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

Subject: Re: [boost] [utility/result_of] decltype-based implementation breaking valid code on msvc-10
From: Eric Niebler (eric_at_[hidden])
Date: 2010-04-06 11:59:41

On 4/6/2010 12:52 AM, Daniel James wrote:
> I'll revert it for now but we
> should probably change config to define BOOST_NO_DECLTYPE for Visual
> C++ 10 if it's buggy on that compiler.

I had a similar thought. Here is the bug I filed against msvc:

   https://connect.microsoft.com/VisualStudio/feedback/details/548883

And for easy reference, here is the code:

   template<class T>
   struct S;

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

   template<class T>
   S<T> wrap(T) { return 0; }

   template<class T>
   struct S
   {
     S(int = 0) {}

     // The following use of decltype causes the compiler to blow up:
     decltype(wrap(pair<T,T>())) foo() { return wrap(pair<T,T>()); }

     // The following, which should be equivalent, compiles without
     // problem:
     //S<pair<T,T> > foo() { return wrap(pair<T,T>()); }
   };

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

As you can see, decltype forces the types used in the expression to be 
complete. I'm not sure if that's conforming, but I hope not. I'd say 
it's premature to turn decltype on across the board for this compiler 
until we have a better idea of its limitations. The safe thing to do for 
1.43 is to turn it off. We can always turn it on for 1.44.

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