Negative cases of getNarrowedType that match the exact type should be filtered out, even when generic #45205
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… filtered out, even when generic
typescript-bot
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Author: Team
For Milestone Bug
This was referenced Aug 18, 2021
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Fixes #44404
The reported regression is caused by the changes in #43763 - in breaking apart constraints in negative cases, we made it so
First(a generic) could never removeFirst(itself), sinceFirstis constrained to{foo: string}(yet obviously isn't related to the generic subtype in question). With this change to that, we do not perform the constraint check if the candidate exactly matches - this is easily foiled (by subtypes and intersections).See, the thing is - after some investigation, I've decided this is a kind of hacky fix layered on a hacky fix.
#43763 is ultimately doing the best it can to work around the fact that the subtype comparison result for
Slice[A][B]andExtract<Slice[A][B]. whatever>is wrong - a root cause bug which #41821 fixes. I think, rather than this, we should be looking to pull in #41821 and completely revert the non-test changes in #43763 (since I can confirm the issue is also fixed by #41821). That combined change can be found here, if we think we'd like to do that, instead (the relevant diff has been merged into #41821).