Assertion function methods complains about missing annotation even with explicit annotation

[jquense]

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

[RyanCavanaugh]

The error is correct; Pet does not have an explicit type annotation for its assert member because its assert member is only synthesized from its union constituents. The implied suggestion is to iterate through any explicitly-declared unions to see if each of its constituents has an explicitly-annotated asserts method. This might be tricky or might be straightforward; I'm not sure.

[jquense]

My guess was the error is correct, I find it confusingly worded for this case. I read "explicit" as "has a type annotation", which this does.

The implied suggestion is to iterate through any explicitly-declared unions to see if each of its constituents has an explicitly-annotated asserts method.

That would be awesome yes.