fix: createAsyncThunk union return values fall back to allowing only single member

[phryneas]

This should fix #1156

[github-actions]

size-limit report 📦

Path	Size
1. entry point: @reduxjs/toolkit (cjs.production.min.js)	12.21 KB (0%)
1. entry point: @reduxjs/toolkit (esm.js)	10.21 KB (0%)
1. entry point: @reduxjs/toolkit/query (cjs.production.min.js)	20.32 KB (0%)
1. entry point: @reduxjs/toolkit/query (esm.js)	17.36 KB (0%)
1. entry point: @reduxjs/toolkit/query/react (cjs.production.min.js)	22.19 KB (0%)
1. entry point: @reduxjs/toolkit/query/react (esm.js)	19.8 KB (0%)
2. entry point: @reduxjs/toolkit (without dependencies) (cjs.production.min.js)	5.55 KB (0%)
2. entry point: @reduxjs/toolkit (without dependencies) (esm.js)	5.53 KB (0%)
2. entry point: @reduxjs/toolkit/query (without dependencies) (cjs.production.min.js)	9.2 KB (0%)
2. entry point: @reduxjs/toolkit/query (without dependencies) (esm.js)	9.58 KB (0%)
2. entry point: @reduxjs/toolkit/query/react (without dependencies) (cjs.production.min.js)	2.36 KB (0%)
2. entry point: @reduxjs/toolkit/query/react (without dependencies) (esm.js)	2.21 KB (0%)
3. createSlice (esm.js)	5.15 KB (0%)
3. createEntityAdapter (esm.js)	5.82 KB (0%)
3. configureStore (esm.js)	5.83 KB (0%)
3. createApi (esm.js)	15.65 KB (0%)
3. createApi (react) (esm.js)	18.03 KB (0%)
3. fetchBaseQuery (esm.js)	10.91 KB (0%)
3. setupListeners (esm.js)	9.78 KB (0%)
3. ApiProvider (esm.js)	16.97 KB (0%)

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[Shrugsy]

❓

For my own learning, do you know why it works with this change, but not otherwise? I'm guessing it's something to do with an interaction with IsUnknown, but my brain is turning to jelly trying to understand this

[phryneas]

https://artsy.github.io/blog/2018/11/21/conditional-types-in-typescript/ - look for this paragraph and a bit above:

All we did is wrap the meat of our logic in a flimsy tortilla of inevitability, since the outer condition A extends any will, of course, always be true

In this case, assume

type Union = 'A' | 'B'

type X = Promise<Union> // will be Promise<'A' | 'B'>
type Y = Union extends any ? Promise<Union> : never // will be Promise<'A'> | Promise<'B'>

we essentially now do it both ways to get to Promise<'A' | 'B'> | Promise<'A'> | Promise<'B'>

[Shrugsy]

aha, so a case of this then: https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types

I'll probably have to encounter this a couple more times before it sinks in to me that it occurs with boolean too (which I guess is 'just' a union of true | false.

Thanks for the explanation & link, I'll go over that article in full to brush up my knowledge

[phryneas]

Yeah, that is happens with boolean was completely new to me as well.

[bfricka]

I just have one question about this now: Isn't T extends any ? Promise<T> : never essentially the only type now? T will always extend any so this will always be Promise<T> and not T, right? I'm probably missing something.

[Shrugsy]

@bfricka it would have to look like this to fit the behaviour you describe:

type MaybePromise<T> = T extends any ? Promise<T> : T

As it is, it's still a union of three possibilities. It might make more sense if you format it a bit differently:

type MaybePromise<T> = 
  | T
  | Promise<T>
  | (T extends any ? Promise<T> : never)

So the fact that T extends any is only relevant for the third scenario. The other unions of T and Promise<T> don't care whether T extends any is true or not