Move Equals constraint into an intersection type.

[DanielRosenwasser]

In microsoft/TypeScript#57117, we noticed that react-redux's build is impacted by a change in in microsoft/TypeScript#56515. What this means is that TypeScript 5.4, you'll get an error in expectExactType.

export function expectExactType<T>(t: T) {
  return <U extends Equals<T, U>>(u: U) => {}
//        ~
// error TS2313: Type parameter 'U' has a circular constraint.
}

The solution I would suggest for now is to just make U extend T and move the constraint to an intersection wherever you use U.

-  return <U extends Equals<T, U>>(u: U) => {}
+  return <U extends T>(u: U & Equals<T, U>) => {}

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[timdorr]

Thanks, Daniel! This is only in our test suite, so it shouldn't be a user-facing issue, but I very much appreciate the attention to detail on your end and the PR!

[EskiMojo14]

React Redux and Reselect don't use expectExactType properly, as noted in microsoft/TypeScript#57117 (comment). Their usage is equivalent to expectType (and should probably be changed to it).

RTK on the other hand does, and this suggestion seems to break the expected behaviour of the utility. As per reduxjs/redux-toolkit@1ae8a62 it appears some errors we want to be raised are no longer raised.

Here's a quick reproduction: https://tsplay.dev/NVePlw

[DanielRosenwasser]

@EskiMojo14 looks like this is from use of any in the conditional type. It seems like using unknown fixes the issue.

Playground link

Let me know if that works!

[EskiMojo14]

that seems to do the trick, thanks!

in the slightly more long term I think we'll switch to vitest's type test utilities, so hopefully those will continue to work :)