Inconsistent type inference on overloaded function types

🔎 Search Terms

type inference of overloaded functions

🕗 Version & Regression Information

The behaviour seemed to change between 4.0.5 and 4.1.5, but it was incorrect nonetheless.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.0.5#code/C4TwDgpgBAwghgGwQIzgYwNYB4BKA+KAXigAoAnCAZwFcFgAuKHASiIIDcB7ASwBMBuALAAoUJCgB5MMG6cAdpSJQA3lDTzgEAB4MocOSCgBfIcJHawnMsChjoAQUog5aAGLUXM+VntRtmuV5FDww5TgB3OQBtAF0AGkk-HQhAxSkvBQT8JWURKCgAegKoAGVwuDBbAAsqaGBwzigEbjkqW0bKCDqaqAAzDzzSADoRuDIAc0pGKJGh+wT4JFRMXDwY5kYuPlN8klmxyenZ+ckFxBR0bHx1zZ4BERMREX7PWTkoMDJOAFtuSm5eiAfEkAkEoCEwpFYgkJCCUmD0m9KFk8CReoxHM43B40BkfDCUaxlEYnsIXri3n0AIwkOSMOTUb7ICBkBJoRiLC4rBlMll4DZQLb3Mk4jLU2n0xnM1lQTiMRHyZFqDnnZbYHnS-m3bbPUWU3o0-YTKZ6AyxIkksyicAOJwuCRkABynGAWFcBGIrjhqSgmJc7le3havRZUAA+idg6GwxIElGyOHsgB+WxkajQRi9RCdUwiT4-P4AkBoqnMXPW8QAFSpSj9aAdztddk4vWpeFM6gUNmAVMY1aUwDTEFMQA

💻 Code

type Callback<R> = (result: R) => void;
type Options = { context: any };

export type AsyncFunction<A extends unknown[], O extends Options, R> = {
  // Swap these two lines to see the fun
  (...args: [...A, Callback<R>]): void;
  (...args: [...A, O, Callback<R>]): void;
};

function promisify<A extends unknown[], O extends Options, R>(f: AsyncFunction<A, O, R>) {}

function f1(n: number, c: Callback<number>): void;
function f1(n: number, o: Options, c: Callback<number>): void;
function f1(...args: any[]) {}

type AsyncOrNot<F> = F extends AsyncFunction<infer _A, infer _O, infer _R> ? true : false;

promisify(f1);

type T1 = AsyncOrNot<typeof f1>;
const t1: T1 = true;

🙁 Actual behavior

In version 4.0.5, either the inference in the function call or the inference in the conditional type succeeds, depending on the order in which the overloads are declared.
In all later versions up to and including 5.4.0 beta, the inference in the conditional type always fails.

🙂 Expected behavior

Type inference for the same type against the same generic type to always succeed or always fail, whether it is done for a generic function call or for evaluating the extends clause in a conditional type.

Additional information about the issue

No response

Start with the provided TypeScript Playground reproduction and compare its generic function-call inference with the conditional-type inference across TypeScript 4.0.5 and current versions. Trace the type-checking path for overloaded function types, then verify that both contexts consistently succeed or fail regardless of overload order.