[Boost-bugs] [Boost C++ Libraries] #4072: [result_of] decltype-based implementation breaking valid code on msvc-10

Subject: [Boost-bugs] [Boost C++ Libraries] #4072: [result_of] decltype-based implementation breaking valid code on msvc-10
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-04-05 22:36:32

#4072: [result_of] decltype-based implementation breaking valid code on msvc-10
----------------------------------------+-----------------------------------
 Reporter:  eric_niebler                |       Owner:  dgregor
     Type:  Bugs                        |      Status:  new    
Milestone:  Boost 1.43.0                |   Component:  utility
  Version:  Boost Release Branch        |    Severity:  Problem
 Keywords:  decltype result_of msvc-10  |  
----------------------------------------+-----------------------------------
 I recently looked into some proto failures on msvc-10 and traced them back
 to the use of decltype in the implementation of boost::result_of. The
 program below demonstrates the problem:

 {{{
     // Uncomment the next line to make the problem go away
     //#define BOOST_NO_DECLTYPE
     #include <boost/utility/result_of.hpp>

     template<class X, class Y> struct pair {};

     template<class Base>
     struct S;

     struct wrapper
     {
       template<class T>
       struct result;

       template<class This, typename That>
       struct result<This(That)>
       {
           typedef S<That> type;
       };

       template<typename That>
       typename result<wrapper(That)>::type
       operator()(That) const
       {
           return 0;
       }
     };

     template<class T>
     struct S
     {
         S(int = 0) {}
         typename boost::result_of<wrapper(pair<T, T>)>::type foo()
         {
           return 0;
         }
     };

     int main()
     {
       S<int> s;
     }
 }}}

 The use of decltype in result_of causes the compiler to recursively
 instantiate templates until it blows its stack. To make the problem go
 away, you must #define BOOST_NO_DECLTYPE.

 I suspect the real problem is in a buggy implementation of decltype on
 msvc-10. For the upcoming boost release, I suggest that we stick with the
 non-decltype implementation of result_of on msvc-10, or risk massively
 breaking users' code, not to mention proto, spirit and xpressive on that
 compiler.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/4072>
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