Typescript 4.4 breaking tuple union inference

Question

// Works fine in 4.3.5, not in 4.4.2

type ArgsTupleUnion<T> =
  | [obj: T, x: (arg: T) => void]
  | [y: (arg: number) => void];

function f<T>(...args: ArgsTupleUnion<T>) {}

f({ k: 15 }, (arg) => { // 4.3.5: arg is {k: number}, 4.4.2: arg is 'any'
  console.log(arg.k);
});

f((a: number) => {});


type ArgsTuple<T> =
  | [obj: T, x: (arg: T) => void]
  // | [y: (arg: number) => void];

function f2<T>(...args: ArgsTuple<T>) {}

f2({ k: 15 }, (arg) => { // Both 4.3.5 and 4.4.2: arg is {k: number}
  console.log(arg.k);
});

Here's the code on TS Playground. I'm not sure which change has made this stop working but it's preventing me from upgrading to 4.4. Anyone else who has run into this or has an idea for a workaround?

Answer 1

A possible work around would be to use functional overload

It seems that typescript is opting in for the more specific type number rather than tracking the number of elements in a given array.

type ArgsTuple<T> = [obj: T, x: (arg: T) => void];
type NumArrFun = [y: (arg: number) => void];

type ArgsTupleUnion<T> =
  | ArgsTuple<T>
  | NumArrFun;

function f<T>(...args: ArgsTuple<T>): void
function f<T>(...args: NumArrFun): void
function f<T>(...args: ArgsTupleUnion<T>) {}

f({ k: 15 }, (arg) => { // 4.3.5: arg is {k: number}, 4.4.2: arg is 'any'
  console.log(arg.k);
});

f((a: number) => {});

playground link