I have the following code (nb. this is a simplified version, there are more types allowed than just number or bigint, like time periods, etc.) :

type One<T> = T extends number ? 1 : T extends bigint ? 1n : never;

function one<T extends number | bigint>(arg: T): One<T> {
  switch (typeof arg) {
    case 'number':
      return 1;    // ts error : Type '1' is not assignable to type 'One<T>'
    case 'bigint':
      return 1n;   // ts error : Type '1n' is not assignable to type 'One<T>'
    default:
      throw new TypeError();
  }
}

const n = 123;
const bigN = 123n;

// But here Type '1' is indeed assignable to type 'One<T>' where T extends number :

const a: One<typeof n> = 1; // ok `const a: 1`

const b = one(n);           // ok `const b: 1`
const c = one(bigN);        // ok `const c: 1n`

TS Playground

I don’t understand why I get those errors for the return statements, while everything outside of the function works as expected. Also, if I write return 1 as One<T> or return 1 as any, the error goes away.

Is this a typescript issue ? What am I missing here ?