chore #7070: warn properties that starts with $ or _

src/core/instance/state.js

@@ -141,7 +141,14 @@ function initData (vm: Component) {
         `Use prop default value instead.`,
         vm
       )
-    } else if (!isReserved(key)) {
+    } else if (isReserved(key)) {
+      warn(
+        `Property "${key}" will not be proxied on the instance because it ` +
+        `may conflict with Vue's internal properties. You will have to access it as ` +
+        `"vm.$data.${key}". Avoid defining component properties that start with _ or $.`,
+        vm
+      )
+    } else {
       proxy(vm, `_data`, key)
     }
   }

test/unit/features/instance/properties.spec.js

@@ -200,4 +200,13 @@ describe('Instance properties', () => {
     vm.$listeners = {}
     expect(`$listeners is readonly`).toHaveBeenWarned()
   })
+
+  it('warn data properties that start with _ or $', () => {
+    new Vue({
+      data: { $a: 'a', _b: 'b' },
+      template: '<div></div>'
+    }).$mount()
+    expect(`Property "$a" will not be proxied on the instance`).toHaveBeenWarned()
+    expect(`Property "_b" will not be proxied on the instance`).toHaveBeenWarned()
+  })
 })