typescriptProblematicOptional.ts

function foo<T>(arg: T) {
  if (arg === undefined)
    doThis()
  else
    doThat(arg)
}

// number
foo<number>(42)            // OK
foo<number>()              // ERROR: 1 argument expected
foo<number>("bar")         // ERROR: 'bar' is not assignable 'number'.

// undefined
foo<undefined>()           // ERROR: 1 argument expected, BUT we would like it to be possible!
foo<undefined>(41)         // ERROR: '42' is not assignable to 'undefined'
foo<undefined>(undefined)  // OK