// I want my function to have a different return type depending on input
export function foo<T extends string|number>(
  val: T                                  // input type is a union
): T extends string ? string : number {   // output type is either string or number
  return val;
} 
// expected this to work, but received:
// Type 'T' is not assignable to type 'T extends string ? string : number'.
//  Type 'string | number' is not assignable to type 'T extends string ? string : number'.
//    Type 'string' is not assignable to type 'T extends string ? string : number'.

// capitalize a string or each element of an array of strings
function capitalize<T extends string | string[]>(
  input: T
): T extends string[] ? string[] : string {
  if (isString(input)) {
    return input[0].toUpperCase() + input.slice(1);
  } else {
	return input.map(elt => capitalize(elt));
  }
}