Typescript loses constraint of satisfies on initial value/accumulator for Array.prototype.reduce

[DesignByOnyx]

🔎 Search Terms

• satisfies
• reducer
• nested array

🕗 Version & Regression Information

• v4.9.4 - I assume this affects >=4.9.0 when the satisfies operator was introduced

💻 Repo

type ParseResult = { errors: Error[] }

'foob@r'.split(' ').reduce((acc, part) => {
    if (/^\w+$/.test(part)) {
        acc.errors.push(new Error('Not a valid term'));
        //              ^^^^ Argument of type 'Error' is not assignable to parameter of type 'never'.
    }
    return acc;
}, { errors: [] } satisfies ParseResult)

Playground

🙁 Actual behavior

The accumulator loses some of that information from the satisfies constraint, especially as it pertains to arrays. Typescript incorrectly thinks the errors array is type never[].

🙂 Expected behavior

The errors array should have the same type as asserted with the satisfies operator. As you can see in the playground, you can use the as operator, but you lose all of the benefits that satisfies provides. In my head, the satisfies operator is the same as as, but with actual type checking happening on the operands.

[DesignByOnyx]

I had too narrow of a search before I opened this. I see the duplicate now that I have expanded my search (#51853). Apologies for the noise.

[fatcerberus]

@DesignByOnyx Your issue got auto-labeled as a dupe because you probably used the template for 9998 dupes (“types not correct in/with callback”, which has to do with type guards and is totally unrelated), but it’s good that you found the actual dupe anyway. 😄