Regression in 4.8 where string union type widens to string
See original GitHub issueBug Report
🔎 Search Terms
string union widening
, string union regression
, union widening
🕗 Version & Regression Information
This regression occurred in 4.8.0, and can be reproduced with the 4.8.0-beta
, and nightly (4.9.0-dev.20220904
) tag in TS Playground. This works with 4.7.4
.
- This changed between versions 4.7.4 and 4.8.0-beta
I feel like this might be related to the Improved inference from binding patterns change
⏯ Playground Link
Playground link with relevant code
To reproduce, hover over the “isWorking” variable to see that the type definition includes a union of the strings. Switch to 4.8.0-beta, and note that the type definition is now just string
.
💻 Code
interface Guard<T> {
(val: unknown): val is T;
}
type ObjectGuard<T> = {
[key in keyof T]: Guard<T[key]>;
};
function isObject(val: unknown): val is Record<string, unknown> {
return val !== undefined && val !== null && typeof val === 'object' && !Array.isArray(val);
}
function isObjectOfShape<T>(
value: unknown,
shape: ObjectGuard<T>,
): value is T {
if (!isObject(value)) {
return false;
}
let validShape = true;
for (const key in shape) {
const guard = shape[key];
if (guard && !guard(value[key])) {
return false;
}
}
return validShape;
}
function createObjectGuard<T>(guard: ObjectGuard<T>) {
return (val: unknown): val is T => isObjectOfShape(val, guard);
}
function asLiteral<T extends (string | boolean | number)[]>(...literals: T) {
return (val: unknown): val is T[number] => {
return literals.includes(val as T[number]);
};
}
// See type of `isWorking` - should include the type key as a union of strings
const isWorking = createObjectGuard({
// ^?
type: asLiteral('these', 'should', 'be', 'a', 'union'),
});
🙁 Actual behavior
String union type is widened to string
.
🙂 Expected behavior
String union type remains a union, rather than widening
Issue Analytics
- State:
- Created a year ago
- Reactions:3
- Comments:6 (3 by maintainers)
Top Results From Across the Web
why is TypeScript converting string literal union type to string ...
The type inferred for a property in an object literal is the widened literal type of the expression unless the property has a...
Read more >Handbook - Unions and Intersection Types - TypeScript
A union type describes a value that can be one of several types. We use the vertical bar ( | ) to separate...
Read more >input — Enter data from keyboard - Stata
You can input string variables by using input, but you must remember to indicate explicitly that the variables are strings by specifying the...
Read more >Type Hinting Unions in doc strings comes up as "_SpecialForm"
If you're doing the type hinting inside of a doc string, with or without params, PyCharm complains that all instances of its use...
Read more >Union type widening - Language Design - Scala Contributors
Currently, the compiler widens union types to a common supertype when ... def foo(x: String | Int) = println(x) val x = if...
Read more >Top Related Medium Post
No results found
Top Related StackOverflow Question
No results found
Troubleshoot Live Code
Lightrun enables developers to add logs, metrics and snapshots to live code - no restarts or redeploys required.
Start FreeTop Related Reddit Thread
No results found
Top Related Hackernoon Post
No results found
Top Related Tweet
No results found
Top Related Dev.to Post
No results found
Top Related Hashnode Post
No results found
Top GitHub Comments
I think the milestone should be adjusted because that ship has sailed… 😃
Thanks. I forgot to update this issue, but we decided that the fix looked a little risky to ship in a patch when weighed against the small number of reports of this issue we’ve gotten.