联合上的错误类型断言

Question

I do not understand why the type assertion is not working in this minimum reproducible example.

Here is the Playground Link

type Letter = "A" | "B"

type Useless = {}

type Container<T> =
    Useless |
    {
        type: "container"
        value: T
    }

function transform<X extends Letter, Y extends Letter>(container: Container<X | Y>): asserts container is Container<X> {
    // custom logic
}

let container: Container<"A" | "B"> = undefined as any

// Asserts no B
transform<"A", "B">(container)

container // Container<"A" | "B">
          // Container<"A"> when Useless is commented
          // I want Container<"A">

I want container to have type Container<"A"> .

When I remove the type Useless altogether, then it works as expected.

When I replace type Useless by type Useless = { key: string } , my container has type Useless instead of Container<"A"> .

How can I obtain the type Container<"A"> ?

Answer 1

im actually not super sure why typescript has some problems here i assume the union with a generic where the T has no affect sometimes trips it up

if you split it up it works for me. i assume here that Useless always asserts?

type Letter = "A" | "B"

type Useless = {}

type Container<T> =
    {
        type: "container"
        value: T
    }

type Input<T> = Useless | Container<T>

function transform<X extends Letter, Y extends Letter>(container: Input<X | Y>): asserts container is Container<X> {
    // custom logic
}

let container: Input<"A" | "B"> = undefined as any

// Asserts no B
transform<"A", "B">(container)

container // Container<"A" | "B">
          // Container<"A"> when Useless is commented
          // I want Container<"A">

Playground