Allow duplicate fields

[romdotdog]

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What's this?

This PR replaces the "duplicate identifier" diagnostic used in the markVirtuals function (which also doesn't catch private duplicate fields) with a few checks in the finishResolveClass function for visibility and assignability, conforming to TS. I also implemented ambient fields, or fields with the declare keyword.

What does this allow?

This allows, in general, duplicate fields, but also allows duplicate fields that are assignable to the respective field in the base class

class A {}
class B extends A {}

class A1 {
  constructor(public a: A) {}
}

class A2 extends A1 {
  constructor(public a: B) {
    super(a)
  }
}

In the end, this feature was implemented in order to allow the VariableDeclaration class to upgrade its name field from an IdentifierExpression to an Expression; this allows array destructuring to be implemented cleanly.

[dcodeIO]

A few observations :)

[dcodeIO]

I have a vague suspicion that this can lead to issues, since typically inherited fields share the same flow flags for example, which is not the case anymore when duplicating the field. I don't see a case right now where this would trigger for sure (perhaps field initialization checks in ctors, hmm), though, but leaving a note here in case we ever encounter something along those lines.

[romdotdog]

Would you like me to insert some sort of comment there?

[dcodeIO]

I guess it's fine without, just wanted to leave the note so you and me are aware in case :)

[dcodeIO]

Thank you!

[dcodeIO]

Turns out that covariance on class fields as introduced here in unsound and is rejected by Wasm GC. Means we'll likely have to revert this. See PR linked above for details.