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Type narrowing context is lost within closures #46118
Description
Bug Report
🔎 Search Terms
typescript union typed narrowing lost in closures
type narrowing closures
🕗 Version & Regression Information
Typescript versions 4.4.3 and still exists on Nightly 4.5.0-dev
⏯ Playground Link
Playground link with relevant code
💻 Code
type MyUnion = { type: 'left', propertyExistsOnLeft: string } | { type: 'right', propertyExistsOnRight: string }
let x = {} as MyUnion;
switch (x.type) {
case 'left':
const y = x.propertyExistsOnLeft; //all good here ...
console.log(() => x.propertyExistsOnLeft) //showing error "Property 'propertyExistsOnLeft' does not exist on type 'MyUnion'"
break;
}🙁 Actual behavior
Type narrowing is working as expected when following control flow, however when the variable is used inside a closures scope (arrow functions or normal functions) it reverts back to it's declared type losing all narrowing inferred from the containing scope.
🙂 Expected behavior
Type narrowing should continue to work inside the context of closures.
Activity
[-]Type narrowing lost within closures[/-][+]Type narrowing context is lost within closures[/+]MartinJohns commentedon Sep 28, 2021
And another duplicate of #9998.
ramyhhh commentedon Sep 28, 2021
Yeah, that issue looks like a master issue collecting all control flow analysis issues :)
MartinJohns commentedon Sep 28, 2021
The exact same issue you describe comes up again and again and again. I probably have seen at least 50 issues already. It's all due to the difficulties described in #9998.
fatcerberus commentedon Sep 28, 2021
More specifically, control flow analysis issues across function boundaries. Which is not an easy problem to solve.
typescript-bot commentedon Oct 3, 2021
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.