Destructuring object with generic key should reject undefined

🔎 Search Terms

destructure, generic key, undefined

🕗 Version & Regression Information

This is the behavior in every version I tried (back to at least 5.0), and I reviewed the FAQ for entries about destructuring

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.0-dev.20241002&ssl=7&ssc=1&pln=7&pc=2#code/JYOwLgpgTgZghgYwgAgCrIN7LgfgFzICuIA1iAPYDuIA3MgEb5GkXV0JPFlUjIC+AKAExiCMMHK8YAHgAyyCAA9IIACYBnZCQgBPcjDQA+ABQkCsgDTIwBdAB9mqiDFARVASgJdWvDAOTICJLqYJjIANokALoEAG78yAC81sgA9KnI6gAW5IQANqoKUFDkUMjGCFBw2RCawAahxE4uIG7u-oHBoQD6sQBMSdaRUWkZEACOhMCxcHkQ4J1OVqrktUUlUB1QEGCEULyxAnxAA

💻 Code

interface T { a?: unknown; b?: unknown; c?: unknown }

function f<L extends keyof T>(k: L, t: T | undefined): unknown {
  const { [k]: v } = t // should error (crashes if t undefined)
  const _v2 = t[k] // equivalent code, does error
  return v
}

🙁 Actual behavior

TS does not error on the destructuring, even though it will crash at runtime if t is undefined, and it errors on equivalent code using object-access syntax.

🙂 Expected behavior

TS should error on the indicated line.

Additional information about the issue

This is simplified from an actual example in our codebase. Obviously in this case it's silly to destructure but this was part of a larger block of destructuring so the code would at a glance seem reasonable.

Note that the destructured value does end up having a funky type (T | undefined)[L], which presumably it's hard to do anything with. But of course it's still assignable to unknown, and thus usable in various ways. (In the actual code in question, the authors were doing other complicated things, running an issue similar to #40312, and ended up casting through unknown to the type they wanted.)

Start with the linked Playground example and compare the checker’s handling of computed-property destructuring with the equivalent indexed access expression. Trace the generic key and possibly undefined object through type checking; done means the destructuring line receives an appropriate error without regressing valid destructuring.