Cannot directly @attach a class function

[Refzlund]

Describe the bug

When using a class function as an attachment, it loses its this reference.

Reproduction

Svelte Playground

<script>
	
	class Example {
		element
		setElement(element) {
			this.element = element
		}
	}

	const example = new Example()
	
	let name = 'world';
	
</script>

<!-- Does not work - loses `this` reference -->
<div {@attach example.setElement} style:color='tomato'>
	Does not work - loses `this` reference
</div>

<!-- Works -->
<div {@attach el => example.setElement(el)} style:color='lightgreen'>
	Works
</div>

Logs

TypeError: Cannot set properties of undefined (setting 'element')
    at setElement (playground:output:2786:18)
    at eval (playground:output:2527:63)
    at update_reaction (playground:output:1478:57)
    at update_effect (playground:output:1645:19)
    at flush_queued_effects (playground:output:1753:6)
    at flush_queued_root_effects (playground:output:1725:6)

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (12) x64 Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz
    Memory: 5.14 GB / 31.93 GB
  Binaries:
    Node: 23.11.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.9.2 - C:\Program Files\nodejs\npm.CMD
    bun: 1.2.16 - ~\.bun\bin\bun.EXE
  Browsers:
    Edge: Chromium (132.0.2957.115)
  npmPackages:
    svelte: ^5.34.1 => 5.34.1

Severity

annoyance

[brunnerh]

The same is true when using class methods in event handlers; it's just how JS works and something to be aware of when using this.