fix(reactivity): ensure the first parameter in the reduce callback is reactive

packages/reactivity/__tests__/reactiveArray.spec.ts

@@ -623,7 +623,7 @@ describe('reactivity/reactive/Array', () => {
       expect(left.value).toBe(shallow[0])
       expect(right.value).toBe(shallow[0])
 
-      const deep = reactive([{ val: 1 }, { val: 2 }])
+      let deep = reactive([{ val: 1 }, { val: 2 }])
       left = computed(() => deep.reduce((acc, x) => acc + x.val, '0'))
       right = computed(() => deep.reduceRight((acc, x) => acc + x.val, '3'))
       expect(left.value).toBe('012')
@@ -632,6 +632,25 @@ describe('reactivity/reactive/Array', () => {
       deep[1].val = 23
       expect(left.value).toBe('0123')
       expect(right.value).toBe('3231')
+
+      deep = reactive([{ val: 1 }, { val: 2 }])
+      const maxBy = (prev: any, cur: any) => {
+        expect(isReactive(prev)).toBe(true)
+        expect(isReactive(cur)).toBe(true)
+        return prev.val > cur.val ? prev : cur
+      }
+      left = computed(() => deep.reduce(maxBy))
+      right = computed(() => deep.reduceRight(maxBy))
+      expect(left.value).toMatchObject({ val: 2 })
+      expect(right.value).toMatchObject({ val: 2 })
+
+      deep[0].val = 23
+      expect(left.value).toMatchObject({ val: 23 })
+      expect(right.value).toMatchObject({ val: 23 })
+
+      deep[1].val = 24
+      expect(left.value).toMatchObject({ val: 24 })
+      expect(right.value).toMatchObject({ val: 24 })
     })
 
     test('some', () => {

packages/reactivity/src/arrayInstrumentations.ts

@@ -280,7 +280,12 @@ function reduce(
   let wrappedFn = fn
   if (arr !== self) {
     if (!isShallow(self)) {
+      let needReactive = args.length === 0
       wrappedFn = function (this: unknown, acc, item, index) {
+        if (needReactive) {
+          needReactive = false
+          acc = toReactive(acc)
+        }
         return fn.call(this, acc, toReactive(item), index, self)
       }
     } else if (fn.length > 3) {