Skip to content

Function parameter type inference error #59949

Closed
@lengyuxuan

Description

@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

Metadata

Metadata

Assignees

No one assigned

    Labels

    No labels
    No labels

    Type

    No type

    Projects

    No projects

    Milestone

    No milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions