Union type member order switches

[cain-wang]

We noticed TypeScript switches the order of the union type members in the error message. And we hope someone can shine a light on this. The error is for React Ref type.

The error message used to be:

"Type 'Ref<unknown>' is not assignable to type 'string | RefObject<HTMLDivElement> | ((instance: HTMLDivElement | null) => void) | null | undefined'."

Now it becomes: (RefObject and ((instance: HTMLDivElement | null) => void) switches)

Type 'Ref<unknown>' is not assignable to type 'string | ((instance: HTMLDivElement | null) => void) | RefObject<HTMLDivElement> | null | undefined

We did not change relevant code, but did add react-transition-group dependency.

What puzzles me is in @types/react, the ref type is defined as:

type Ref<T> = { bivarianceHack(instance: T | null): void }["bivarianceHack"] | RefObject<T> | null;

Why the order of the bivarianceHack and RefObject would switch all of sudden?

[AnyhowStep]

It's just a thing that happens.

#29255

#29146

[cain-wang]

Thanks a lot for the pointers! I've been searching a lot on the internet and even digging up TS code base to find an answer!!

[AnyhowStep]

One more,
#32224