Assertion function methods complains about missing annotation even with explicit annotation

This may be working as intended but the error message is at least unclear for this case. It's not obvious why explicit annotation with a Union is different than explicit annotation with a class type.

TypeScript Version: 3.9.4

Search Terms:

• assertion function union
• assertion function

Code

type Pet = Cat | Dog;

type Filter<V extends Pet, T extends 'dog' | 'cat' > = V extends { type: T } ? V : never

abstract class Base {
    abstract type: 'dog' | 'cat'   
  
    assert<T extends 'dog' | 'cat'>(this: Pet, type: T): asserts this is Filter<Pet, T>  {
        if (this.type === type) return
        throw new Error('nope')
    }
}

class Dog extends Base {
    type = 'dog' as const
    bark() { }
}       
class Cat extends Base  {
    type = 'cat' as const
    meow() { }
}

const a: Pet = new Dog() as any

const b: Dog = new Dog() as any

a.assert('dog')
a.bark()

b.assert('dog')
b.bark()

Expected behavior:

a.assert() to narrow the type and not complain about an explicit type annotation

Actual behavior:

a.asserts is marked as implicitly typed

Playground Link:

https://www.typescriptlang.org/play/index.html?ssl=38&ssc=9&pln=3&pc=1#code/LAKFBcE8AcFMAIAKtzwLzwMIENUB94ARAewHMBuUCGBAMQEsAbcWAJwB4A1eWADxYB2AEwDOSFABp4AFR79YwsQHIhZJfAJKAxrnUA+dPG59Bo+AG94UOAC4Z8AL7wA-Efh2BsAG5sqIbABGIuCs2FqoWozYImIAQtEI5qDwKfCBwaHhVjR2Kmoa8Nq6qcnwpSnRImzg7LImCmZ5pOqaOuBKegAU4AAW9CJ2yOBS1rB20gCUdpXVYr398AsMzGzsQ1LSBhblqSn0AGbw3X0iAHSj6GgYoxPwrCgArqwCO7u9rMQA7vCe3wCirA+rE6SgExDgSgmOwcoBhYBA+weAnC9GIAjSMWq0hotTkpmUqmaBSK7S6XkGkmythkUwxVVY4DEXkWYmWLA46xkWySIF2ByOXnONEu1xot3u4CeL15qXeXx+sH+gOIwNB4NgkNhfkilSIZDxDTiCW2MpSFwwTXU0XgWjRwR2AWwrAA1p1bpY4U5dilQDqYlhcAbFPB4lUUjy3sKLW0rWJbQJ7ab4ABbWBfN0WRxa+G+u2obAU1AYX560gZ63YASQXMJ1ABOwkUiGEuN8tiSvV+HYU4zBkgwma-ynR0ut1+AI9zF9y1QkATkeu2dAA

Related Issues:

#36931
#37241

リンクされた TypeScript Playground で、提供されている Pet、Base、Dog、Cat の例を使って報告された動作を再現します。まず、コンパイラーがアサーションメソッドの明示的な asserts this is Filter<Pet, T> アノテーションをどのように検査するかを追跡し、次にクラス型の場合と比較します。ユニオンアノテーションが一貫して処理されるか、拒否される理由を診断が明確に説明し、関連するコンパイラーテストにリグレッションカバレッジが追加されれば完了です。