narrowing-problem.ts

type A = number;
type B = string;

type Lists = {
  a: A[];
  b: B[];
};

export const selectList = <ListName extends keyof Lists>(
  listName: ListName
): Lists[ListName] => {
  switch (listName) {
    case "a":
      return [1, 2, 3] as Lists[ListName]; // Gives error if i don't cast
    case "b":
      return ["a", "b", "c"] as Lists[ListName]; // Gives error if i don't cast
  }
  return []; // Gives error if i don't return (which I shouldn't need to since above switch exhausts all return options)
};

This works:

type A = number;
type B = string;

type Lists = {
  a: A[];
  b: B[];
};

function selectList<E extends keyof Lists>(listName: E): Lists[E];
function selectList(
  listName: keyof Lists
): Lists[typeof listName] {
  switch (listName) {
    case "a":
      return [1, 2, 3];
    case "b":
      return ["a", "b", "c"]
  }
};

const s = selectList("a")