test: add test case for type narrowing

Author: mikesposito

packages/keyring-controller/src/KeyringController.test.ts

@@ -3156,6 +3156,26 @@ describe('KeyringController', () => {
           expect(result).toBe('foo');
         });
       });
+
+      it('should narrow the return type based on the handler return type', async () => {
+        const messenger = buildKeyringControllerMessenger<
+          | KeyringControllerActions
+          | KeyringControllerWithKeyringAction<MockKeyring, string>
+          | KeyringControllerWithKeyringAction<MockKeyring, number>
+        >();
+        await withController({ messenger }, async ({ controller }) => {
+          const selector = { type: MockKeyring.type };
+          jest.spyOn(controller, 'withKeyring').mockResolvedValue('foo');
+
+          const result: number = await messenger.call(
+            'KeyringController:withKeyring',
+            selector,
+            async (_keyring: EthKeyring<Json>) => 10,
+          );
+
+          expect(result).toBe(10);
+        });
+      });
     });
 
     describe('prepareUserOperation', () => {