Optional parameters based on conditional types

You can do this in 3.1 using Tuples in rest parameters and spread expressions

const foo = <T extends string | number>(
  first: T, 
  ...a: (T extends string ? [boolean] : [undefined?])
) => undefined;

foo('foo', true); // ok, as intended
foo(2, true); // not ok, as intended
foo(2, undefined); // ok, as intended
foo(2); // ok

But the better way is to use overloads.

function foo2(first: string, second: boolean) : undefined
function foo2(first: number, second?: undefined): undefined
function foo2<T>(first: T, second?: boolean): undefined{
  return undefined
}

foo2('foo', true); // ok, as intended
foo2(2, true); // not ok, as intended
foo2(2, undefined); // ok, as intended
foo2(2); // ok