`noUncheckedIndexAccess` behavior with for...in loops with a constant

[charles-toller]

Bug Report

Using a for...in loop with a constant key variable errors with noUncheckedIndexAccess.

🔎 Search Terms

for...in
noUncheckedIndexAccess
for(const in)

🕗 Version & Regression Information

Seen in v4.1.5 (oldest playground with noUncheckedIndexAccess) and next (v4.4.0-dev.20210723 at time of writing)

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = {[key: string]: number};
declare var a: A;
for (const key in a) {
    const value: number = a[key];
}

🙁 Actual behavior

Error:

Type 'number | undefined' is not assignable to type 'number'.
  Type 'undefined' is not assignable to type 'number'.(2322)

🙂 Expected behavior

No error, key is in object by the for...in loop and clearly isn't modified as variable is const

[RyanCavanaugh]

This is the documented behavior of the flag; see #39560. It's entirely possible to read an undefined out of this loop if mutation were to occur to the object prior to the read.

[typescript-bot]

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