Re: [boost] [result_of] now uses decltype on release branch

Subject: Re: [boost] [result_of] now uses decltype on release branch
From: Joel de Guzman (djowel_at_[hidden])
Date: 2012-09-04 10:57:32

On 9/3/2012 12:07 PM, Joel de Guzman wrote:
> On 9/3/2012 11:10 AM, Michel Morin wrote:

>> Joel, Fusion's invoke test (libs/fusion/test/invoke.test) fails
>> with decltype-based result_of.
>>
>> The reason of the test failure is that fusion::invoke tries to
>> instantiate boost::result_of with uncallable signatures.
>> (Just making boost::result_of SFINAE-able does not solve the problem.)
>>
>> Here is a minimal test case:
> [snip]
>>
>> And here is a related ticket
>>      https://svn.boost.org/trac/boost/ticket/6915

Ok I fixed the fusion::invoke test. Tricky!

That also fixes the first test from Ticket #6915. The second, OTOH,
I think is an error. Essentially, what the OP wants is something like
this to work (i.e. compile and call the fallback overload):

     #include <boost/utility/result_of.hpp>

     int foo(int);
     struct junk {};

     template <typename F, typename Arg>
     typename boost::result_of<F(Arg)>::type
     call(int)
     {
         return F(Arg());
     }

     template <typename F, typename Arg>
     void call(long)
     {
         // fallback
     }

     int main()
     {
         call<int(*)(int), junk>(123);
     }

And so you get: error: cannot convert 'junk' to 'int' in argument passing
That IMO is the correct behavior. If not, then boost::result_of is
wrong.

However! This makes the current result_of code not an exact replacement
to decltype which allows this variation of above:

     #include <boost/utility/result_of.hpp>

     int foo(int);
     struct junk {};

     template <typename F, typename Arg>
     decltype(F(Arg()))
     call(int)
     {
         return F(Arg());
     }

     template <typename F, typename Arg>
     void call(long)
     {
         // fallback
     }

     int main()
     {
         call<int(*)(int), junk>(123);
     }

So, there's your problem. If Fusion's invoke used decltype instead of
result_of, it would have worked.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com