Satisfies does not work with const assertion

Bug Report

🔎 Search Terms

satisfies, as const, literal

🕗 Version & Regression Information

• This is a crash
• I was unable to test this on prior versions because satisfies is new in 4.9.0 beta

⏯ Playground Link

Playground Link

💻 Code

type Colors = "red" | "green" | "blue";
type RGB = readonly [red: number, green: number, blue: number];
type Palette = Readonly<Record<Colors, string| RGB>>;

const palette1 = {
    red: [255, 0, 0],
    green: "#00ff00",
blue: [0, 0, 255]   
    // Expect to pass but throws error
} satisfies Palette as const;

🙁 Actual behavior

Fails with error:

‘const’ assertions can only be applied to references to enum members, or string, number, boolean, array, or object literals.

🙂 Expected behavior

No error because it is a literal and satisfies should not change the type. Also, other type assertions work fine, so const should as well.

[fatcerberus]

That’s the part I’m stuck on. Why would it ever apply to the satisfies expressions if they are purely checks?

It wouldn’t—which is the problem here. as const applies to object/array literals, so even if satisfies T as const worked now, it wouldn’t do what you want—it would just infer the readonly types to begin with and subsequently fail the check. In order to do what you want the narrowing implied by a const assertion would need to be done as a separate step, which requires an architectural change to the compiler.

(in simpler terms { … } as const is one atomic expression potentially with a different type from { … } and breaking it apart, e.g. by putting a satisfies clause in the middle, makes no sense with the current architecture)

[steverep]

Try reversing the order of satisfies and as const. Having the satisfies clause first makes the compiler think you’re trying to apply as const to an expression instead of a bare literal, so it’s rejected for the same reason as (1 + 1) as const would be.

Yes, it works when reversed as I show in the playground link, but that just creates an unnecessary restriction to make sure everything in the satisfies type is readonly because the check will fail otherwise.

The compiler may be treating it as an expression, but it’s not, and therein lies the bug.