fix(readonly): align behavior with vue-next. (#765)

Author: ygj6

src/reactivity/readonly.ts

@@ -44,6 +44,9 @@ type UnwrapNestedRefs<T> = T extends Ref ? T : UnwrapRef<T>
 export function readonly<T extends object>(
   target: T
 ): DeepReadonly<UnwrapNestedRefs<T>> {
+  if (__DEV__ && !isObject(target)) {
+    warn(`value cannot be made reactive: ${String(target)}`)
+  }
   return target as any
 }
 

test/v3/reactivity/readonly.spec.ts

@@ -6,6 +6,7 @@ import {
   reactive,
   watch,
   nextTick,
+  readonly,
 } from '../../../src'
 
 const Vue = require('vue/dist/vue.common.js')
@@ -386,6 +387,9 @@ describe('reactivity/readonly', () => {
       // @ts-ignore
       shallowReadonly(25)
       expect(`value cannot be made reactive: 25`).toHaveBeenWarned()
+      // @ts-ignore
+      readonly(25)
+      expect(`value cannot be made reactive: 25`).toHaveBeenWarned()
     })
 
     // #669