Fix keepLatest types

.changeset/sour-coats-shop.md

@@ -0,0 +1,9 @@
+---
+"ember-resources": patch
+---
+
+The `keepLatest` utility previously incorrectly had a `| undefined` type for the return value.
+That's been removed.
+
+`| undefined` is still a valid type if the passed value is possibly `| undefined`.
+This made the `| undefined` on `keepLatest` redundant.

ember-resources/src/util/keep-latest.ts

@@ -69,7 +69,7 @@ interface Options<T = unknown> {
  */
 export function keepLatest<Return = unknown>({ when, value: valueFn }: Options<Return>) {
   return resource(() => {
-    let previous: Return | undefined;
+    let previous: Return;
 
     return () => {
       let value = valueFn();

ember-resources/type-tests/keep-latest.test.ts

@@ -0,0 +1,12 @@
+import { expectTypeOf } from 'expect-type';
+
+import { keepLatest } from '../src/util/keep-latest';
+
+const foo: boolean = true;
+
+const value = keepLatest({
+  value: () => foo,
+  when: () => true,
+});
+
+expectTypeOf(value).toMatchTypeOf<boolean>();