warn on duplicate props (reduxjs#508)

Author: ynonp

src/connect/mergeProps.js

@@ -1,8 +1,24 @@
 import shallowEqual from '../utils/shallowEqual'
 import verifyPlainObject from '../utils/verifyPlainObject'
+import warning from '../utils/warning'
 
 export function defaultMergeProps(stateProps, dispatchProps, ownProps) {
-  return { ...ownProps, ...stateProps, ...dispatchProps }
+  if (process.env.NODE_ENV !== 'production') {
+    const stateKeys = Object.keys(stateProps)
+
+    for ( let key of stateKeys ) {
+      if (typeof ownProps[key] !== 'undefined') {
+        warning(false, `Duplicate key ${key} sent from both parent and state`)
+        break
+      }
+    }
+  }
+
+  return {
+    ...ownProps,
+    ...stateProps,
+    ...dispatchProps
+  }
 }
 
 export function wrapMergePropsFunc(mergeProps) {

test/components/connect.spec.js

@@ -79,6 +79,32 @@ describe('React', () => {
       expect(container.context.store).toBe(store)
     })
 
+
+    it('should warn if same key is used in state and props', () => {
+      const store = createStore(() => ({
+        abc: 'bar'
+      }))
+
+      @connect(({ abc }) => ({ abc }))
+      class Container extends Component {
+        render() {
+          return <Passthrough {...this.props} />
+        }
+      }
+
+      const errorSpy = expect.spyOn(console, 'error')
+
+      TestUtils.renderIntoDocument(
+        <ProviderMock store={store}>
+        <Container abc="buz" />
+        </ProviderMock>
+      )
+      errorSpy.destroy()
+      expect(errorSpy).toHaveBeenCalled()
+    })
+
+
+
     it('should pass state and props to the given component', () => {
       const store = createStore(() => ({
         foo: 'bar',