Re: PR 60519: Debug mode should check comparators for irreflexivity (Was: pdqsort - a faster std::sort)

[Jonathan Wakely <jwakely at redhat dot com>]

On 11/04/15 13:44 +0100, Jonathan Wakely wrote:
> we can't just add the check in the constructor of _Iter_to_comp_iter
> where we have a dereferencable iterator and the functor:
>
> --- bits/predefined_ops.h.orig  2015-04-11 13:28:30.063009877 +0100
> +++ bits/predefined_ops.h       2015-04-11 13:28:33.645013410 +0100
> @@ -268,7 +268,13 @@
>
>      _Iter_comp_to_iter(_Compare __comp, _Iterator1 __it1)
>       : _M_comp(__comp), _M_ref(*__it1)
> -      { }
> +      {
> +#ifdef _GLIBCXX_DEBUG
> +       // Comparison must define a Strict Weak Ordering, like "less than",
> +       // so must be irreflexive, i.e. x < x must be false.
> +       __glibcxx_assert( !_M_cmp(_M_ref, _M_ref) );
> +#endif
> +      }
>
>      template<typename _Iterator2>
>       bool
>
> That won't help std::sort, because it doesn't use _Iter_comp_to_iter.

Oh, that *definitely* won't work because _Iter_comp_to_iter is only
used for equality comparisons not strict weak orderings, so checking
for irreflexivity there is completely wrong :-)

So ignore the patch above, but I think my main point is still valid, 
doing the check in the wrapper means we have to repeat it many times,
for very little benefit.