fix(shallowReactive): align behavior with vue-next (#696)

Author: ygj6

src/reactivity/reactive.ts

@@ -141,9 +141,11 @@ function mockObserver(value: any = {}): any {
 
 export function shallowReactive<T extends object = any>(obj: T): T
 export function shallowReactive(obj: any): any {
-  if (__DEV__ && !obj) {
-    warn('"shallowReactive()" is called without provide an "object".')
-    return
+  if (!isObject(obj)) {
+    if (__DEV__) {
+      warn('"shallowReactive()" is called without provide an "object".')
+    }
+    return obj as any
   }
 
   if (

test/v3/reactivity/reactive.spec.ts

@@ -207,4 +207,34 @@ describe('reactivity/reactive', () => {
       expect(isReactive(props.n)).toBe(true)
     })
   })
+
+  test('should shallowReactive non-observable values', () => {
+    const assertValue = (value: any) => {
+      expect(shallowReactive(value)).toBe(value)
+    }
+
+    // number
+    assertValue(1)
+    // string
+    assertValue('foo')
+    // boolean
+    assertValue(false)
+    // null
+    assertValue(null)
+    // undefined
+    assertValue(undefined)
+    // symbol
+    const s = Symbol()
+    assertValue(s)
+
+    expect(warn).toBeCalledTimes(6)
+    expect(
+      warn.mock.calls.map((call) => {
+        expect(call[0]).toBe(
+          '[Vue warn]: "shallowReactive()" is called without provide an "object".'
+        )
+      })
+    )
+    warn.mockReset()
+  })
 })