Typing mixins does not work as mentioned in the docs

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Hello,

The documentation contains information on typing the subclass returned by a mixin: https://lit.dev/docs/composition/mixins/#typing-the-subclass

However, this does not seem to work in call instances: When you copy the example code into the playground: [link](https://lit.dev/playground/#project=W3sibmFtZSI6InNpbXBsZS1ncmVldGluZy50cyIsImNvbnRlbnQiOiJ0eXBlIENvbnN0cnVjdG9yPFQgPSB7fT4gPSBuZXcgKC4uLmFyZ3M6IGFueVtdKSA9PiBUO1xuaW1wb3J0IHtMaXRFbGVtZW50fSBmcm9tICdsaXQtZWxlbWVudCc7XG5cbmV4cG9ydCBkZWNsYXJlIGNsYXNzIE15TWl4aW5JbnRlcmZhY2Uge1xuICBoaWdobGlnaHQ6IGJvb2xlYW47XG4gIHByb3RlY3RlZCByZW5kZXJIaWdobGlnaHQoKTogdW5rbm93bjtcbn1cbuKAi1xuZXhwb3J0IGNvbnN0IE15TWl4aW4gPSA8VCBleHRlbmRzIENvbnN0cnVjdG9yPExpdEVsZW1lbnQ-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-XG4gIDxzaW1wbGUtZ3JlZXRpbmcgbmFtZT1cIldvcmxkXCI-PC9zaW1wbGUtZ3JlZXRpbmc-XG48L2JvZHk-XG4ifV0)

The compiler errors with something like `Conversion of type '{ new (...args: any[]): MyMixinClass; prototype: MyMixin.MyMixinClass; } & T' to type 'Constructor & T' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first. Type '{ new (...args: any[]): MyMixinClass; prototype: MyMixin.MyMixinClass; } & T' is not comparable to type 'Constructor'. Type 'MyMixinClass & LitElement' is not comparable to type 'MyMixinInterface'. Property 'renderHighlight' is protected but type 'MyMixinClass' is not a class derived from 'MyMixinInterface'.`.

Is it necessary to convert the class to `unknown` first maybe?

Thanks,
Christian

Start with the “Typing the subclass” example in the linked Lit documentation and reproduce it in the linked TypeScript playground. Compare the compiler error with the documented cast and determine whether the example or its explanation needs updating. Done means the example compiles as documented or clearly explains the required type conversion.

Written by the indexing model from the issue text.