Mark: I outlined two of these use cases in https://esdiscuss.org/topic/subclassing-es6-objects-with-es5-syntax#content-50
One is `WeakPromise` which is a promise holding a weak reference to its resolved value. This is the closest analogy with the canonical Smalltalk motivating example for species. Another is `TimeoutPromise` which is a promise that rejects in a fixed time if not resolved before then. Both of these would set `WeakPromise[Symbol.species] = TimeoutPromise[Symbol.species] = Promise;` so that the weak reference/timeout is not "contagious" across `then()`. `WeakPromise.all([...])` reads like it should return a `WeakPromise` (ie, ignore species). This is consistent with the "static methods ignore species" rule. *However*, `WeakPromise.resolve([....]).all()` makes it clearer that weak reference held by `WeakPromise` is actually just to the initial array, and that the result of `WeakPromise.prototype.all` should in fact be a `Promise` (ie, honor species). Similarly for `TimeoutPromise`: `TimeoutPromise.resolve([...]).all()` implies a timeout to resolve the initial array-of-promises, but the result of the `all()` is a normal Promise and won't timeout. But on the other hand, you wouldn't be wrong if you thought that `TimeoutPromise.all([...])` should timeout the result of the all. So, to me it seems like if you think that `Promise.all(x)` is shorthand for a future `this.resolve(x).all()`, then honoring the species is not a terrible thing. In ES7-ish, when you allow `Promise.all` and `Promise.race` to accept Promises as their arguments (in addition to iterables), then you could clarify the spec to indicate that the species is ignored when resolving the Promise argument, then honored when processing the `all` or `race`. But if you want a crisp semantics and weren't afraid of more last-minute changes to the Promise spec, then you'd define `Promise.all` and `Promise.race` to ignore species, and handle the species issue in ES7 when you define `Promise.prototype.all` and `Promise.prototype.race`. IMO, at least. --scott
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