type gets widened on declaration emit when inferred in destructuring context

[h-joo]

🔎 Search Terms

declaration, widened, d.ts, destructuring

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries

⏯ Playground Link

https://www.typescriptlang.org/play?jsx=0&module=1&ts=5.1.6#code/GYVwdgxgLglg9mABMOcAUBKAXIg3gTxwEYBfPAKESsQCcBTKEGpXRQxIxE878ugDwAOcGlEQQEAZzGt8iALyIADFwXJUmANxA

💻 Code

function foo(): {y: 1} {
    return { y: 1 }
}

export const { y = 0 } = foo();

🙁 Actual behavior

y is number in d.ts files, although the language service says y is 1|0

🙂 Expected behavior

y is 1|0 in d.ts files.

Additional information about the issue

Without the = 0 assignment y properly becomes type 1 in d.ts

[MartinJohns]

Workaround: 0 as const.

[dragomirtitian]

@Andarist I found a very similar issue. In the code below x is typed in ts as 0 | 1, but is typed as number in declarations.

export const x = Math.random() ? 1 : 0 // 0 | 1

Playground

Will your fix work for this example as well ?

[Andarist]

Yes, I think so - what Jake commented on here is basically the same as your case above: https://github.com/microsoft/TypeScript/pull/55445/files#r1300377161

[dragomirtitian]

@Andarist Oh. Yeah. Forgot about that. Thanks 😅