fix: don't invoke Vue getters in settter

Fixes vuejs#498

src/reactivity/reactive.ts

@@ -80,12 +80,14 @@ export function defineAccessControl(target: AnyObject, key: any, val?: any) {
     }
   }
 
+  let valIsRef = isRef(val)
+
   setupAccessControl(val)
   proxy(target, key, {
     get: function getterHandler() {
       const value = getter ? getter.call(target) : val
       // if the key is equal to RefKey, skip the unwrap logic
-      if (key !== RefKey && isRef(value)) {
+      if (key !== RefKey && valIsRef) {
         return value.value
       } else {
         return value
@@ -94,16 +96,17 @@ export function defineAccessControl(target: AnyObject, key: any, val?: any) {
     set: function setterHandler(newVal: any) {
       if (getter && !setter) return
 
-      const value = getter ? getter.call(target) : val
       // If the key is equal to RefKey, skip the unwrap logic
       // If and only if "value" is ref and "newVal" is not a ref,
       // the assignment should be proxied to "value" ref.
-      if (key !== RefKey && isRef(value) && !isRef(newVal)) {
-        value.value = newVal
+      if (key !== RefKey && valIsRef && !isRef(newVal)) {
+        val.value = newVal
       } else if (setter) {
         setter.call(target, newVal)
+        valIsRef = isRef(newVal)
       } else {
         val = newVal
+        valIsRef = isRef(newVal)
       }
       setupAccessControl(newVal)
     },