Consider: class C1 extends ArrayBuffer { constructor(n) { super(n) } } var c1 = new C1(10); c1 instanceof ArrayBuffer // true c1 instanceof C1 // true But then: class C2 extends SharedArrayBuffer { constructor(n) { super(n) } } var c1 = new C2(10); c1 instanceof SharedArrayBuffer // true c1 instanceof C2 // ***false*** Probably just needs a small adjustment somewhere.

A number of test cases for this will land with bug 1232966 in jit-test/tests/sharedbuf/methods.js but could usefully be broken out and/or generalized when we fix the present bug.

Copied from the ArrayBuffer case. Plus test cases.

Comment on attachment 8698982 [details] [diff] [review] SharedArrayBuffer subclassing + tests Review of attachment 8698982 [details] [diff] [review]: ----------------------------------------------------------------- Looks perfect. If it's a cargo-cult, it's a very well understood one :P

For extra credit, you can even add testBuiltin(SharedArrayBuffer) to tests/ecma_6/Class/extendBuiltinConstructors.js, if you want, though the current test coverage is sufficient.

https://hg.mozilla.org/integration/mozilla-inbound/rev/70cfa74485fd6927c6547e2f5df4c3848e382757 Bug 1233011 - SharedArrayBuffer subclassing + tests. r=efaust

Note, this change was incomplete, we also need a SpeciesConstructor thingie. See bug 1314564.