Unions seem to sidestep `--noUncheckedIndexedAccess` and lose the possibility of undefined

🔎 Search Terms

noUncheckedIndexedAccess, union, undefined

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link

💻 Code

const nums: { [k: string]: number } = Math.random() < 0.5 ? { a: 1 } : { b: 2 };
const str = { a: "hello" }

// with --noUncheckedIndexedAccess on
const hmm = (Math.random() < 0.5 ? nums.a : str.a) // string | number | undefined
const wha = (Math.random() < 0.5 ? nums : str).a // string | number <-- 😕

🙁 Actual behavior

The type of wha is string | number, completely ignoring the possibility that it might be undefined, even though --noUncheckedIndexedAccess is enabled. Looks like unions of types with index signatures and known keys lose the --noUncheckedIndexedAccess behavior.

🙂 Expected behavior

wha should be of type string | number | undefined, just like hmm, since indexed access into a union should look like a union of indexed accesses.

Additional information about the issue

This is related to #50474. It’s also related to #47531, which had specifically to do with never[] and so the focus was on avoiding never[] as opposed to investigating what happened to the indexed access. ( https://github.com/microsoft/TypeScript/issues/47531#issuecomment-1229522810 )

Ran into this when looking at a Stack Overflow question

Beginne mit der verknüpften TypeScript Playground-Reproduktion unter Verwendung von --noUncheckedIndexedAccess und vergleiche die beiden Indexed-Access-Ausdrücke im Issue. Das Issue nennt keine Repository-Datei und keinen Test; verfolge, wie Indexed Access für Unions aufgelöst wird, füge dann eine Abdeckung hinzu, die zeigt, dass der zweite Ausdruck undefined einschließt, und führe die relevanten Compiler-Tests aus.