[Boost-bugs] [Boost C++ Libraries] #5098: [result_of] decltype-based result_of cannot handle rvalue-reference correctly

Subject: [Boost-bugs] [Boost C++ Libraries] #5098: [result_of] decltype-based result_of cannot handle rvalue-reference correctly
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-01-20 09:56:22

#5098: [result_of] decltype-based result_of cannot handle rvalue-reference
correctly
-------------------------------------+--------------------------------------
 Reporter:  mimomorin@…              |       Owner:  no-maintainer
     Type:  Bugs                     |      Status:  new          
Milestone:  To Be Determined         |   Component:  utility      
  Version:  Boost Development Trunk  |    Severity:  Problem      
 Keywords:  result_of decltype       |  
-------------------------------------+--------------------------------------
 The implementation of decltype-based boost::result_of is essentially the
 same as
 {{{
 // This code is taken from N1454 (author: Douglas Gregor)
 template <typename F, typename T1, typename T2, ..., typename TN>
 class result_of<F(T1, T2, ..., TN)> {
     static F f;
     static T1 t1;
     static T2 t2;
          .
          .
          .
     static TN tN;

 public:
     // Note: even if Ti is rvalue reference, ti is treated as lvalue.
     typedef decltype(f(t1, t2, ..., tN)) type;
 };
 }}}
 So, the implementation cannot handle rvalue-reference correctly (it
 becomes lvalue).

 For example, in the following code
 {{{
 struct func
 {
     unsigned char operator()(int&&     ) { return 0; }
       signed char operator()(int&      ) { return 0; }
              char operator()(int const&) { return 0; }
 };
 }}}
 `boost::result_of<func(int)>::type` is `signed char` (which should be
 `unsigned char`).

 I will attach a patch for fixing this issue.
 The patch uses `declval` rather than static members.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5098>
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