Encourage warning messages for onFulfilled or onRejected being promises

[ForbesLindesay]

This is probably the most common error I see from beginners using promises on stack overflow (e.g. http://stackoverflow.com/questions/17517593/more-complex-deferred-promise-usage-and-chains-just-arent-clicking-would-love/17525172). They call the function, rather than passing the function to .then. Since I can't see why anyone would intentionally pass a promise to a .then handler and these sorts of functions almost always return promises, I think this could be a big win for helping people get started faster.

P.S. the PR is just there as a discussion point, I'm happy to re-word/move it as appropriate.

[bergus]

Actually I think we should recommend this for all truthy, non-function values. There are many callbacks that produce non-promise values, and they should trigger a warning as well.

[domenic]

I don't think the spec should be in the business of handing out warnings. I admit the circular thenable chain scenario is a bit of a slipper slope, but I'd rather leave this up to implementers.

If anything I'd go in the direction of creating a best-practices document for implementers, with suggestions like "warn on non-undefined non-functions passed to then," or "provide a resolve method but not a reject one," and then we could move the circular thenable chain error into that document.

[ForbesLindesay]

My feeling at the moment is that there's an implicit "and do nothing else" clause effectively associated with every point of the spec. Over-zealous warnings could lead to a very broken promise library. I think there should at least be some advice on the sorts of things it is and isn't ok to produce a warning for.

I like the idea of a separate document for these sorts of things, but I think it belongs in this repository so that it at least gets the promises/A+ rubber stamp :)

[briancavalier]

I'd be ok with putting this kind of thing in a separate doc.

[domenic]

Let's close this and open a more general best-practices-for-implementers doc issue.