Destructured union of tuples with non-uniform length results in diagnostic when narrowing

Bug Report

🔎 Search Terms

tuple type union destructuring narrowing control flow analysis non-uniform length varying length

🕗 Version & Regression Information

• This appeared in TS 4.6 when the feature was introduced.

⏯ Playground Link

Playground link with relevant code

💻 Code

declare const [a, b, c]: ["a", number] | ["b", number, boolean];
if (a === "a") {
    // If this line is commented, the diagnostic is not thrown
    c;
} else {
    c;
}

🙁 Actual behavior

When c is accessed in the branch for ["a", number], it throws a diagnostic on the original declaration for c since the control flow analysis tries to access narrowedType[2] which does not exist on ["a", number]

🙂 Expected behavior

There should be no diagnostic thrown and c should simply be undefined (like it already is) or the diagnostic should be thrown where c is accessed rather than defined.

[RyanCavanaugh]

I don’t see how either proposed behavior is necessarily better than the current one. It’s very weird to get an error about “no element at position 2” on a bare identifier reference – if you were many lines down in the function, that would seem like a bug, or possibly extremely misleading if you wrote something like arr[c].

[Fireboltofdeath]

Tuples can’t have excess members. This is not the case with objects, so it can’t reasonably conclude that z is undefined but in the tuple case it is safe to.

playground