1512780 - MozPromise shouldn't require callbacks to return a MozPromise to be chainable

Status: NEW

(Core :: XPCOM, defect)

Description

[Jean-Yves Avenard [:jya]]

https://searchfox.org/mozilla-central/rev/ff46b36ac2ebb243ec95fdab01340391963d62e5/xpcom/threads/MozPromise.h#121-123

The documentation states:
If the resolve/reject callback returns void, the
 *       completion promise is resolved/rejected with the same value that was
 *       passed to the callback.

That is, you should be able to do something like:

RefPtr<GenericPromise> p = GenericPromise::CreateAndResolve(true, __func__);

return p->Then(mThread, []() {}, []() {});

however, this fails with:
"The resolve/reject callback needs to return a RefPtr<MozPromise> in order to do promise chaining."


I believe this is a regression, and it used to work.
This can be particularly useful when you want to easily retain an object alive for the duration of the promise: e.g.

RefPtr<GenericPromise> p = GenericPromise::CreateAndResolve(true, __func__);
RefPtr<blah> kungFuDeathGrip = object;
return p->Then(mThread, [kungFuDeathGrip]() {}, [kungFuDeathGrip]() {});