From fb6bccdb14531b93e989c00e36ed278e0d365720 Mon Sep 17 00:00:00 2001
From: Mark Erikson <mark@isquaredsoftware.com>
Date: Tue, 16 Nov 2021 22:14:50 -0500
Subject: [PATCH] Treat empty objects as `never` to force errors

---
 src/types.ts            | 19 ++++++++++++-------
 typescript_test/test.ts | 32 ++++++++++++++++++++++++++++----
 2 files changed, 40 insertions(+), 11 deletions(-)

diff --git a/src/types.ts b/src/types.ts
index 5b20a5c90..e44504d9c 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -125,20 +125,23 @@ export type MergeParameters<
   LongestParamsArray extends readonly any[] = LongestArray<TuplifiedArrays>
 > =
   // After all that preparation work, we can actually do parameter extraction.
-  // These steps work somewhat inside out:
-  // 10) Finally, after all that, run a recursive expansion on the values to make the user-visible
+  // These steps work somewhat inside out (jump ahead to the middle):
+  // 11) Finally, after all that, run a shallow expansion on the values to make the user-visible
   //     field details more readable when viewing the selector's type in a hover box.
   ExpandItems<
-    // 9) Tuples can have field names attached, and it seems to work better to remove those
+    // 10) Tuples can have field names attached, and it seems to work better to remove those
     RemoveNames<{
       // 5) We know the longest params array has N args. Loop over the indices of that array.
       // 6) For each index, do a check to ensure that we're _only_ checking numeric indices,
       //    not any field names for array functions like `slice()`
       [index in keyof LongestParamsArray]: LongestParamsArray[index] extends LongestParamsArray[number]
-        ? // 8) Then, intersect all of the parameters for this arg together.
-          IntersectAll<
-            // 7) Since this is a _nested_ array, extract the right sub-array for this index
-            LongestParamsArray[index]
+        ? // 9) Any object types that were intersected may have had
+          IgnoreInvalidIntersections<
+            // 8) Then, intersect all of the parameters for this arg together.
+            IntersectAll<
+              // 7) Since this is a _nested_ array, extract the right sub-array for this index
+              LongestParamsArray[index]
+            >
           >
         : never
     }>
@@ -158,6 +161,8 @@ type EmptyObject = {
   [K in any]: never
 }
 
+type IgnoreInvalidIntersections<T> = T extends EmptyObject ? never : T
+
 /** Extract the parameters from all functions as a tuple */
 export type ExtractParams<T extends readonly UnknownFunction[]> = {
   [index in keyof T]: T[index] extends T[number] ? Parameters<T[index]> : never
diff --git a/typescript_test/test.ts b/typescript_test/test.ts
index b0eeb5cf5..1916c787b 100644
--- a/typescript_test/test.ts
+++ b/typescript_test/test.ts
@@ -708,13 +708,13 @@ function testDynamicArrayArgument() {
   )
 
   const s = createSelector(
-    data.map(obj => (state: {}, fld: keyof Elem) => obj[fld]),
+    data.map(obj => (state: StateA, fld: keyof Elem) => obj[fld]),
     (...vals) => vals.join(',')
   )
-  s({}, 'val1')
-  s({}, 'val2')
+  s({ a: 42 }, 'val1')
+  s({ a: 42 }, 'val2')
   // @ts-expect-error
-  s({}, 'val3')
+  s({ a: 42 }, 'val3')
 }
 
 function testStructuredSelectorTypeParams() {
@@ -1365,3 +1365,27 @@ function rtkIssue1750() {
     return null
   }
 }
+
+function handleNestedIncompatTypes() {
+  // Incompatible parameters should force errors even for nested fields.
+  // One-level-deep fields get stripped to empty objects, so they
+  // should be replaced with `never`.
+  // Deeper fields should get caught by TS.
+  // Playground: https://tsplay.dev/wg6X0W
+  const input1a = (_: StateA, param: { b: number }) => param.b
+
+  const input1b = (_: StateA, param: { b: string }) => param.b
+
+  const testSelector1 = createSelector(input1a, input1b, () => ({}))
+
+  // @ts-expect-error
+  testSelector1({ a: 42 }, { b: 99 }) // should not compile
+
+  const input2a = (_: StateA, param: { b: { c: number } }) => param.b.c
+  const input2b = (_: StateA, param: { b: { c: string } }) => param.b.c
+
+  const testSelector2 = createSelector(input2a, input2b, (c1, c2) => {})
+
+  // @ts-expect-error
+  testSelector2({ a: 42 }, { b: { c: 99 } })
+}