Function parameter type inference error

[lengyuxuan]

🔎 Search Terms

Function parameter

🕗 Version & Regression Information

• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/zh/play/?#code/KYOwrgtgBAKgngB2FA3gWAFBSgQQDSbYBCBGAvppgJYgAuwATgGYCGAxsvEgLIsKqEoAbS7AAdDgC6ALigBnWgxoBzANyCRicURlRwEAEaN15ShlpaoADQA8MKMAAe9EABM5sLQD4oAXmyivAgikiaYTGAgbLRUAPYgUBFRdg7OoO6eSF4AFBZIsjB4qS6usrYwXgCUAlhQVExQuZa+LZniONXotdgA9D1QAOoAFnBQAEROJWNQechUHiCxtPKKKlAA-IK9-dkILAwsEMD0DNWT6bIKSiDKUAA+epBGDPdQBrGxADbALCBbUGx4nIvuJPrFlE0kEVzm5KiZsBRTBhwpE2NlRBIigByLFwlFRdFaMQkKAARjhQA

💻 Code

enum Type {
 A,
 B,
}

interface TypeMap {
 [Type.A]: string;
 [Type.B]: number;
}

type X<T extends Type> = TypeMap[T];

function func<T extends Type>(type: T, extend: X<T>) {
 if (type === Type.A) {
 // Why "extend" type is not string ?
 // (parameter) extend: string | number | boolean
 console.log(type, extend);
 }
}

func(Type.A, '');
func(Type.B, 1);

🙁 Actual behavior

In the example， Why "extend" type is not string ?

🙂 Expected behavior

The "type" constrained to "Type.A"，so "extend" shuld be "string".

Additional information about the issue

No response

[MartinJohns]

Because checking what value type is does not have any relevance for what type the value of extend is.

This is a perfectly valid call: func<Type.A | Type.B>(Type.A, 1);

[lengyuxuan]

Because checking what value type is does not have any relevance for what type the value of extend is.

This is a perfectly valid call: func<Type.A | Type.B>(Type.A, 1);

How should I constrain the type of "extend" ? Do you have any suggestions ? Thank you so much !

[jcalz]

Duplicate #33014

For now, if you need this behavior you either need to rewrite in terms of #47109 and use generic indexing instead of control flow anlaysis:

function func<T extends Type>(type: T, extend: X<T>) {
 const x: { [K in Type]: (type: K, x: X<K>) => void } = {
 [Type.A](type, extend) {
 console.log(type, extend);
 // ^? (parameter) extend: string
 },
 [Type.B](type, extend) {

 }
 }
 x[type](type, extend);
}
func(Type.A, '');
func(Type.B, 1);
func(Math.random() < 0.99 ? Type.A : Type.B, 1); // still allowed, #47109 technically unsound this way

or, you need to use control flow analysis on a discriminated union and not use generics:

function func(...[type, extend]: { [T in Type]: [type: T, extend: X<T>] }[Type]) {
 if (type === Type.A) {
 console.log(type, extend);
 // ^? (parameter) extend: string
 }
}

func(Type.A, '');
func(Type.B, 1);
func(Math.random() < 0.99 ? Type.A : Type.B, 1); // error

Playground link to code

[lengyuxuan]

Duplicate #33014

For now, if you need this behavior you either need to rewrite in terms of #47109 and use generic indexing instead of control flow anlaysis:

function func<T extends Type>(type: T, extend: X<T>) {
 const x: { [K in Type]: (type: K, x: X<K>) => void } = {
 [Type.A](type, extend) {
 console.log(type, extend);
 // ^? (parameter) extend: string
 },
 [Type.B](type, extend) {

 }
 }
 x[type](type, extend);
}
func(Type.A, '');
func(Type.B, 1);
func(Math.random() < 0.99 ? Type.A : Type.B, 1); // still allowed, #47109 technically unsound this way

or, you need to use control flow analysis on a discriminated union and not use generics:

function func(...[type, extend]: { [T in Type]: [type: T, extend: X<T>] }[Type]) {
 if (type === Type.A) {
 console.log(type, extend);
 // ^? (parameter) extend: string
 }
}

func(Type.A, '');
func(Type.B, 1);
func(Math.random() < 0.99 ? Type.A : Type.B, 1); // error

Playground link to code

Good job