On 29/11/2022 20:03, Nikolay Nikolov via fpc-devel wrote:
if (a<b) and (b<c) then (a<c),

That's the big one that sorting algorithms rely on... the transitive law.  If that is violated, then you cannot guarantee a sorted result.

It doesn't matter if (a < b) or (b < a) return False for equal elements, or use (a <=b) or (b <= a) instead, as long as it's consistent.  Also have to watch out for more subtle instances of it, like "if (a < b) then DoX else DoY;" and then having "if (b < a) then DoZ else DoQ;".

While I do wish people would fix any bugs that are found in their own code, sometimes we do have to accept that this isn't always possible.  I think the most famous example I can think of is with SimCity 2000... there was a critical bug in it where memory was used after it was deallocated.  Under DOS and Windows 3.1, this wasn't an issue because the memory wouldn't be reused by another application, but under Windows 95 this assumption could no longer hold.  So, under the guidance of Raymond Chen, Microsoft programmed Windows 95 to delay actually releasing that block of memory... only for SimCity 2000!


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