From a200305b9f29458eded4e060b025a5ca98492d29 Mon Sep 17 00:00:00 2001
From: HcySunYang <HcySunYang@outlook.com>
Date: Tue, 4 Sep 2018 03:53:21 +0800
Subject: [PATCH 1/4] fix(#8730): improve error reporting at compile time

---
 src/compiler/parser/index.js              | 15 +++++++++++++--
 test/unit/modules/compiler/parser.spec.js | 11 +++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/src/compiler/parser/index.js b/src/compiler/parser/index.js
index a603b36bb55..99bc651918b 100644
--- a/src/compiler/parser/index.js
+++ b/src/compiler/parser/index.js
@@ -331,8 +331,19 @@ export function processElement (element: ASTElement, options: CompilerOptions) {
 function processKey (el) {
   const exp = getBindingAttr(el, 'key')
   if (exp) {
-    if (process.env.NODE_ENV !== 'production' && el.tag === 'template') {
-      warn(`<template> cannot be keyed. Place the key on real elements instead.`)
+    if (process.env.NODE_ENV !== 'production') {
+      if (el.tag === 'template') {
+        warn(`<template> cannot be keyed. Place the key on real elements instead.`)
+      }
+      if (el.for) {
+        const iterator = el.iterator2 || el.iterator1
+        if (iterator && iterator === exp && el.parent.tag === 'transition-group') {
+          warn(
+            `Do not use sequential index on the child of the <transtion-group> component, ` +
+            `this may cause some rendering errors.`
+          )
+        }
+      }
     }
     el.key = exp
   }
diff --git a/test/unit/modules/compiler/parser.spec.js b/test/unit/modules/compiler/parser.spec.js
index 750e371105f..41a5152d461 100644
--- a/test/unit/modules/compiler/parser.spec.js
+++ b/test/unit/modules/compiler/parser.spec.js
@@ -242,6 +242,17 @@ describe('parser', () => {
     expect('<template> cannot be keyed').toHaveBeenWarned()
   })
 
+  it('warn the child of the <transition-group> component has sequential index', () => {
+    parse(`
+      <div>
+        <transition-group>
+          <i v-for="(o, i) of arr" :key="i"></i>
+        </transition-group>
+      </div>
+    `, baseOptions)
+    expect('Do not use sequential index on the child of the <transtion-group> component').toHaveBeenWarned()
+  })
+
   it('v-pre directive', () => {
     const ast = parse('<div v-pre id="message1"><p>{{msg}}</p></div>', baseOptions)
     expect(ast.pre).toBe(true)

From 9a9094710f29f57fb0d730050b8ebe16db4bf5c9 Mon Sep 17 00:00:00 2001
From: HcySunYang <HcySunYang@outlook.com>
Date: Tue, 4 Sep 2018 03:58:24 +0800
Subject: [PATCH 2/4] fix(flow): possibly undefined

---
 src/compiler/parser/index.js | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/compiler/parser/index.js b/src/compiler/parser/index.js
index 99bc651918b..870ab61c3cd 100644
--- a/src/compiler/parser/index.js
+++ b/src/compiler/parser/index.js
@@ -337,7 +337,8 @@ function processKey (el) {
       }
       if (el.for) {
         const iterator = el.iterator2 || el.iterator1
-        if (iterator && iterator === exp && el.parent.tag === 'transition-group') {
+        const parent = el.parent
+        if (iterator && iterator === exp && parent && parent.tag === 'transition-group') {
           warn(
             `Do not use sequential index on the child of the <transtion-group> component, ` +
             `this may cause some rendering errors.`

From 69f910181d19d4eff56ccc5fb1b57cf625f498a3 Mon Sep 17 00:00:00 2001
From: Evan You <yyx990803@gmail.com>
Date: Tue, 23 Oct 2018 18:06:58 -0400
Subject: [PATCH 3/4] Update index.js

---
 src/compiler/parser/index.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/compiler/parser/index.js b/src/compiler/parser/index.js
index 870ab61c3cd..4a1bfe43309 100644
--- a/src/compiler/parser/index.js
+++ b/src/compiler/parser/index.js
@@ -340,8 +340,8 @@ function processKey (el) {
         const parent = el.parent
         if (iterator && iterator === exp && parent && parent.tag === 'transition-group') {
           warn(
-            `Do not use sequential index on the child of the <transtion-group> component, ` +
-            `this may cause some rendering errors.`
+            `Do not use v-for index as key on <transtion-group> children, ` +
+            `this is the same as not using keys.`
           )
         }
       }

From 1dd779799d72d735984b14f65247e870c549efcd Mon Sep 17 00:00:00 2001
From: Evan You <yyx990803@gmail.com>
Date: Tue, 23 Oct 2018 18:07:25 -0400
Subject: [PATCH 4/4] Update parser.spec.js

---
 test/unit/modules/compiler/parser.spec.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/unit/modules/compiler/parser.spec.js b/test/unit/modules/compiler/parser.spec.js
index 41a5152d461..7a2a32a1a43 100644
--- a/test/unit/modules/compiler/parser.spec.js
+++ b/test/unit/modules/compiler/parser.spec.js
@@ -250,7 +250,7 @@ describe('parser', () => {
         </transition-group>
       </div>
     `, baseOptions)
-    expect('Do not use sequential index on the child of the <transtion-group> component').toHaveBeenWarned()
+    expect('Do not use v-for index as key on <transtion-group> children').toHaveBeenWarned()
   })
 
   it('v-pre directive', () => {