Conditional return type wrong on function, but correct on caller

[metehus]

Bug Report

I'm working on a function, that should return ResponseA when a parameter is true, or ResponseB otherwise. The return on the execution works fine, but when returning it inside the function, there's a type error, needing to use as any to fix it

🔎 Search Terms

I've tried to search about conditional returns by functions, but didn't find any regarding this issue

🕗 Version & Regression Information

Tried on 4.7.4 and the nightly version

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

⏯ Playground Link

Playground link with relevant code

💻 Code

interface ResponseA {
    fooBar: string
}

interface ResponseB {
    specialNumber: number
}


type ShouldGetFirst<R extends [unknown, unknown], SHOULD extends boolean>
    = SHOULD extends true ? R[0] : R[1]


function getInfo<B extends boolean>(format: B): ShouldGetFirst<[ResponseA, ResponseB], B> {
    if (format) {
        return {
            fooBar: 'hello world' // Type '{ fooBar: string; }' is not assignable to type 'ShouldGetFirst<[ResponseA, ResponseB], B>'
        }
    } else {
        return {
            specialNumber: 42 // Type '{ specialNumber: number; }' is not assignable to type 'ShouldGetFirst<[ResponseA, ResponseB], B>'
        }
    }
}

const a = getInfo(true) // a is ResponseA
const b = getInfo(false) // b is ResponseB

🙁 Actual behavior

The return statements are returning an not assignable error, even that, when executing the function(as seen on the end), the response types are correct.

🙂 Expected behavior

The problem should not exist, so it's not needed to use as any

[MartinJohns]

Resolving of conditional types involving unbound generic type arguments is deferred. At that point the compiler doesn't know what ShouldGetFirst<[ResponseA, ResponseB], B> resolves to, because the compiler doesn't know what type B is. Constraints are not taken into consideration. Same as #48243.

When actually calling the function the type argument is not unbound anymore, so the conditional type can be resolved.

[RyanCavanaugh]

Also #33014

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.