#sync breaks when mixing Promise classes about promise.rb HOT 1 CLOSED

Yes, that is the problem with mixing promise classes. This can be avoided by using p1 = MyPromise.resolve(123) so that you don't mix promise classes.

However, I don't think this is a bug.

To start with, you have an incorrect assumption that the block passed to then is executed immediately when the receiver Promise instance has already been resolved. The promise spec expects the onFulfilled or onRejected to be deferred. Promise#defer can be overridden to implement this behaviour and conform to the spec as documented in the README's usage section. This prevents the Promise#then method from being able to determine the class of a promise that may be returned from the block.

The other problem with what you are suggesting is that it would introduce racey behaviour in the type of value that is returned from Promise#then in code where the Promise instance could be resolved by another fiber or thread that is running in parallel. I would much prefer to have Promise#then deterministically return a result based on the type of the receiver so that it is easier to catch errors from misuse during testing.

from promise.rb.