Type bug: using mapped type (Partial) & this type & lookup type

TypeScript Version: nightly

Search Terms: mapped type, Partial, this, lookup type

Code

class A {
  state = { foo: "foo", bar: 42 };

  something(): Partial<A["state"]> {
    return { foo: "changed" }; // works
  }
}

class B {
  state = { foo: "foo", bar: 42 };

  something(): Partial<this["state"]> {
    return { foo: "changed" }; // fails
  }
}

class C {
  state = { foo: "foo", bar: 42 };

  something(): Partial<this["state"]> {
    return { foo: "changed", bar: 123 }; // works
  }
}

Expected behavior:

Should type check.

Actual behavior:

Type '{ foo: "changed"; }' is not assignable to type 'Partial<this["state"]>'.(2322)

Playground Link: Example

Related Issues: Tried searching for Partial this, did not find anything related.