warn against non-primitive key (#5816)

src/core/vdom/create-element.js

@@ -61,6 +61,16 @@ export function _createElement (
     // in case of component :is set to falsy value
     return createEmptyVNode()
   }
+  // warn against non-primitive key
+  if (process.env.NODE_ENV !== 'production' &&
+    isDef(data) && isDef(data.key) && !isPrimitive(data.key)
+  ) {
+    warn(
+      'Avoid using non-primitive value as key, ' +
+      'use string/number value instead.',
+      context
+    )
+  }
   // support single function children as default scoped slot
   if (Array.isArray(children) &&
     typeof children[0] === 'function'

test/unit/modules/vdom/create-element.spec.js

@@ -153,6 +153,15 @@ describe('create-element', () => {
     expect('Avoid using observed data object as vnode data').toHaveBeenWarned()
   })
 
+  it('warn non-primitive key', () => {
+    new Vue({
+      render (h) {
+        return h('div', { key: {}})
+      }
+    }).$mount()
+    expect('Avoid using non-primitive value as key').toHaveBeenWarned()
+  })
+
   it('nested child elements should be updated correctly', done => {
     const vm = new Vue({
       data: { n: 1 },