Sorting floating-point values, and NaN

[tn]

On Tuesday, 12 November 2013 at 15:56:25 UTC, Andrei Alexandrescu 
wrote:
> On 11/12/13 12:54 AM, tn wrote:
>> ...
>> An alternative would be to check for nans explicitly.
>
> I think NaNs are singular unordered values that make invalid 
> inputs for either sort or isSorted. We should simply add an 
> assert to isSorted.

But sort and isSorted both allow user to provide a custom "less" 
function. What if the user needs NaNs and, for example, wants to 
sort them before all the other values? Then he calls 
"arr.sort!((a,b) => a < b || (isnan(a) && !isnan(b)))" and the 
code asserts?