microsoft / microsoft/TypeScript
C extends A but C does not extend A | B
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Domain: check: Variance Relationships
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Possible Improvement
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Description
🔎 Search Terms
union extends
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about unions and extends
⏯ Playground Link
💻 Code
type A = { a: 1, b: string} | { a: 2 , b: string};
type B = { a: 2, c: string };
type C = { a: 1 | 2, b: string };
type Works<T extends A = C> = T;
type DoesNotWork<T extends A | B = C> = T;
🙁 Actual behavior
An error tells that C does not extend A | B
🙂 Expected behavior
No error since C extends A and therefore extends A | B
Additional information about the issue
No response
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked TypeScript Playground example and reproduce the generic constraint error for DoesNotWork<T extends A | B = C>. Trace the type-checking path for union constraints and verify the change against this example, with the goal that C is accepted because it extends A.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 35/100