Fix UPGRADING and #9556 "iterable" alias "array|Traversable" breaks PHP 8.1 code

[Girgias]

This kinda address #9556 by making it a documentation issue, and adds tests to verify the behaviour.

[cmb69]

Yeah, looks good to me. Thank you!

@cziegenberg, what do you think?

[cziegenberg]

Yes, Looks good. Thanks!

[nikic]

IMHO a redundancy should only be reported it one type is a strict subset of another, such as in array|iterable, but not if they only have a non-empty intersection, as in object|iterable. If distinguishing these cases is too hard, I would err on the side of not reporting an error. Relaxing the redundancy checks was always going to become necessary once we support userland type aliases, because we certainly don't want A1|A2 where A1 = A|C1, A2 = A|C2 to complain that this is redundant and you must spell it A|C1|C2 instead, that would be ridiculous.

[Girgias]

IMHO a redundancy should only be reported it one type is a strict subset of another, such as in array|iterable, but not if they only have a non-empty intersection, as in object|iterable. If distinguishing these cases is too hard, I would err on the side of not reporting an error. Relaxing the redundancy checks was always going to become necessary once we support userland type aliases, because we certainly don't want A1|A2 where A1 = A|C1, A2 = A|C2 to complain that this is redundant and you must spell it A|C1|C2 instead, that would be ridiculous.

Okay, I'll have another look as I basically stopped when I got to the point that I needed to modify a bunch of functions, but if the guideline is "only warn on strict subset" I can probably do some ground work in that direction which will make the type check redundancy easier to amend if/when we get type aliases

[Girgias]

Turns out the fix is rather simple, just took me a long detour to get there, I maybe even should revert b7da08e as that's probably a pointless move too.

[iluuu1994]

Seems reasonable

[iluuu1994]

Unrelated?

[iluuu1994]

===DONE=== is kinda pointless, since Fatal error indicates termination. It's also used inconsistently.

[cmb69]

That ===DONE=== has been used in the past to easily see segfaults (which may not have been reported), but this is indeed obsolete since we now report any unexpected exit ("Termsig") on all platforms. However, according to https://qa.php.net/write-test.php#lastbit, there is another use case, and it seems this is still supported by run-tests.php. Still, without any code following ===DONE===, there's no need to use it. (And frankly, I doubt that ===DONE=== is actually useful for that purpose.)