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Incorrect overload resolution in 3.2.0-rc #28567
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The error is not that we're picking the wrong overload, but rather that none of the overloads are applicable.
Here's a simplified repro:
declare function test<T>(f: (x: T | undefined) => T, enhancer: () => void): T; declare function test<T>(f: (x: T | undefined) => T, state: T): T; test(x => x, { a: 'hello' }); // Ok in 3.1, error in 3.2 test(x => x || { a: 'xxx' }, { a: 'hello' }); // Ok in both
The correct inference for
Tis{ a: string }and therefore it is an error for the arrow function to returnx({ a: string } | undefinedis not assignable to{ a: string }).The reason it succeeds in 3.1 is somewhat subtle: We do multiple passes on the overloaded signatures, initially making an inference of
{ a: string }forT. In a later pass we end up with a contra-variant inference of{ a: string }and a co-variant inference of{ a: string } | undefined. In 3.1 we'd then go with the less specific inference of{ a: string } | undefined, but in 3.2 we now prefer the more specific and more correct{ a: string }. This changed in the commit here as part of #27028.The fact that 3.1 gets it wrong becomes obvious if you comment out the first overload (which shouldn't really matter as it is not applicable). 3.1 then behaves the same as 3.2.
I see. This is likely then an error with the
reduxtype definitions -- if there is an initial state, then the reducer won't be called withundefinedbut rather with... the initial state.Reacted by Andrii DieievThis issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
- locked as resolved and limited conversation to collaborators
on Oct 21, 2025
TypeScript Version: 3.2.0-rc
Search Terms:
Overload
Code
Expected behavior:
The second overload of
createStoreshould be chosen, asinitialStoreis not aStoreEnhancerbut apreloadedState.Actual behavior:
The
StoreEnhancer, a function, overload ofcreateStoreis chosen despiteinitialStorebeing absolutely, definitely not a function, causing the type ofxto be incorrectly inferred as{} | undefined.