fix: allow mutation of private derived state

[paoloricciuti]

Closes #15227

I simplified the code to check for the error a bit since there's no need to recurse up, you can just check if the argument type and property are a this and a private identifier.

However this made me wonder: why do we allow reassignment of public deriveds? Is it just because someone could still reassign it from the outside? It feels weird that this is not an error even today

class Test{
    der = $derived({test: 0});

    set test(v){
        this.der = 45;
    }
}

while this is

class Test{
    #der = $derived({test: 0});

    set test(v){
        this.#der = 45;
    }
}

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Playground

pnpm add https://pkg.pr.new/svelte@15228

[dummdidumm]

It feels weird that this is not an error even today

It seems it fails at runtime, but it probably makes sense to fail at compile time already

[paoloricciuti]

It feels weird that this is not an error even today

It seems it fails at runtime, but it probably makes sense to fail at compile time already

Aight gonna add that too

[paoloricciuti]

I've added checks both for public derived state and literals this["der"]...technically you could still try to do something funky like this[some_variable] but that would be a runtime error.