Type narrowing discards template type on this-typed return value

Bug Report

🔎 Search Terms

type narrowing template constraint

🕗 Version & Regression Information

Bug occurs in every version on the playground

⏯ Playground Link

Playground link with relevant code

💻 Code

abstract class Foo {
  abstract clone(): this;
}

function cloneMe<T extends Foo|Date>(x: T): T {
  if (x instanceof Foo) return x.clone();
  // These casts are necessary because of https://github.com/microsoft/TypeScript/issues/38309.
  return new Date(x as Date) as T;
}

🙁 Actual behavior

Type 'Foo & Foo' is not assignable to type 'T'. 'Foo & Foo' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Foo | Date'.

🙂 Expected behavior

No errors; just like the instanceof check narrows x to Foo & T, the this return type of x.clone() should also be Foo & T rather than degrading to Foo & Foo.

Start with the linked TypeScript Playground example and inspect the type narrowing and polymorphic this return behavior involved in cloneMe. The issue is done when the x.clone() return is preserved as Foo & T and the example type-checks without casts or errors.