Microsoft/TypeScript
View on GitHubTypes that depend on constants are not narrowing correctly
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#56,417 opened on Nov 15, 2023
BugDomain: Conditional TypesHelp Wanted
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Description
🔎 Search Terms
"ternary narrowing", "ternary constant", "constant narrowing", "constant dependent"
🕗 Version & Regression Information
- This is the behavior in every version I tried (including nightly), and I reviewed the FAQ.
⏯ Playground Link
💻 Code
const BOOL = true as const
type T_True = { x: string, T: string }
type T_False = { x: string, F: string }
type T = (typeof BOOL) extends true ? T_True : T_False
function f(): T {
const varT: T_True = { x: 'varT', T: 'good' }
const varF: T_False = { x: 'varF', F: 'okay' }
return BOOL ? varT : varF
}
🙁 Actual behavior
Typechecking fails with the error Type 'T_True | T_False' is not assignable to type 'T_True'. Property 'T' is missing in type 'T_False' but required in type 'T_True'. on the return statement.
🙂 Expected behavior
Typechecking should pass, since the const BOOL = true as const should mark the false ternary case as unreachable.
I also tried making T generic with respect to BOOL, and splitting the ternary operator into an if/else, but the same error persisted regardless.
Additional information about the issue
No response