Re: [boost] [result_of] ambiguous instantiation for function pointers

From: shunsuke (pstade.mb_at_[hidden])
Date: 2007-03-04 23:12:15

Eric Niebler wrote:
> shunsuke wrote:
>> Hi,
>>
>> CVS HEAD can't compile this under maybe any conforming compiler.
>> (RC_1_34 is ok.)
>>
>>
>> 	#include <boost/utility/result_of.hpp>
>>
>> 	int my_fun(int) { return 0; }
>>
>> 	template< class F >
>> 	void test(F )
>> 	{
>> 	    typedef typename
>> 	        boost::result_of<F()>::type
>> 	    result_t; // shall be 'void'.
>> 	}
>>
>> 	int main()
>> 	{
>> 	    ::test(&my_fun);
>> 	}
>>
>>
>> Specializations are not enough?
> 
> 
> I'm a bit confused. The type of F is int(*)(int). You're asking what the 
> return type is if you call it with no parameters. But you can't do that! 
> It takes an int. result_of<F(int)>::type compiles fine. I'm not sure 
> this is a bug. Can you justify why you think this code should compile?

AFAIK, nullary instantiations of 'result_of' must always succeed; for 
function forwarding.
See the Rationale at
  http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1454.html

According to 20.5.4, I was slightly wrong.
'result_t' shall be 'int'? (I'm sure nobody cares, though.)

Regards,

-- 
Shunsuke Sogame