fix(reactivity): trigger reactivity for Map key `undefined` (#12055)

close #12054

packages/reactivity/__tests__/reactive.spec.ts

@@ -409,4 +409,14 @@ describe('reactivity/reactive', () => {
     e.effect.stop()
     expect(targetMap.get(obj)?.get('x')).toBeFalsy()
   })
+
+  test('should trigger reactivity when Map key is undefined', () => {
+    const map = reactive(new Map())
+    const c = computed(() => map.get(void 0))
+
+    expect(c.value).toBe(void 0)
+
+    map.set(void 0, 1)
+    expect(c.value).toBe(1)
+  })
 })

packages/reactivity/src/dep.ts

@@ -340,7 +340,7 @@ export function trigger(
       })
     } else {
       // schedule runs for SET | ADD | DELETE
-      if (key !== void 0) {
+      if (key !== void 0 || depsMap.has(void 0)) {
         run(depsMap.get(key))
       }