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Covariance, contravariance and assignability #55603
Description
🔎 Search Terms
covariance
contravariance
🕗 Version & Regression Information
(Please suggest a better title 😅)
Recently Andrew Burgess posted a video related to covariance and contravariance, and also the in and out keywords
Noticed issue related to assignability while playing around with the code
- This is the behavior in every version I tried
⏯ Playground Link
💻 Code
type Job = {
enqueuedAt: Date;
transactionId: string;
name: string;
};
type PriorityJob = Job & {
priority: true;
level: 1 | 2 | 3;
};
type Queue<J> = {
enqueue: (j: J) => void;
getJob: (id: string) => J;
// enqueue(j: J): void;
// getJob(id: string): J;
};
declare const jobQueue: Queue<Job>;
declare const priorityJobQueue: Queue<PriorityJob>;
const q3: Queue<Job> = jobQueue;
const q4: Queue<Job> = priorityJobQueue;
const q5: Queue<PriorityJob> = jobQueue;
const q6: Queue<PriorityJob> = priorityJobQueue;🙁 Actual behavior
If the methods defined in the Queue type (without using in and out keywords) used the arrow function syntax, there are two of the four assignments which are errors.
But if using the normal function syntax, only one is erroring.
🙂 Expected behavior
Consistent behaviour is expected in all cases.
Additional information about the issue
No response
Activity
[-]Covariance contravariance and assignability[/-][+]Covariance, contravariance and assignability[/+]fatcerberus commentedon Sep 1, 2023
Methods are intentionally bivariant, this is mentioned in the FAQ:
https://github.com/microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs
And from the linked issue:
aswinsvijay commentedon Sep 1, 2023
Agreed. But then, in the latter case, the missing error pops back up when the generic is specified to be
in out.Does that mean those keywords override bivariance 🤔 ?
whzx5byb commentedon Sep 1, 2023
Aswin S Vijay (@aswinsvijay)
Don't rely too much on these variance annotations, as they may produce unexpected behavior when fall back to structure checking. You can see the discussion here.
andrewbranch commentedon Sep 1, 2023
Yes.
typescript-bot commentedon Sep 4, 2023
This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.