XS fails to cache next for array-destructuring

[gibson042]

Environment: XS 14.2.0 32 4

Description
Array-destructuring does not cache an iterator's next method as required by the ECMAScript specification.

Steps to Reproduce

Object.defineProperty(Array.prototype, Symbol.iterator, {
  get() {
    print("get Symbol.iterator", this);
    return function makeIterator() {
      print("@@iterator", this, ...arguments);
      let i = 0;
      const src = this;
      return {
        [Symbol.iterator]: () => iterator,
        get next() {
          print("get", "next");
          return () => {
            if (i >= src.length) return { done: true, value: undefined };
            const value = src[i];
            print("yield", i, value);
            i += 1;
            return { done: false, value };
          };
        },
      };
    };
  },
});
const [x, y] = ["a", "b", "c"];

Actual behavior

get Symbol.iterator a,b,c
@@iterator a,b,c
get next
yield 0 a
get next
yield 1 b

Expected behavior

get Symbol.iterator a,b,c
@@iterator a,b,c
get next
yield 0 a
yield 1 b

DestructuringAssignmentEvaluation on an |ArrayAssignmentPattern| with argument value (the iterable responsible for providing values) is specified to always start with GetIterator(value, SYNC), which gets the Symbol.iterator method of value and uses GetIteratorFromMethod to invoke it and return a corresponding Iterator Record capturing the resulting iterator and its next method for use by IteratorDestructuringAssignmentEvaluation. But XS incorrectly gets the next method before each step (and fails to get it at all for const [] = …).

[bakkot]

Note that the current behavior (at least for this test) was correct prior to tc39/ecma262#1021.

[phoddie]

Thanks for the test case. It is a little odd that this isn't covered by test262, but that's a different discussion.

We have a fix for this that will be part of Modable SDK 4.2.1, likely later this week.

[phoddie]

Closing, as the fix for this is live.