"it can return a promise" -> actually a thenable

[ricea]

https://streams.spec.whatwg.org/#ref-for-pull-source-1

it can return a promise

Actually a thenable. The same applies to all sink and source methods that can return a Promise.

I noticed this when I was looking at the Closure compiler extern definitions for Streams, at https://github.com/google/closure-compiler/blob/master/externs/browser/streamsapi.js

For example, the type of ReadableStreamSource.prototype.start is declared as function((!ReadableByteStreamController|!ReadableStreamDefaultController)):(!Promise<*>|undefined)). At first I thought this was a bug in Closure's extern definitions, but then I realised that this is what the Streams standard actually says.

Editorially, the simplest fix would be to say "thenable" where we currently say "Promise", but I am concerned that might be too hard to understand.

[domenic]

I think it is fine as-is. The notes explaining usage are meant to guide you down the right path, not state what is technically allowed.

As a separate example, we would say "can return a number" instead of "can return any object that coerces to a number". (I.e., there's no need to point out that not only 1, but also { valueOf() { return 1; } } or "1" work.)

[domenic]

Closing, but happy to reopen if you disagree.