support function overloads #7

async function fn(): Promise<symbol>
async function fn(p: string): Promise<number>
async function fn(p: number): Promise<string>
async function fn(p?: string | number) {
  switch (typeof p) {
    case 'string':
      return Number(p)

    case 'number':
      return String(p)

    default:
      return Symbol()
  }
}

fn()     // Promise<symbol>
fn(1)    // Promise<string>
fn('1')  // Promise<number>

const wrap = oi(fn)
wrap.init()     // Expected 1 arguments, but got 0.
wrap.init(1)    // Promise<string>
wrap.init('1')  // Argument of type 'string' is not assignable to parameter of type 'number'.ts(2345)