Lookup types and polymorphic this issue

[dsehnal]

TypeScript Version: 2.1.4

Code

interface A<T> { value: T }
interface B extends A<string> { }

abstract class C<T extends A<any>> {
 private t: T;
 get a(): this['t']['value'] { return this.t.value };
}

class D extends C<B> {
 get b() { return this.a; }
}

Expected behavior:

I would expect that the type of D.b would be string.

Actual behavior:

Type of D.b is any

[dsehnal]

Here is a more simplified version of the issue:

interface A<T> { t: T }
function f<T extends A<any>>(t: T['t']): T {
 return { t };
}

Here t is of type any instead of T['t']. So I suppose that what's going on is that T['t'] is evaluated immediately instead of keeping the generic value.

So my question would be: Will the type system ever support this behaviour?

[mhegazy]

This is a bug, should be fixed shortly.

[ahejlsberg]

The issue is fixed by #12770, but note that your example only works if you remove the private modifier from t because indexed access types only operate on public properties.