array.sort behavior can differ between MSVC and GCC compilers

[NotHyper-474]

class Main {
	static function main() {
		var sortThing:Array<Null<Int>> = [0, 0, 1000, null, 800, 801, 802, 850, 850, 900, 1000, 1001, 10000, 10000, 10000];

		sortThing.sort((a, b) -> {
			if (a == null || b == null) return 0;
			return Reflect.compare(a, b);
		});

		trace(sortThing);
	}
}

I've noticed this behaved differently between the compilers.
On MSVC, sorting results in [0,0,1000,null,800,801,802,850,850,900,1000,1001,10000,10000,10000], but on GCC the same array instead becomes [0,0,850,1000,null,800,801,802,850,900,1000,1001,10000,10000,10000].

It's a minimal recreation of how objects in a game's scene are ordered, which also caused a bug.
While it uncovers a flaw with how the game's sorting objects, I considered that the discrepancy in how the sorting behaved between compilers could be a bug.

[Aidan63]

This will be because your comparison function doesn't adhere to "weak strict ordering", under the hood hxcpp's array sort uses std::sort which has this requirement. Using a comparison function which doesn't follow this is undefined behaviour per the cpp spec, this can result in inconsistent results, random values in the results, or crashes.

https://en.cppreference.com/cpp/named_req/Compare

We discovered this during some coroutine work and I think we decided to make haxe follow the same requirements, but seems we forgot to update the docs.