microsoft / microsoft/TypeScript
Types `number` and explicitly constrained `T extends unknown` shouldn't be comparable
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Description
const num = 1;
function check<T extends unknown>(x: T) {
return x === num;
}
check(num);
Expected: Error: This condition will always return 'false' since the types 'T' and '1' have no overlap.
Actual: No error.
Contrast this with the following example from #32768.
const num = 1;
function check<T>(x: T) {
return x === num;
}
check(num);
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 by reproducing the two TypeScript snippets in the issue and compare how the checker handles unconstrained T versus T extends unknown during the equality comparison. Done means the explicitly constrained example reports that T and 1 have no overlap, while the behavior in the contrast example remains correct.
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
- Mostly clear
- Newbie friendliness
- 35/100