I want to define class with generic parameter T and some methods operating with instances of the same class with the same parameter T.
But it doesn’t produce type errors as I expect.

Basically I want to something like this:

class C<T extends "a" | "b"> {
  foo(b: C<T>): C<T> {
    return b;
  }
}

I want this to ensure safety, but typescript doesn’t produce an error on the last line as I expect.
a.foo has type (C<"a">) => C<"a"> but accepts C<"b">

const a = {} as C<"a">;
const b = {} as C<"b">;
a.foo(b);

Simpler example without class in argument produces error.

class D<T extends "a" | "b"> {
  foo(b: T): T {
    return b;
  }
}

const a = {} as D<"a">;
a.foo("b");