Incorrect narrowing in control flow analysis when destructuring union of tuples of different lengths #55661

🔎 Search Terms

Tuple union, Tuple destructuring assignment, Array destructuring assignment, control flow analysis, non-uniform length, varying length, never

🕗 Version & Regression Information

This behaviour appears to exist since the introduction of control flow analysis on destructured data in TS 4.6, and still appears in nightly.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.2#code/GYVwdgxgLglg9mABFOcByBDATluB3ACgG0APAGkQE8BdALkSIEYLHrEAfB5xAJjc6bUAlIgDeAKERTEMYIgKVEAXhWJwAEwCmwGGE3qRE6ccQQEAZyiJzACzggANurRwoAIU0B5ANb09AN00sZUQSAG5JaQBfcSigA

💻 Code

function tooNarrow([x, y]: [1, 1] | [1, 2] | [1]) {
    if (y === undefined) {
        const shouldNotBeOk: never = x;
    }
}

🙁 Actual behavior

Outside the conditional, x has type 1 and y has type 1 | 2 | undefined, which is what I expect.

Inside the conditional, the type of x is narrowed down to never, which is not expected. The assignment type-checks.

Interestingly, we seem to need three tuples to trigger the issue. Using [1, 1] | [1] or [1, 2] | [1] results in correct behaviour.

🙂 Expected behavior

x should have type 1 inside the conditional. The assignment should then not type-check.

Additional information about the issue

Possibly relates to:

• Destructured union of tuples with non-uniform length results in diagnostic when narrowing #49626
• Type narrowing not working for unions of tuples with object literals #31613