false ? ... behavior is weird.

[sparecycles]

🔎 Search Terms

ternary false inference

🕗 Version & Regression Information

Looks like this has been the case forever. (since oldest playground version 3.3333̅)

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABMAFAfQFyIM5QE4xgDmAlIgN4C+A3AFAA2AplIgB5a4HGIA+i4AE0bBCjAXQgJciAI5YARnDhMAhkgC8ifCEYSpLGRESbgK+tkaIV2RJLC46tGYgD8bV8hSsyWMCHr0iL7+9HSGHqweqN5BiH4BsfGhtKbmlm6RbtE+cSGJIdRAA

💻 Code

function f(_: string) {};
let x: string | undefined;
const q: boolean = true;
const qc = false as const;

q ? x ? f(x) : null : null;
qc ? x ? f(x) : null : null; // no error as below despite qc being `const false` so it should behave the same?
false ? x ? f(x) : null : null; // error on `x` being string | undefined in impossible branch after x is checked.

🙁 Actual behavior

🙂 Expected behavior

Either disable typechecking in unreachable code or still apply narrowing (as is done in the qc ? ... line)?

Additional information about the issue

Just sharing since it surprised me a little.

[MartinJohns]

Either disable typechecking in unreachable code or still apply narrowing

#26914

And #39995 (for why line 7 doesn't behave the same way).

[typescript-bot]

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