Repository navigation
Regression: Indexed Access of Partial Mapped Type with Union from Generic Function is missing undefined #57487
Description
🔎 Search Terms
generic function, partial, optional, mapped type, indexed access, undefined, bug, regression
🕗 Version & Regression Information
- This is a bug
- This changed between versions 4.5.5 and 4.6.2
- This is the behavior in every version I tried (all playground versions)
⏯ Playground Link
💻 Code
type Obj = {
a: 1,
b: 2
};
const mapped: {
[K in keyof Partial<Obj>]: Obj[K]
} = {}
const resolveMapped = <K extends keyof typeof mapped>(key: K) => mapped[key].toString();
// ^-- Error expected (`mapped[key]` is possibly undefined)🙁 Actual behavior
TypeScript does not recognize mapper[key] might be undefined and emits broken code without errors
🙂 Expected behavior
Correct type inference from pre-v4.6.x
Additional information about the issue
No response
Activity
RyanCavanaugh commentedon Feb 22, 2024
Isn't the bug earlier? The type
{ [K in keyof Partial<Obj>]: Obj[K] }should be{ a: number, b: number }and the initialization from{ }should fail.3x071c commentedon Feb 22, 2024
Ryan Cavanaugh (@RyanCavanaugh) The
mappeddefinition seems to work as intended ({ a?: number, b?: number }), as it does in all minor TypeScript versions since 3.3.3 (from the playground). Kinda surprised if that actually isn't expected behavior.RyanCavanaugh commentedon Feb 22, 2024
Bisects to #47109
jcalz commentedon Feb 22, 2024
From here