jest-haste-map: add test case for broken handling of ignore pattern

packages/jest-haste-map/src/__tests__/index.test.js

@@ -548,6 +548,36 @@ describe('HasteMap', () => {
       });
   });
 
+  it('discards the cache when configuration changes (broken)', () => {
+    return new HasteMap(defaultConfig)
+      .build()
+      .then(() => {
+        fs.readFileSync.mockClear();
+
+        // Explicitly mock that no files have changed.
+        mockChangedFiles = Object.create(null);
+
+        // Watchman would give us different clocks.
+        mockClocks = object({
+          '/fruits': 'c:fake-clock:3',
+          '/vegetables': 'c:fake-clock:4',
+        });
+
+        const config = Object.assign(
+          {},
+          defaultConfig,
+          {ignorePattern: /kiwi|pear/},
+        );
+        return new HasteMap(config)
+          .build()
+          .then(({moduleMap}) => {
+            // `getModule` should actually return `null` here, because Pear
+            // should get ignored by the pattern.
+            expect(typeof moduleMap.getModule('Pear')).toBe('string');
+          });
+      });
+  });
+
   it('ignores files that do not exist', () => {
     const watchman = require('../crawlers/watchman');
     const mockImpl = watchman.getMockImplementation();