[1.4.9] $compile:nonassign error in directive

[s-KaiNet]

I created directive, isolated scope has a bi-directional binding isOpen: '='. In html I didn't set is-open attribute, but do it in a click handler. In that case $compile:nonassign error is thrown.
Here the a simple plunk demonstrating the issue - plunk (error is in console)

For version 1.4.8 all works good - here the same plunk but for 1.4.8 version.
This can be fixed but making binding optional isOpen: '=?', but is this a bug in 1.4.9 or not?

[gkalpak]

This seems like an intentional change made in 7bb2414 (with a relevant test added in 45c5688) to fix #13367 (which is about a regression that was introduced in 1.4.0-beta.3 as part of 6a38dbf).

[wesleycho]

I encountered this too when upgrading from 1.4.7 to 1.4.9 - that was actually the main reason I opened #13827.

While this is a regression, I think the proper fix is to indeed mark it with ? because this involves setting two watchers internally if there is a two way binding, and it doesn't make sense to do so if the attribute isn't set.

[lgalfaso]

Directives that use = binding the parameter is mandatory. A change so this is clear landed with 4ff9c02

[AckerApple]

This issue breaks my 1.4.6 angular code. I can't go look at every single controller. I will just be putting a question mark after EVERY scope attribute just so I can move on without error.