How is assignability b/w two uninstantiated generics checked?

[juhort]

Acknowledgement

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Comment

I have the following AllKeys type that returns all the keys of union type.

type AllKeys<T> = T extends unknown ? keyof T: never;

Now, I'm unsure why keyof T is not assignable to AllKeys<T>, whereas AllKeys<T> is assignable to keyof T. Shouldn't the opposite of this be true?

type AllKeys<T> = T extends unknown ? keyof T: never;

// KeysOfUnion<T> should not be assignable to keyof T
type WithKeyOfConstraint<T, K extends keyof T> = unknown;
type Test1<T> = WithKeyOfConstraint<T, AllKeys<T>>; // should error but doesn't

// keyof T should be assignable to KeysOfUnion<T>
type WithAllKeysConstraint<T, K extends AllKeys<T>> = unknown;
type Test2<T> = WithAllKeysConstraint<T, keyof T>; // shouldn't error but does

Playground

Would love to understand how is assignability b/w two uninstantiated generics checked?

[Andarist]

When it comes to this specific case you can read around the comment I touched here: 69e4dcf . In general, the containing function is responsible for a lot of the assignability rules so if you are interested in how TS breaks down through the relationships you can read about it there.

That said, this PR wasn't correct and I already have closed it 😅