For TypedArrays read byteOffset before getting start and final

[nickva]

Previously, in issue #418 and pr #417 I misread the spec that in subarray we should be able to access the byteOffset from a detached buffer. Thinking more about it, something didn't seem right and I started a discussion in the TC39 group [1].

It turns out we shouldn't be able to read the byteOffset from detched buffers. Instead, the spec says we should just read the byteOffset value before we access start and finish. In the test262 test [2] the buffer is detached when accessing the end inside the valueOf() conversion, and the test expects to see the byteOffset before it was detached.

So to fix it, ensure we access and save the byteOffset value first, then get the start and finish.

[1] https://es.discourse.group/t/typedarray-subarray-byteoffset-with-detached-buffers/2381

[2] https://github.com/tc39/test262/blob/main/test/built-ins/TypedArray/prototype/subarray/byteoffset-with-detached-buffer.js

[nickva]

Ran in CI with test262 https://github.com/nickva/quickjs/actions/runs/15958281778/job/45007403622 on top of the "Run test262 tests in CI" PR #408

[bellard]

There is still a small problem in your fix: you read ByteOffset before converting 'start' to integer as in the spec. I made another patch instead.

[nickva]

There is still a small problem in your fix: you read ByteOffset before converting 'start' to integer as in the spec. I made another patch instead.

You're right, I agree. I see the steps are ToIntegerOrInfinity(start) then srcByteOffset = O.[[ByteOffset]] then ToIntegerOrInfinity(end). If the buffer is detached during start parsing with the valueOf() {$DETACHBUFFER(..); return ...} trick, then ByteOffset will read a detached buffer's offset. Seems like a spec omission but your fix matches the spec. I'll close this PR then. Thank you for noticing the issue.

For reference if others see this: https://tc39.es/ecma262/#sec-%25typedarray%25.prototype.subarray

...
8. Let relativeStart be ? ToIntegerOrInfinity(start).
...
13. Let srcByteOffset be O.[[ByteOffset]].
...
16. Else,
    a. If end is undefined, let relativeEnd be srcLength; else let relativeEnd be ? ToIntegerOrInfinity(end).
...