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CFA for destructured discriminated unions failed with generic constraints #50206
Description
Bug Report
🔎 Search Terms
CFA, destructure, discriminated union, generic
🕗 Version & Regression Information
- This is the behavior in every version I tried
⏯ Playground Link
Playground link with relevant code
💻 Code
Here are two examples taken from #46266. Either of them works well.
type Action =
| { kind: 'A', payload: number }
| { kind: 'B', payload: string };
function f10({ kind, payload }: Action) {
if (kind === 'A') {
payload.toFixed();
}
if (kind === 'B') {
payload.toUpperCase();
}
}
function f11(action: Action) {
const { kind, payload } = action;
if (kind === 'A') {
payload.toFixed();
}
if (kind === 'B') {
payload.toUpperCase();
}
}However, when the argument action is not a certain type, but a generic type with a constraint, only the latter works.
I'm not sure whether it is a bug but I believe these two functions should have the same behavior.
type Action =
| { kind: 'A', payload: number }
| { kind: 'B', payload: string };
function _f10<T extends Action>({ kind, payload }: T) {
if (kind === 'A') {
payload.toFixed(); // doesn't work
}
if (kind === 'B') {
payload.toUpperCase(); // doesn't work
}
}
function _f11<T extends Action>(t: T) {
const { kind, payload } = t;
if (kind === 'A') {
payload.toFixed();
}
if (kind === 'B') {
payload.toUpperCase();
}
}🙁 Actual behavior
CFA fails in _f10
🙂 Expected behavior
No error.
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