fix(reactivity): ensure readonly refs can be replaced with new refs in reactive objects (#5310)

fix #5307

packages/reactivity/__tests__/readonly.spec.ts

@@ -476,7 +476,7 @@ describe('reactivity/readonly', () => {
     expect(isReadonly(rr.foo)).toBe(true)
   })
 
-  test('attemptingt to write to a readonly ref nested in a reactive object should fail', () => {
+  test('attemptingt to write plain value to a readonly ref nested in a reactive object should fail', () => {
     const r = ref(false)
     const ror = readonly(r)
     const obj = reactive({ ror })
@@ -486,4 +486,15 @@ describe('reactivity/readonly', () => {
 
     expect(obj.ror).toBe(false)
   })
+  test('replacing a readonly ref nested in a reactive object with a new ref', () => {
+    const r = ref(false)
+    const ror = readonly(r)
+    const obj = reactive({ ror })
+    try {
+      obj.ror = ref(true) as unknown as boolean
+    } catch (e) {}
+
+    expect(obj.ror).toBe(true)
+    expect(toRaw(obj).ror).not.toBe(ror) // ref successfully replaced
+  })
 })