Re: [boost] [utility][enable_if]Fiddly function lookup failure on msvc11

Subject: Re: [boost] [utility][enable_if]Fiddly function lookup failure on msvc11
From: Edward Diener (eldiener_at_[hidden])
Date: 2013-10-07 09:33:08

On 10/7/2013 8:06 AM, 山本賢一 wrote:
> 2013/10/7 Gavin Lambert <gavinl_at_[hidden]>
> 
>> On 10/7/2013 9:41 AM, Quoth 山本賢一:
>>
>>   template <class T>
>>> struct always_true :
>>>       boost::mpl::true_
>>> {
>>> };
>>>
>> [...]
>>
>> Does the behaviour change if you make always_true not a template, or if
>> you make some member of it actually depend on the template argument?
>>
> 
> The behaviour changed and the code compiled if I changed always_true not a
> template.
> 
> struct always_true :
>      boost::mpl::true_
> {
> };
> 
> template <class T> inline
> typename boost::enable_if_c<
>      !boost::mpl::not_<
>          always_true
>      >::type::value
> , int
> > ::type

How about trying another "boost::mpl::not_" rather than "!" in the above
code. Does that make any difference ?

> get_one(T)
> {
>      return 1;
> }
> 
> The behaviour didn't change and the code didn't compile if I made some
> member of always_true actually depend  on the template argument.
> 
> template <class T>
> struct always_true :
>      boost::mpl::true_
> {
>      typedef typename boost::add_const<T>::type unused_type;
> };
> 
> The actual condition to reproduce the issue seems very complex. My best
> guess is that msvc11 gives up resolving template-parameter-dependent
> condition and marks it as failure (in the sense of SFINAE) under a specific
> circumstance.