 > template<typename T>
 > struct fresult;
 >
 > template<>
 > struct fresult<A>{
 >     typedef void type;
 > };
 >
 > template<typename T>
 > typename fresult<T>::type f(T &){}
 > template<typename T>
 > typename fresult<T const>::type f(T const &){}
 >
 > int main(){
 >     A a;
 >     f(a);
 > }
 >
 > }}}
 >
 > the undefined fresult<T const> is not instantiated for the call to the
 non-const overload. I don't know what exactly causes the instantiation of
 result_of::fold<Seq const>, but this suggests there might a solution other
 than removing the const overload.


 That sample compiles fine as SFINAE is taking care of the non-existing
 specialization.
 SFINAE does not help in your fusion usecase, though. There is simply no
 specialization of F::result that matches the template arguments in the
 first place - and that very instantiation is directly referenced by the
 boost::result_of implementation.
 I do not think that this issue can be worked-around.

 Here is a minimal non-fusion testcase.
 {{{
 #include <boost/utility/result_of.hpp>

 struct A{};

 struct F
 {
         template<typename> struct result;
         template<typename Self>struct F::result<Self(A&)>
         {typedef void type;};
 #if 0
         template<typename Self>struct F::result<Self(A const&)>
         {typedef void type;};
 #endif
 };

 template<typename C>
 typename boost::result_of<F(C const&)>::type
 bub(C const&){}

 template<typename C>
 typename boost::result_of<F(C&)>::type
 bub(C&){}

 int main()
 {
         A a;
         bub(a);
 }
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/4540#comment:3>
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