unsound TS function type annotations

index.ts

// copyright 2022 Zach Hardesty

// want to check this out in the TypeScript playground?
// visit the following link to automatically see the latest version!
// https://www.typescriptlang.org/play?jsx=0#gist/737d914974c9ae26935a5c3fcfdc0289

// TODO: write functions that mimic React for better example
// see the comment toward the bottom for the issue

type SetStateAction<S> = S | ((prevState: S) => S)
type Dispatch<A> = (value: A) => void
// function useState<S>(initialState: S | (() => S)): [S, Dispatch<SetStateAction<S>>];

type IOne = {
  a: number
}

type ITwo = {
  a: number
  b: string | undefined
}

type IThree = {
  a: number
  b: string
}

const genericFunction = <T>(input: T): T => {
  return input
}

const DEFAULT_1: IOne = { a: 1 }
const DEFAULT_2: ITwo = { a: 2, b: undefined }

// removing `<IOne>` will show the correct error
const out = genericFunction<IOne>(DEFAULT_2)

const OBJ_3: IThree = { a: 3, b: "obj 3" }
const newOut: IThree = { ...OBJ_3, ...out }

console.log("out", out)
console.log("new out", newOut)

newOut.b.substring(1, 2)