Fix PayloadAction inference (#138)

* Fix PayloadAction inference

By wrapping the match in a single-element tuple, it prevents the
conditional from being checked distributively, which otherwise has the
effect of converting a union into an intersection when used with
contra-variant types.

* Add a descriptive comment for the esoteric tuple

Author: Justin

src/createSlice.ts

@@ -5,8 +5,12 @@ import { createSliceSelector, createSelectorName } from './sliceSelector'
 
 /**
  * An action creator atttached to a slice.
+ *
+ * The `P` generic is wrapped with a single-element tuple to prevent the
+ * conditional from being checked distributively, thus preserving unions
+ * of contra-variant types.
  */
-export type SliceActionCreator<P> = P extends void
+export type SliceActionCreator<P> = [P] extends [void]
   ? () => PayloadAction<void>
   : (payload: P) => PayloadAction<P>
 

type-tests/files/createSlice.typetest.ts

@@ -60,12 +60,16 @@ import {
     reducers: {
       increment: state => state + 1,
       decrement: state => state - 1,
-      multiply: (state, action: PayloadAction<number>) => state * action.payload
+      multiply: (state, { payload }: PayloadAction<number | number[]>) =>
+        Array.isArray(payload)
+          ? payload.reduce((acc, val) => acc * val, state)
+          : state * payload
     }
   })
 
   counter.actions.increment()
   counter.actions.multiply(2)
+  counter.actions.multiply([2, 3, 4])
 
   // typings:expect-error
   counter.actions.multiply()