Split out optional last argument (memoizeOptions) into it's own overload

.eslintrc

@@ -54,7 +54,7 @@
       }
     },
     {
-      "files": ["**/test/**/*.ts"],
+      "files": ["**/test/**/*.ts", "**/typescript_test/**/*.ts"],
       "rules": {
         "consistent-return": "off",
         "max-lines": "off",
@@ -67,7 +67,8 @@
         "@typescript-eslint/no-unused-vars": "off",
         "@typescript-eslint/camelcase": "off",
         "import/max-dependencies": "off",
-        "sonarjs/no-duplicate-string": "off"
+        "sonarjs/no-duplicate-string": "off",
+        "@typescript-eslint/no-shadow": "off"
       }
     }
   ]

src/index.ts

@@ -14,7 +14,8 @@ export type {
   EqualityFn,
   SelectorArray,
   SelectorResultArray,
-  ParametricSelector
+  ParametricSelector,
+  OutputParametricSelector
 } from './types'
 
 import {
@@ -52,7 +53,7 @@ type DropFirst<T extends unknown[]> = T extends [unknown, ...infer U]
   : never
 
 export function createSelectorCreator<
-  F extends(...args: unknown[]) => unknown,
+  F extends (...args: unknown[]) => unknown,
   MemoizeFunction extends (func: F, ...options: any[]) => F,
   MemoizeOptions extends unknown[] = DropFirst<Parameters<MemoizeFunction>>
 >(
@@ -98,7 +99,7 @@ export function createSelectorCreator<
     // we wrap it in an array so we can apply it.
     const finalMemoizeOptions = Array.isArray(memoizeOptions)
       ? memoizeOptions
-      : ([ memoizeOptions ] as MemoizeOptions)
+      : ([memoizeOptions] as MemoizeOptions)
 
     const dependencies = getDependencies(funcs)
 
@@ -160,13 +161,25 @@ interface CreateSelectorFunction<
 > {
   /** Input selectors as separate inline arguments */
   <Selectors extends SelectorArray, Result>(
-    ...items:
-      | [...Selectors, (...args: SelectorResultArray<Selectors>) => Result]
-      | [
-          ...Selectors,
-          (...args: SelectorResultArray<Selectors>) => Result,
-          CreateSelectorOptions<MemoizeOptions>
-        ]
+    ...items: [
+      ...Selectors,
+      (...args: SelectorResultArray<Selectors>) => Result
+    ]
+  ): OutputSelector<
+    Selectors,
+    Result,
+    GetParamsFromSelectors<Selectors>,
+    ((...args: SelectorResultArray<Selectors>) => Result) &
+      ReturnType<MemoizeFunction>
+  >
+
+  /** Input selectors as separate inline arguments with memoizeOptions passed */
+  <Selectors extends SelectorArray, Result>(
+    ...items: [
+      ...Selectors,
+      (...args: SelectorResultArray<Selectors>) => Result,
+      CreateSelectorOptions<MemoizeOptions>
+    ]
   ): OutputSelector<
     Selectors,
     Result,

typescript_test/test.ts

@@ -5,7 +5,8 @@ import {
   createSelectorCreator,
   createStructuredSelector,
   ParametricSelector,
-  OutputSelector
+  OutputSelector,
+  SelectorResultArray
 } from '../src/index'
 
 import microMemoize from 'micro-memoize'
@@ -162,8 +163,8 @@ function testInvalidTypeInCombinator() {
 
   // does not allow heterogeneous parameter type
   // selectors when the combinator function is typed differently
+  // @ts-expect-error
   createSelector(
-    // @ts-expect-error
     (state: { testString: string }) => state.testString,
     (state: { testNumber: number }) => state.testNumber,
     (state: { testBoolean: boolean }) => state.testBoolean,
@@ -990,3 +991,32 @@ function createSelectorConfigOptions() {
     }
   )
 }
+
+// Issue #526
+function testInputSelectorWithUndefinedReturn() {
+  type Input = { field: number | undefined }
+  type Output = string
+  type SelectorType = (input: Input) => Output
+
+  const input = ({ field }: Input) => field
+  const result = (out: number | undefined): Output => 'test'
+
+  // Make sure the selector type is honored
+  const selector: SelectorType = createSelector(
+    ({ field }: Input) => field,
+    args => 'test'
+  )
+
+  // even when memoizeOptions are passed
+  const selector2: SelectorType = createSelector(
+    ({ field }: Input) => field,
+    args => 'test',
+    { memoizeOptions: { maxSize: 42 } }
+  )
+
+  // Make sure inference of functions works...
+  const selector3: SelectorType = createSelector(input, result)
+  const selector4: SelectorType = createSelector(input, result, {
+    memoizeOptions: { maxSize: 42 }
+  })
+}