doesn't work return params for function

[ilchenkoArtem]

Bug Report

🔎 Search Terms

doesn't work return params for function
function return parmas doesn't work
return function params with generic type don't work

🕗 Version & Regression Information

• This is a crash
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about 'return type function type with generic'

⏯ Playground Link

Playground link with relevant code #1

Playground link with relevant code #2

💻 Code

#1

type Types = string | number;

type Obj1 = { x: 10 };
type Obj2 = { x: 20 };

type GetType<T extends Types> = T extends string 
    ? Obj1
    : T extends number 
        ? Obj2 
        : never


const fn = <T extends Types>(a: T):GetType<T> => {
    if (typeof a === 'string') {
        return {x: 10} //there should be no mistake. Should be has correct type by generic
    }

    if (typeof a === 'number') {
        return {x: 20} //there should be no mistake. Should be has correct type by generic
    }

    throw Error('test')
}

const x = fn(10)

#2

type Obj1 = { x:10 }
type Obj2 = { x: 20 }
type Obj3 = { x: 30 }

function fn(a: string): Obj1
function fn(a: number): Obj2
function fn(a: boolean): Obj3
function fn(a: unknown): Obj1 | Obj2 | Obj3 | never {
    if (typeof a === 'string') return {x: 10};
    if (typeof a === 'number') return {x: 20}
    if (typeof a === 'boolean') return {x: 20} //Must be erorr :(

    throw new Error("test")
}

const x = fn(true)

🙁 Actual behavior

#1 has error Type '{ x: 10; }' is not assignable to type 'GetType'.(2322);

#2 has not error with wrong return type

🙂 Expected behavior

#1 there should be no mistake. Should be has correct type by generic
#2 has error with wrong return type

[MartinJohns]

This is a crash

This is not a crash.

---

Number 2 is working as intended. The implementation is not checked against the overloaded signatures. It's a duplicate of #13235.

[ilchenkoArtem]

This is a crash

This is not a crash.

Number 2 is working as intended. The implementation is not checked against the overloaded signatures. It's a duplicate of #13235.

Thanks. What about the first option?

[jcalz]

That’s a duplicate of #33912 I think

[4lessandrodev]

I also have a similar error

const data = ["a", "b", "c", 1, 2, 3];

// Type '(string | number)[]' is not assignable to type 'number[]'
const result: number[] = data.filter((i) => typeof i === 'number');

console.log(result); // [1, 2, 3] 

[MartinJohns]

@4lessandrodev That's completed unrelated, and is working as intended. Your inner function is not a type guard, it's just a function returning a boolean. As a result it's not used to narrow the type. You can add an explicit type annotation to make it a type guard.

[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.