typescriptProblematicOptionalFix.ts

type OptionalSpread<T> =
    T extends undefined
    ? []
    : [T]

function foo<T = undefined>(...args: OptionalSpread<T>){
    const arg = args[0] // Type of: T = undefined

    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 explicit
foo<undefined>()            // OK Now is possible! Yay! 
foo<undefined>(42)          // ERROR: 0 arguments expected
foo<undefined>(undefined)   // ERROR: 0 arguments expected Even more safety! 

// undefined inferred
foo()                       // OK
foo(42)                     // OK single argument type is inferred, can't do anything about it