🔎 Search Terms

boolean invariant type parameter literal widen union

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.0-dev.20240824&ssl=14&ssc=130&pln=1&pc=1#code/PTAECEHsA8B4BUB8oAuBPADgU1ASwM54B2AbgIYBOuZRKxo8AsAFAvrYQwLIC8oA3i1CgA5lhQAuUAAoAlKB7J4AbiGh84qdPIAbAK5Yp8eYtAlIuACarmAXxstLWAMY7KOAGZ6izlLkhEoABGXEjaZPqGDLJSUHBIDszOAfh0QUQAjArBMNL8oBgUkBhSAAygtrLKoCCccPmFxVJEegC2QVgUFYgsyUSpwUQATLFcDUUloC3tnd3ZIdB5BRNlFVU1YADyANYsvSlpQVl8C0uNkx4RGmvVtXGw4005kDpYNN37-YcjdQ-LTyEXm9ArZeDlFo8LlccJVbmAAKIUIoUAA04L+5yklx011BU0gdDI+HwuBERDIQVeqEgv0hUkBr3eoJYQA

💻 Code

// Box<T> type is invariant in T

type Box<T> = {
  get: () => T;
  set: (value: T) => void;
};

declare function box<T>(value: T): Box<T>;

const bn1 = box({ prop: 0 }); // Box<{ prop: number }>
const bn2: Box<{ prop: number }> = box({ prop: 0 }); // Ok

const bb1 = box({ prop: false }); // Box<{ prop: boolean }>
const bb2: Box<{ prop: boolean }> = box({ prop: false }); // Error, box<{ prop: false }> not assignable to Box<{ prop: boolean }>

🙁 Actual behavior

Error on bb2 is surprising and inconsistent with lack of error on bn2.

🙂 Expected behavior

Expected the box(false) expression to have type Box<boolean> for both bb1 and bb2.

Additional information about the issue

This issue is a small variation of #48363 that was fixed by @ahejlsberg in #48380