Allow non-polymorphic ("private") extension of base classes about typescript HOT 4 OPEN

This is also possibly related to #4628. Not in the sense that my issue is affected by static inheritance, but because if this issue were resolved by the addition of something like the extends private mechanism described here, that could also be used to resolve 4628, by allowing decoupling of inherited members vs exposed members.

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It occurs to me that one radical way to solve this problem would be to add some syntax that allows you to suppress creating the interface portion of the class altogether. Then you could simply define the interface separately with whatever subset of the actual functionality you want to expose, and then the ES6 class definition just gets matched against that interface, rather than generating the interface itself.

...Huh. This actually feels like a useful and actionable feature. You could rewrite the original example like this:

// This line is the only definition of the MyTuple<T> type
export interface MyTuple<T> extends ReadonlyArray<T> { }
// A private class is one whose types are not visible to the outside. It has signature { } or possibly object.
export private class MyTuple<T> extends Array<T>
// This one implements the MyTuple<T> interface to ensure that it matches its public API
    implements MyTuple<T> {
  constructor(...args: readonly T[]) {
    super();
    this.push(...args);
    Object.freeze(this);
  }
}

What do you think?

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@dmchurch You can use a class expression to make it "private".

// This line is the only definition of the MyTuple<T> type
export interface MyTuple<T> extends ReadonlyArray<T> { }
// A private class is one whose types are not visible to the outside. It has signature { } or possibly object.
export const MyTuple = class<T> extends Array<T>
// This one implements the MyTuple<T> interface to ensure that it matches its public API
    implements MyTuple<T> {
  constructor(...args: readonly T[]) {
    super();
    this.push(...args);
    Object.freeze(this);
  }
}

Note that you MUST NOT use the name MyTuple for the class expression. Use any other name or anonymous instead, otherwise the interface MyTuple<T> declared in the outer scope will be shadowed.

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@whzx5byb Yes, I've used that workaround before, and it's not applicable in this case. I need the emit to contain an ECMAScript top-level class export, which has slightly different semantics than an anonymous class assigned to a const (or a let, or a var, etc etc). If you have any suggestions that actually fit my needs, I'm all ears.

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