Duplicate of #47669. Arrow functions do not support overloading yet.

[craigphicks]

🔎 Search Terms

is: issue: in-title: defined overload type
is:issue label:bug in-title: overload type

🕗 Version & Regression Information

Same in versions 3.75 ~ 5.3.3
Slight differences in errors before that.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=3.7.5&ssl=24&ssc=27&pln=23&pc=27#code/PTAEBMFMDMEsDtLlAewG6QE4BsUENkAXATwAdIAoE80AWWIHkAZUAXlFAG8KOOAKAB4AuAIwBKUQG4evQUIBME+dN6g5AZgnrpAXwoUAxingBnQqAC2xaHiH1mbNbZEAfeS81sAfFwohVAH5BAX5gACpkkKAA5Hy2oCKgLqDySaCerD6JyanJ6mkArvBQcIjg0aCwJqDwKOZ4JiawAObweABG2FGEKKDUUdH2TNEAdKG8ETTR2Slp+clFJQhIFVU1daANTa0dXX29-TEio+OqkwOLMMvlldW19Y0tbZ3dB5FHo4QmfPLq8ooyDgmADusEIBgAFnExJxQAYGlEREJQJhIIQCph4AlJHCESlkaj0ZiUjj4SYouoCWiMVjtKA9Ko8Dj-IgMJgADSgACq8FReEhuyiRigEDRkAMhCQIy+fAA7AAGeSysQUPRGUzmKzQdp2RgsdhxURuDxiby+fy8YIhfznGKGhJpXLpU2ZB05OaFYpXMqrO4bLZPQX7PrvQZ6k4W0C26aOj0LL2lFa3dYPbbPPY9ENTY5jSMcaOXRM3Nb3TaPHYvYOHaafb6-f4qvOqZuWq1tq3jABy-uw2DhKBFpDwhAh1UJNM2oDQeGwBUgIzl8vU8tNfGMoAABobXO4Mj5OBuVapYNAnGxWOxxCjqcSRCpeCezxf2Ipr0SscpAZVT3Fz+xPOOxLaF+TKgCykBspy7QFOYpYmBCKDAliDSgEUfICi8qr6OqZiWNYBjIv0KCnlqyAGs4xqaJksIsr0WCYCgmCnBwHY2qG9ozE6e5urMeS+imZZpkGmbVrCHESNiTjIoo0k4vamjInSOgRmA+ahpxcz8aWAYVhmbzZipzbRvIWn+uW6avFmAw5j8fwAqoIJgpC0KwmSiJUu+klufib4TsouLkukHkTkpWHjFABjYHgqLIOgWC4AQoDQEUEqwOubJNOuEIobUoCkAxLwWBQyXwKl66kduEh3sVKWEGlWIVbYMmfiVZUNdY4CGgpwGtXV5UddulEwmBYC5fRjFfo54JQngw3eUivm3qSeLyMFxL+d5lKLbSOJ6HoFBAA

💻 Code

// defined overload type
type MyOL =  {
    (x:1):1;
    (x:2):2;
    (x:3):3;
}

const myfa:MyOL = (a:1|2|3) => {
//    ~~~~
// Type '(a: 1 | 2 | 3) => 1 | 2 | 3 | undefined' is not assignable to type 'MyOL'.
//   Type '1 | 2 | 3 | undefined' is not assignable to type '1'.
//     Type 'undefined' is not assignable to type '1'.ts(2322)
    switch(a){ case 1: return 1; case 2: return 2; case 3: return 3; }
    a; // never, Unreachable code detected.ts(7027)
}
const myfb:MyOL = (a:1|2|3) => {
//    ~~~~
// Type '(a: 1 | 2 | 3) => 1 | 2 | 3 | undefined' is not assignable to type 'MyOL'.
//   Type '1 | 2 | 3 | undefined' is not assignable to type '1'.
//     Type 'undefined' is not assignable to type '1'.ts(2322)
//                ~~~~~~~~~~~~~~
// Not all code paths return a value.(7030) (on `(a:1|2|3) => {`)
    if (a === 1) return 1;
    if (a === 2) return 2;
    if (a === 3) return 3;
    a; // never, but not shown as unreachable
}

const myfc: typeof myfd = (a:1|2|3)=>{ // no error
//    ~~~~
// Type '(a: 1 | 2 | 3) => 1 | 2 | 3' is not assignable to type '{ (a: 1): 1; (a: 2): 2; (a: 3): 3; }'.
//   Type '1 | 2 | 3' is not assignable to type '1'.
//     Type '2' is not assignable to type '1'.(2322)
    switch(a){ case 1: return 1; case 2: return 2; case 3: return 3; }
}

// declared overload function version has no problem
function myfd(a:1):1;
function myfd(a:2):2;
function myfd(a:3):3;
function myfd(a:1|2|3){ // no error
    switch(a){ case 1: return 1; case 2: return 2; case 3: return 3; }
}

🙁 Actual behavior

Errors on the implementations of myfa and myfb and myfc, which are typed by a defined function overload type.

However, no errors on the declared function overload myfd.

🙂 Expected behavior

No errors at all.

Additional information about the issue

Workaround:

const myWorkaround = ((a:1|2|3) => {
    switch(a){ case 1: return 1; case 2: return 2; case 3: return 3; }
}) as any as MyOL;

Advice welcome on a better title.

[MartinJohns]

Duplicate of #47669. Arrow functions do not support overloading yet.

[craigphicks]

Closing as duplicate.