fix(watch): should trigger watcher callback on triggerRef when watching ref source

fix #1736

packages/runtime-core/__tests__/apiWatch.spec.ts

@@ -12,7 +12,8 @@ import {
   ITERATE_KEY,
   DebuggerEvent,
   TrackOpTypes,
-  TriggerOpTypes
+  TriggerOpTypes,
+  triggerRef
 } from '@vue/reactivity'
 
 // reference: https://vue-composition-api-rfc.netlify.com/api.html#watch
@@ -637,4 +638,27 @@ describe('api: watch', () => {
     v.value++
     expect(calls).toBe(1)
   })
+
+  test('should force trigger on triggerRef when watching a ref', async () => {
+    const v = ref({ a: 1 })
+    let sideEffect = 0
+    watch(v, obj => {
+      sideEffect = obj.a
+    })
+
+    v.value = v.value
+    await nextTick()
+    // should not trigger
+    expect(sideEffect).toBe(0)
+
+    v.value.a++
+    await nextTick()
+    // should not trigger
+    expect(sideEffect).toBe(0)
+
+    triggerRef(v)
+    await nextTick()
+    // should trigger now
+    expect(sideEffect).toBe(2)
+  })
 })