Double implicit cast of optional function arguments allows a callback to receive the wrong types

[keeslinp]

Bug Report

🔎 Search Terms

optional argument cast callback

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about substitutability

There is a FAQ for half of this bug https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-functions-with-fewer-parameters-assignable-to-functions-that-take-more-parameters.

⏯ Playground Link

Playground link with relevant code

💻 Code

const test3 = (cb: (arg1: string) => void) => {
  cb("test")
}

const test2 = (cb: () => void) => {
  test3(cb)
}

const test = () => {
  test2((val?: number) => {
    console.log(typeof val) // Expect number but it prints string
  })
}
test()

🙁 Actual behavior

This type checks just fine and prints out "string"

🙂 Expected behavior

This should fail somehow because it is unsound. I'm not sure where it should fail and maybe this is just an accepted bug, but if that is the case then the FAQ should be updated to reflect that you can make unsound behavior by implicitly casting functions with optional parameters.

[MartinJohns]

Duplicate of #13043. Used search terms: callback optional unsound

[keeslinp]

This doesn't seem to be an issue in practice, and our appetite for breaking changes diminishes every day.

I guess there's no desire to fix this. Thanks for linking the other issue 👍

[MartinJohns]

I guess there's no desire to fix this.

As far as I understood there simple is no good way to fix this, without introducing a whole other can of issues.