WIP: Improve type narrowing for .includes

src/tests/array-includes.ts

@@ -59,6 +59,47 @@ doNotExecute(async () => {
   }
 });
 
+doNotExecute(async () => {
+  /**
+   * type narrowing for numbers in readonly array
+   */
+  let arr = [1, 2, 3] as const;
+
+  let member = 1 as 1 | 2 | 4;
+  if (arr.includes(member)) {
+    type tests = [Expect<Equal<typeof member, 1 | 2>>];
+  }
+});
+
+doNotExecute(async () => {
+  /**
+   * type narrowing for objects in readonly array
+   */
+  const arr = [{ a: 1 }, { a: 2 }, { a: 3 }] as const;
+
+  arr.includes(
+    // @ts-expect-error
+    4,
+  );
+
+  let member = { a: 1 } as const;
+  if (arr.includes(member)) {
+    type tests = [Expect<Equal<typeof member, { a: 1 }>>];
+  }
+});
+
+doNotExecute(async () => {
+  /**
+   * type narrowing for objects in writable array
+   */
+  let arr: Array<1 | 2 | 3> = [1, 2, 3];
+
+  let member = 1 as 1 | 2 | 4;
+  if (arr.includes(member)) {
+    type tests = [Expect<Equal<typeof member, 1 | 2>>];
+  }
+});
+
 doNotExecute(async () => {
   const arr: Array<"1" | "2" | "3"> = ["1", "2", "3"];
 
@@ -73,3 +114,12 @@ doNotExecute(async () => {
     true,
   );
 });
+
+doNotExecute(async () => {
+  const arr: Array<1 | 2 | 3> = [1, 2, 3];
+
+  let member = 4 as 3 | 4;
+  if (arr.includes(member)) {
+    type tests = [Expect<Equal<typeof member, 3 | 4>>];
+  }
+});