Improve error message on indexed access to private members of type parameters

[andrewbranch]

Fixes #30882, assuming it's a bug. Noticing that this works (has always worked?):

class A {
  private a: string;
}

type X = A["a"]; // indexed access can access private members

I assumed that there’s no reason why the same shouldn’t work when A is the constraint of a type parameter:

type X<T extends A> = A["a"];

---

EDIT: #30882 is not a bug, as @weswigham explains below. This PR now just improves the error message.

[weswigham]

I assumed that there’s no reason why the same shouldn’t work when A is the constraint of a type parameter:

The type of the private is erased in a declaration file, so the index will become any. So

export class A {
  private a: string;
}

export type X<T extends A> = T["a"];

emitted with declarations becomes

export class A {
  private a;
}

export type X<T extends A> = T["a"];

wherein T["a"]` loses any constraint it previously had.

So yeah, there is a reason we can't allow accessing privates on generics - since we need to defer resolving the type and we know that the type information for privates is erased, the type information for the privates of generics can't be considered constant.

[andrewbranch]

Ah, ok. So the observable effect of this problem is that you’d have different results depending on whether you were consuming this type as a .ts or a .d.ts. 😱

[andrewbranch]

@weswigham I reverted my previous change and am issuing a more specific error message instead now—thanks for the explanation.

[weswigham]

I'd edit the PR title and OP just for ease of browsing the repo history, too.

[andrewbranch]

Oh, I edited the title but forgot about the original post. Thanks!