Add irreflexive comparison debug check

[François Dumont]

On 01/08/2015 02:25, Jonathan Wakely wrote:
> On 31 July 2015 at 20:49, François Dumont wrote:
>> On 30/07/2015 12:30, Jonathan Wakely wrote:
>>> On 29/07/15 22:08 +0200, François Dumont wrote:
>>>>    Standard algos signatures are such that there is no guaranty that
>>>> the operator < or predicate to compare the iterator value type will
>>>> exist. So I had to check if the call is valid using C++11 features
>>>> declval and decltype.
>>> Surely if someone calls an algorithm that requires a
>>> StrictWeakOrdering then we know that it's valid to compare the value
>>> types using the comparison implied by that particular call (either
>>> operator< or the supplied predicate), and we should only be adding
>>> this assertion to algorithms that require a StrictWeakOrdering.
>>>
>>> Am I missing something?
>>     If you consider for instance lower_bound signature:
>>
>>   template<typename _ForwardIterator, typename _Tp>
>>     inline _ForwardIterator
>>     lower_bound(_ForwardIterator __first, _ForwardIterator __last,
>>         const _Tp& __val)
>>
>>     This algo require expressions *__first < __val or __val < *__first
>> to be well defined. But what the check is doing is *__first < *__first
>> that might not exist as the algo won't need it. The range [__first,
>> __last[ might have been sorted using another functor that we don't know
>> at the time of the call to lower_bound.
>>
>>     There might be situations where the check will be useless but I
>> prefer to keep this debug check simple and so always go through the check.
> OK, but that is only true for some algos, it's not a problem for many
> of them, e.g. std::sort we know that we only ever do homogeneous
> comparisons, so we don't need the SFINAE tricks to detect if the
> comparison is valid, and so could check for irreflexivity in C++03
> mode.
>
> So I think ideally we would be able to check for irreflexivity for
> most algos in C++03, and for some (such as lower_bound) we would only
> be able to check for C++11 and later.
>
Ok, it looks like you really want to provide this check in pre-C++11
mode so here it is.

Tested under Linux x86_64.

Ok to commit ?

François

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