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

Reproduisez le comportement signalé dans le TypeScript Playground lié en utilisant l’exemple fourni avec Pet, Base, Dog et Cat. Commencez par suivre la manière dont le compilateur vérifie l’annotation explicite asserts this is Filter<Pet, T> de la méthode d’assertion, puis comparez-la au cas du type de classe. Le travail est terminé lorsque l’annotation d’union est traitée de manière cohérente ou que le diagnostic explique clairement pourquoi elle est rejetée, et qu’une couverture de régression a été ajoutée dans les tests pertinents du compilateur.