Fix computed values breaking when the store is passed to Object.freeze

vuejs/core#3024

src/impl/computed.ts

@@ -4,16 +4,25 @@ export function addComputed(instance: object, descriptors: [string, PropertyDesc
   descriptors.forEach(([key, desc]) => {
     const {get, set} = desc
     if (get) {
-      let ref = set
-          ? computed({get: get.bind(instance), set: set.bind(instance)})
-          : computed(get.bind(instance))
-
-      Object.defineProperty(instance, key, {
-        value: ref, // vue unwraps this Ref automatically when accessing it
-        writable: desc.writable,
-        enumerable: desc.enumerable,
-        configurable: true
-      })
+      // We *could* just set `value: ref` in the descriptor and Vue will automatically unwrap the ref, however that
+      // causes errors if the object is passed through `Object.freeze()`, since the underlying value is a ref, but the
+      // proxy returns a different value: https://github.com/vuejs/core/issues/3024
+      if (set) {
+        const ref = computed({get: get.bind(instance), set: set.bind(instance)})
+        Object.defineProperty(instance, key, {
+          get: () => ref.value,
+          set: (v) => ref.value = v,
+          enumerable: desc.enumerable,
+          configurable: true
+        })
+      } else {
+        const ref = computed(get.bind(instance))
+        Object.defineProperty(instance, key, {
+          get: () => ref.value,
+          enumerable: desc.enumerable,
+          configurable: true
+        })
+      }
     }
   })
 }

tests/vuestore_create.spec.ts

@@ -66,6 +66,21 @@ describe("createStore", () => {
     expect(spies.late).to.be.called.with(400, 40)
   });
 
+  it("computed properties should work after Object.freeze", async () => {
+    class Store {
+      plain = 10
+
+      get computed() {
+        return this.plain + 1
+      }
+    }
+
+    let store = createStore(new Store())
+    expect(store.computed).to.equal(11)
+    Object.freeze(store)
+    expect(() => store.computed).not.to.throw()
+  });
+
   testWatches(c => c, Object, createStore)
 
   it("watches should be created for indirect (string) references", async () => {

tests/vuestore_extends.spec.ts

@@ -40,6 +40,22 @@ describe("@VueStore + extends VueStore", () => {
     expect(spies.late).to.be.called.with(400, 40)
   });
 
+  it("computed properties should work after Object.freeze", async () => {
+    @VueStore
+    class Store extends VueStore {
+      plain = 10
+
+      get computed() {
+        return this.plain + 1
+      }
+    }
+
+    let store = new Store()
+    expect(store.computed).to.equal(11)
+    Object.freeze(store)
+    expect(() => store.computed).not.to.throw()
+  });
+
   testWatches(VueStore, VueStore, v => v)
 
   it("methods should be accessible and reactive", async () => {