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No error on unconstrained type parameter in > comparison #50603
Description
Bug Report
🔎 Search Terms
object is possibly undefined 4.8
🕗 Version & Regression Information
- This changed between versions 4.7.4 and 4.8.2
⏯ Playground Link
TS playground on 4.8.0-beta link.
(The playground doesn't seem to have 4.8.x releases, but I tested on 4.8.0-beta and nightly (4.9.0-dev.XXXXXXX).)
💻 Code
export function min<T>(it: Iterable<T>): T | null {
let result: T | null = null;
for (const v of it) {
if (result === null || result > v) {
result = v;
}
}
return result;
}error TS2532: Object is possibly 'undefined'.
if (result === null || result < v) {
~~~~~~
On that line, hovering over the first result shows T | null. Hovering over the second result shows T & ({} | undefined).
🙁 Actual behavior
I get a type error.
🙂 Expected behavior
No error.
Activity
jakebailey commentedon Sep 2, 2022
This is expected behavior in 4.8. See this section in the blog post: https://devblogs.microsoft.com/typescript/announcing-typescript-4-8/#unconstrained-generics-no-longer-assignable-to
You probably want to change your type variable to be constrained to
{}by writingT extends {}, or check for undefined.cakoose commentedon Sep 2, 2022
Ah, interesting! Where's the
{}requirement coming from, the comparison?For example, I'm confused why there's no error here:
jakebailey commentedon Sep 2, 2022
Honestly, I didn't even notice that the operator you used was
>. It does seem weird that the above compiles, yet, this does not.In that it's odd that we allow
>and<on an unconstrained type variable like this.andrewbranch commentedon Sep 2, 2022
The current behavior is clearly inconsistent. If you can’t use
undefinedin a greater than or less than check, you shouldn’t be able to use an unconstrained type parameter either.[-]New error in TS 4.8: Object is possibly 'undefined' (after null check on `T | null`)[/-][+]No error on unconstrained type parameter in `>` comparison[/+]ahejlsberg commentedon Jul 29, 2024
Note that the issue reported here also exists for the
+,-, and~unary operators. For example:With #59437, errors are reported in both cases above.