function overloads implemented with interface will throw error

[Singloo]

🔎 Search Terms

"function overloads", "overloads with interface"

🕗 Version & Regression Information

overloads implemented with interface behaves differently

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.0-dev.20230814#code/PTAEHUFMBsGMHsC2lQBd5oBYoCoE8AHSAZVgCcBLA1UABWgEM8BzM+AVwDsATAGiwoBnUENANQAd0gAjQRVSQAUCEmYKsTKGYUAbpGF4OY0BoadYKdJMoL+gzAzIoz3UNEiPOofEVKVqAHSKymAAmkYI7NCuqGqcANag8ABmIjQUXrFOKBJMggBcISGgoAC0oACCbvCwDKgU8JkY7p7ehCTkVDQS2E6gnPCxGcwmZqDSTgzxxWWVoASMFmgYkAAeRJTInN3ymj4d-jSCeNsMq-wuoPaOltigAKoASgAywhK7SbGQZIIz5VWCFzSeCrZagNYbChbHaxUDcCjJZLfSDbExIAgUdxkUBIursJzCFJtXydajBBCcQQ0MwAUVWDEQC0gADVHBQGNJ3KAALygABEAAkYNAMOB4GRonzFBTBPB3AERcwABS0+mM9ysygc9wASmCGQUZGSDCWADEuLBQABvRQlFVkZj5UAARh1Tr5zr5AG5baB7Y7QAAmN38wPexQAX2CEPFNBlNGSFqd5vMPL9jmYOp5AD5rb6EemHTzubzXaAnKh8V4PXyxMJ4z6SgX-cXecHy5BK2Rq2G62jKahG1g2BJ+pBRzSyGwyEq+ZxIHoyHydT6Iz7FInzPVGjiLYH-U7g+6wz7N7Bt14z-uM07Xe7PaeLRfd+Zrw7b6AAD5BrM2pupFsS1LLMKyrflPT7Bt8wAjNWx-Dsux7WsGHrRoqSHLJ4FHecJyncUlQAcnnRcCL1KNFBjMgaCtF9YEDUAoxUHBmg8btQEQcVnGBdgaC+NwzGYdgGGYSB+FgaB1ESDl4D0ERqzpBkmUEWtxX5cAHFQAjhAAOXHPkghUAB5L4yHeQRRK0TsrlQRwFFcaQ8A7DidGGLB9BQBBEGhYRLj4iRxWiEQDA4bEFiYVgOB4IISiAA

💻 Code

interface Func {
  (arg: 1): "1";
  (arg: 2): "2";
}

export const func: Func = (arg) => {
  if (arg === 1) return "1" as const;
  if (arg === 2) return "2" as const;
  throw new Error("never");
};

function func2(arg: 2): "2";
function func2(arg: 1): "1";
function func2(arg: 1 | 2) {
  if (arg === 1) return "1" as const;
  if (arg === 2) return "2" as const;
  throw new Error('never')
}

export { func2 }

🙁 Actual behavior

overload implemented with interface will cause error

Type '(arg: 1 | 2) => "1" | "2"' is not assignable to type 'Func'.
  Type '"1" | "2"' is not assignable to type '"1"'.
    Type '"2"' is not assignable to type '"1"'.ts(2322)

🙂 Expected behavior

these two implementations both should work

[MartinJohns]

Duplicate of #47669. Overloading is currently not supported for arrow functions.