Using with `mut` results in render error

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
25/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
typescript
Domain
frontend

Research direction

Start by reproducing the issue with components/test/template.hbs and the shown conditional Test component, comparing fn with action and ember-fn-helper-polyfill. Inspect the render-modifier handling around did-insert and will-destroy, then add a regression test covering mut during destruction; done means the test passes without the render error.

Written by the indexing model from the issue text.

Description

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.

Dominant language
TypeScript
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Forks
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