change inference behaviour in old TS versions

src/createAction.ts

@@ -1,4 +1,5 @@
 import { Action } from 'redux'
+import { IsUnknownOrNonInferrable } from './tsHelpers'
 
 /**
  * An action with a string type and an associated payload. This is the
@@ -46,7 +47,16 @@ export type ActionCreatorWithoutPayload<
 export type ActionCreatorWithPayload<
   P,
   T extends string = string
-> = WithTypeProperty<T, <PT extends P>(payload: PT) => PayloadAction<PT, T>>
+> = WithTypeProperty<
+  T,
+  IsUnknownOrNonInferrable<
+    P,
+    // TS < 3.5 infers non-inferrable types to {}, which does not take `null`. This enforces `undefined` instead.
+    <PT extends unknown>(payload: PT) => PayloadAction<PT, T>,
+    // default behaviour
+    <PT extends P>(payload: PT) => PayloadAction<PT, T>
+  >
+>
 
 /**
  * An action creator that produces actions with a `payload` attribute.

src/tsHelpers.ts

@@ -0,0 +1,36 @@
+// taken from https://github.com/joonhocho/tsdef
+// return True if T is `any`, otherwise return False
+export type IsAny<T, True, False = never> = (
+  | True
+  | False) extends (T extends never ? True : False)
+  ? True
+  : False
+
+// taken from https://github.com/joonhocho/tsdef
+// return True if T is `unknown`, otherwise return False
+export type IsUnknown<T, True, False = never> = unknown extends T
+  ? IsAny<T, False, True>
+  : False
+
+export type IsEmptyObj<T, True, False = never> = T extends any
+  ? {} extends T
+    ? IsUnknown<T, False, IsAny<T, False, True>>
+    : False
+  : never
+
+/**
+ * returns True if TS version is above 3.5, False if below.
+ * uses feature detection to detect TS version >= 3.5
+ * * versions below 3.5 will return `{}` for unresolvable interference
+ * * versions above will return `unknown`
+ * */
+export type AtLeastTS35<True, False> = IsUnknown<
+  ReturnType<<T>() => T>,
+  True,
+  False
+>
+
+export type IsUnknownOrNonInferrable<T, True, False> = AtLeastTS35<
+  IsUnknown<T, True, False>,
+  IsEmptyObj<T, True, False>
+>

type-tests/files/createSlice.typetest.ts

@@ -141,10 +141,10 @@ function expectType<T>(t: T) {
   expectType<number>(counter.actions.concatMetaStrLen('test').meta)
 
   // typings:expect-error
-  expectType<string>(counter.actions.strLen('test').payload)
+  expectType<string>(counter.actions.incrementByStrLen('test').payload)
 
   // typings:expect-error
-  expectType<string>(counter.actions.strLenMeta('test').meta)
+  expectType<string>(counter.actions.concatMetaStrLen('test').meta)
 }
 
 /*
@@ -169,3 +169,30 @@ function expectType<T>(t: T) {
     }
   })
 }
+
+
+/*
+ * Test: if no Payload Type is specified, accept any payload
+ * see https://github.com/reduxjs/redux-starter-kit/issues/165
+ */
+{
+  const initialState = {
+    name: null
+  };
+
+
+  const mySlice = createSlice({
+    initialState,
+    reducers: {
+      setName: (state, action) => {
+        state.name = action.payload;
+      }
+    }
+  });
+
+  const x = mySlice.actions.setName;
+
+  mySlice.actions.setName(null);
+  mySlice.actions.setName("asd");
+  mySlice.actions.setName(5);
+}