Cannot use `this` to index private or protected properties

[Pauan]

TypeScript Version: 2.4.1

Code

This works fine:

class Foo {
  _foo: number;

  foo(): this["_foo"] {
    return this._foo;
  }
}

This works fine:

class Foo {
  protected _foo: number;

  foo(): Foo["_foo"] {
    return this._foo;
  }
}

I expect this to work, but it does not work:

class Foo {
  protected _foo: number;

  foo(): this["_foo"] {
    return this._foo;
  }
}

It fails with this error:

(4,10): error TS2536: Type '"_foo"' cannot be used to index type 'this'.

[weswigham]

I think this is expected given our rules on indexed access types - given that the indexed access operator is defined in terms of keyof T (see here) and keyof T is:

If T has no apparent string index signature, keyof T is a type of the form "p1" | "p2" | ... | "pX", where the string literals represent the names of the public properties of T. If T has no public properties, keyof T is the type never.

So this will require adding an exception whereby private names are actually accessible when indexing this.

[Pauan]

@weswigham But Foo["_foo"] is the same situation, and it works fine.

So does indexing on classes have an exception for private/protected members, but that exception wasn't applied to this?

[mhegazy]

As @weswigham noted, this is the intended behavior. it becomes very hard to reason about these types in higher order if you have to carry around a context to know if the name is visible or not.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.