Using with mut results in render error

[tstormk]

Hello!

Say I have a component Test:
components/test/template.hbs
<div {{did-insert (fn (mut this.element))}} {{will-destroy (fn (mut this.element) null)}} />

When that component is destroyed, it results in the following error:
You modified "element" twice on [object Object] in a single render. It was rendered in undefined and modified in undefined. [...]

If I instead perform the registering/unregistering like so:
components/test/template.hbs
<div {{did-insert this.register}} {{will-destroy this.unregister}} />

components/test/component.js

@action
register(element) {
  this.element = element;
}

@action
unregister() {
  this.element = null;
}

Destroying the component doesn't result in an error.

This happens when mut-ing any property with any value on destroy, by the way, not just an element or a property that has been set before.

For reference, here's the code I used to test it:

{{#if this.show}}
  <Test />
{{/if}}

<Button @onClick={{toggle-action 'show' this}}>Toggle</Button>

Also, I'm not sure it makes any difference, but I'm on a canary build of Ember:
"ember-source": "https://s3.amazonaws.com/builds.emberjs.com/canary/shas/55f876ebc10bd1645e1e62fc5e0408266952259b.tgz"
"ember-cli": "github:ember-cli/ember-cli#ba9e3ea9bcad1c6e2299e40fe265cffe61d7a25b"

Any help would be appreciated!

UPDATE: It works if I use the action helper instead of fn. It also passes if I use ember-fn-helper-polyfill for fn. If I use the Ember 3.11 version of fn however, it fails with the error specified above.

[rwjblue]

@tstormk - Hmm, seems somewhat surprising. Can you submit a failing test to this repo (it would make it a bit easier to dig into)? I think a rendering test would reproduce right?

[tstormk]

@rwjblue Any news on this?

[yratanov]

@rwjblue hi. Can you please look at this issue again? It still happens in 3.18.

[yratanov]

it looks like it's rather fn problem: emberjs/ember.js#18804

[vijayendranj]

Hi, I'm seeing this error with Ember 3.25. any ideas to get around this issue? Thanks