Re: [boost] [Fit]Â formal review starts today

Subject: Re: [boost] [Fit] formal review starts today
From: Paul Fultz II (pfultz2_at_[hidden])
Date: 2016-03-05 11:36:59

On Saturday, March 5, 2016 at 10:15:59 AM UTC-6, Steven Watanabe wrote:
>
> AMDG
>
> On 03/05/2016 01:36 AM, Vicente J. Botet Escriba wrote:
> > Le 05/03/2016 01:11, Paul Fultz II a écrit :
> >>
> >> On Friday, March 4, 2016 at 5:19:52 PM UTC-6, Steven Watanabe wrote:
> >>> 
> >>> On 03/04/2016 11:55 AM, Vicente J. Botet Escriba wrote:
> >>>> 
> >>> #include <boost/fit/conditional.hpp>
> >>>
> >>> struct identity {
> >>>      template<class T>
> >>>      T operator()(T t) { return t; }
> >>> };
> >>>
> >>> int main() {
> >>>      boost::fit::conditional(identity(), identity())(0);
> >>> }
> >>>
> >>> 1> boost\fit\conditional.hpp(84): error C2500:
> >>> 'boost::fit::detail::conditional_kernel<F1,F2>': 'identity' is already 
> a
> >>> direct base class
> >>> 1>          with
> >>> 1>          [
> >>> 1>              F1=identity,
> >>> 1>              F2=identity
> >>> 1>          ]
> >>> 1>
> >>>
> >> This is by design. I should document this. It makes absolutely no
> >> sense to
> >> pass the same function to conditional, and is most likely a mistake in
> >> the
> >> users code.
> >>   
> >
>
>   I disagree that this makes no sense.  It
> would be true if we were talking about match,
> but for conditional, this degenerate case has
> a clear meaning, since conditional chooses
> the first possible function.  Forbidding it
> creates a special case, and special cases
> are bad for generic code.  Of course, my
> example is a bit silly, because I was
> trying to create a minimal reproduction,
> but it's quite possible to have a situation
> where you don't know and don't care whether
> the arguments are the same type.
>

I see, so in generic code this could be a problem. I'll open an issue to 
update
this.