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Yielding an assertion function call does not narrow the asserted type #55205
Description
Bug Report
🔎 Search Terms
generator, yield, assertion function
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about assertion functions and generators.
⏯ Playground Link
Playground link with relevant code
💻 Code
function * testFunc() {
type SomeType = {a: number};
let count = 0;
function assertAThing(obj: unknown): asserts obj is SomeType {
// don't care about the impl
count++;
}
const bla: unknown = {a: 3};
assertAThing(bla);
// no type error here
console.log(bla.a);
const bla2: unknown = {a: 4};
yield assertAThing(bla2);
// bla2 is unknown, but should be SomeType as asserted by the assertAThing call.
if (bla2.a !== 4) {
throw new Error("bla2.a should be 4.");
}
if (count !== 2) {
throw new Error("Count should be two.");
}
console.log("Javascript success")
}
for (const item of testFunc()) {
//
}🙁 Actual behavior
The variable bla2 is still unknown even after passing it through the assertion function assertAThing.
🙂 Expected behavior
bla2 should be narrowed to be SomeType like bla1.
Workaround
We are effectively yielding void since assertion functions cannot return anything. This means there is a simple workaround:
assertAThing(bla2);
yield;
Activity
MartinJohns commentedon Jul 30, 2023
This is not a bug as per #32695:
RyanCavanaugh commentedon Aug 1, 2023
This doesn't seem like something you would likely (ever?) actually do in real code, and making new CFA graph nodes slows down everything else.
typescript-bot commentedon Aug 4, 2023
This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.