This works:

type OptionalInit = void | string

function foo(url: string, init: OptionalInit) {
    console.log(url)
}

foo("hello", "world")
foo("hello")

This works:

type lookup = {
    "a": void | string
}

function foo2(url: string, init: lookup["a"]) {
    console.log(url)
}

foo2("hello", "world")
foo2("hello")

But this doesn’t work:

type lookup = {
    "a": void | string
}

function foo3<T extends keyof lookup>(url: string, init: lookup[T]) {
    console.log(url)
}

foo3<"a">("hello", "world")
foo3<"a">("hello")

test.ts:33:1 - error TS2554: Expected 2 arguments, but got 1.

33 foo3("hello")
   ~~~~

  test.ts:28:52
    28 function foo3<T extends keyof lookup>(url: string, init: lookup[T]) {
                                                          ~~~~~~~~~~~~~~~
    An argument for 'init' was not provided.

What gives? Is this a bug with tsc?

For context, I’m trying to make a function with a conditionally optional second parameter. My idea was to just do a generics lookup to determine whether second parameter should be optional (by adding the | void:

type lookup = {
    "a": void | string,
    "b": string
}