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.

リンクされている TypeScript Playground の例から始め、cloneMe に関係する型の絞り込みとポリモーフィックな this の戻り値の挙動を調査してください。x.clone() の戻り値が Foo & T として保持され、キャストやエラーなしで例の型チェックが通れば、issue は完了です。