Not all parameter assignments prevent incorrect dependent narrowings

[Andarist]

🔎 Search Terms

parameter symbol assignment binding pattern dependent narrowing variables

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20231105#code/GYVwdgxgLglg9mABFApgZygCgNoA8A0iAngLoBci2AjIQEwmIA+lAzIQCwkCUiA3gFCJEMYIkwBZAIZQAFgDoATpLAATOAFtMXHgKFCiiALyJaAbkGIAvhZFiDhhyZ0WhuF4gD0HxAD0A-BbW1vxeiMqIKLiS6gAOADYoiABuknEwKogQqXHIMtKI6jAA5jJQiAmSGVBwYYgK4LDqiSgKCnAKwkiyiVlxCRmgkLAI-KgYOGyInFymQA

💻 Code

function test([x, y]: [1, 2] | [3, 4]) {
  if (Math.random()) {
    y = 2;
  }
  if (y === 2) {
    x
    // ^? (parameter) x: 1
  }
}

// an example valid call that might lead to a runtime error in the called function
test([3, 4]);

🙁 Actual behavior

Assignment to y keeps narrowing x as if y was never mutated. Effectively the binding pattern in this parameter is treated as const variable

🙂 Expected behavior

I'd expect this assignment to prevent dependent narrowing altogether.

Additional information about the issue

Assignment to x actually prevents dependent narrowing in a scenario like this: TS playground. It's just that those assignments are tracked per symbol and not per the root declaration. I believe that the simplest fix would be to just mark the whole root declaration as reassigned and treat all contained variables as non-const based on that.