Presence of export affects narrowing

[nullromo]

🔎 Search Terms

presence of export narrowing typescript typecheck type checking export statement

🕗 Version & Regression Information

• This changed between versions 5.4.5 and 5.5.3

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.3#code/MYewdgzgLgBAHgLhgBQIYCcoEtUBsA8ASgKajoAm+YArgLYBGx6ANDAN6pLTpZgDmAXwB8QmAF52AgNwAoGbmKws4mAFZZWAGYAKOAG0sAXQCU7GTHgHDAOlQqA5AAtiuXCHuyLoSLE0r9RrIA9EEWYTAAegD8MgJyITBQ6ACeMNRgALSgtLTEYNj8iY5YEDC4vMQyCcRwAA4gmDDe0Glg6C6oUMTkACrFhRIALFJAA

💻 Code

const x: Partial<Record<number, {a: string}>> = {};

let i = 5;
if(x[i]) {
  x[i].a = 'hello';
}

// try un-commenting this line
// export const unrelatedThing = 4;

🙁 Actual behavior

The type narrowing for an index type did not work unless an export was included.

🙂 Expected behavior

The export should not affect the compilation or typechecking of a file at all.

Additional information about the issue

This could possibly be a bug with just the TS playground; I did not test it on my machine.

[nullromo]

As an additional note, if you change let to const, then there is no error. The problematic behavior only occurs with let.

[jcalz]

Behavior is intentional; implemented in #57847, and see #58972

[nullromo]

Hmmm, okay. This comment summarizes it well. Adding export actually encapsulates the file, making static analysis possible.