Increment expression of for loops don't invalid the type of variables typed as literal number values

[MichaelMitchell-at]

Bug Report

🔎 Search Terms

increment decrement for loop variable mutate const literal number invalidate invalidation stale

🕗 Version & Regression Information

This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

for (let i = 0 as const; i < 10;) {
  // error - expected
  const x: 0 = i++;
}

for (let i = 0 as const; i < 10;) {
  i++;
  // error - expected
  const x: 0 = i;
}

for (let i = 0 as const; i++ < 10;) {
  // error - expected
  const x: 0 = i;
}

for (let i = 0 as const; i < 10; i++) {
  // no error - unexpected!
  const x: 0 = i;
}

🙁 Actual behavior

No type error

🙂 Expected behavior

Type error, since this is obviously unsound.

[MartinJohns]

Duplicate of #14745.

[MichaelMitchell-at]

I'd contend that this isn't a duplicate on the grounds that even though #14745 was closed as "Won't fix", the example that it referenced was actually fixed at some point in the last five years, but the example I provided still doesn't work.

[MartinJohns]

There's also #47027, which is essentially about the same issue and is still open. The issue is that the operators that modify the unary / left-side argument can be applied on literal typed variables.

[fatcerberus]

The increment and subsequent assignment being allowed is #47027, but there seems to be a second bug here: removing i++ from the loop header makes i be treated as number inside the loop (thus all the other assignments being errors), while having it there preserves the literal type. That is… quite bizarre and if anything I’d expect it to be the other way around.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.