[boost] [utility/result_of] decltype-based implementation breaking valid code on msvc-10

Subject: [boost] [utility/result_of] decltype-based implementation breaking valid code on msvc-10
From: Eric Niebler (eric_at_[hidden])
Date: 2010-04-05 17:59:11

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.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com