as expression and array of union types

[agentcooper]

I'm not 100% sure this is a bug, but the behavior is not clear to me.

As a part of the state object I have an array that I want to be inferred as an array of the union type. I know this can be done by providing a type for the whole state (const state: State = ...), but I would like to rely on type inference since there are many other state properties of primitive types.

type Fruit = "banana" | "apple";

// option 1

const state1 = {
    // OK: works as expected
    fruits: (["banana", "apple"] as Array<Fruit>),
    expression: 1 + 2
};

// option 2

const state2_1 = {
    // OK: error is expected
    fruits: [1, 2, 3] as Array<Fruit>,
    expression: 1 + 2
}

const state2_2 = {
    // NOT OK: error is expected, but not shown!
    fruits: ["banana", "apple", "potato"] as Array<Fruit>,
    expression: 1 + 2
}

// option 3

function inferAsUnionArray<K, T extends K = K>(input: Array<T>) {
    const out: Array<K> = input;
    return out;
}

const state3 = {
    // OK: error as expected. can this be done easier?
    fruits: inferAsUnionArray<Fruit>(["banana", "apple", "potato"]),
    expression: 1 + 2
};

Playground link.

[AnyhowStep]

There is no "static assert" in the C++ sense in TS. Although I wish there was...

I use a similar "hack" as your state3, sometimes

[weswigham]

Casts allow any types that possibly overlap to be converted. What you want is indeed a kind of type assertion expression. The function call is currently the best safe way to do it, IMO. We have an open issue tracking related suggestions: #26064 I'd chime in there.

[typescript-bot]

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