Using slots on svelte:element is buggy in v.3.48.0

[MathiasWP]

Describe the bug

Setting a slot prop on the svelte:element tag makes it behave in a broken way.

I'm not sure if the case I encountered is the only case where behaviour is buggy.

See the REPL for more info, but this is what made me find the bug:

1. Have a dynamic class on the svelte:element tag
2. Pass down a slot prop in the svelte:element tag
3. Update the value of the slot prop
4. The dynamic class is now gone (even though it had nothing to do with the slot pop)

Reproduction

https://svelte.dev/repl/af99dfa4e9ef45f6bce713e90af2de38?version=3.48.0

Logs

No response

System Info

System:
    OS: macOS 11.4
    CPU: (8) x64 Intel(R) Core(TM) i5-8259U CPU @ 2.30GHz
    Memory: 48.09 MB / 8.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.13.0 - /usr/local/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 8.1.0 - /usr/local/bin/npm
  Browsers:
    Brave Browser: 101.1.38.119
    Chrome: 102.0.5005.61
    Edge: 101.0.1210.53
    Firefox: 100.0
    Safari: 14.1.1
  npmPackages:
    svelte: ^3.47.0 => 3.47.0

Severity

blocking an upgrade

[MathiasWP]

Note: This bug was introduced in version 3.48.0, it does not occur in 3.47.0: https://svelte.dev/repl/af99dfa4e9ef45f6bce713e90af2de38?version=3.47.0

[schibrikov]

It seems to be the case even without slot props. I have Text component which looks roughly like this:

<svelte:element
	this={tag}
	class="text"
	class:bold
	class:color-basic={color === 'basic'}
	class:color-accent={color === 'accent'}
	class:color-invert={color === 'invert'}
	class:color-muted={color === 'muted'}
	class:color-invalid={color === 'invalid'}
	class:size-s={size === 's'}
	class:size-m={size === 'm'}
	class:size-l={size === 'l'}
	class:size-xl={size === 'xl'}
	class:size-header={size === 'header'}
	class:block
	class:breakWord
>
	<slot />
</svelte:element>

and it seems to remove dynamic classes when slot content is changed.
Thanks to this issue I was able to workaround the issue by sticking to 3.47.

[baseballyama]

This is already fixed by #7531
Still this is not released but you can check this out when next version is released.