feat: warn if both v-model and v-bind:value used on same element (#7056)

* test($compile): warn if v-model and :value used on same text input

#7048

* test($compile): make v-model and v-bind:value warning apply to all but exceptions

#7048

* test($compile): move v-model/:value conflict warner to model.js

#7048

* style: split long warning messages onto new lines

Author: Robert Pemberton

src/platforms/web/compiler/directives/model.js

@@ -21,6 +21,7 @@ export default function model (
   const modifiers = dir.modifiers
   const tag = el.tag
   const type = el.attrsMap.type
+  const attrsMap = el.attrsMap
 
   if (process.env.NODE_ENV !== 'production') {
     // inputs with type="file" are read only and setting the input's
@@ -31,6 +32,20 @@ export default function model (
         `File inputs are read only. Use a v-on:change listener instead.`
       )
     }
+
+    // warn if v-bind:value conflicts with v-model
+    if (
+      (attrsMap['v-bind:value'] || attrsMap[':value']) &&
+      type !== 'checkbox' &&
+      type !== 'radio' &&
+      tag !== 'select'
+    ) {
+      const vBindValue = attrsMap['v-bind:value'] ? 'v-bind:value' : ':value'
+      warn(
+        `${vBindValue} conflicts with v-model on the same element ` +
+        'because the latter already expands to a value binding internally'
+      )
+    }
   }
 
   if (el.component) {

test/unit/features/directives/model-text.spec.js

@@ -250,6 +250,32 @@ describe('Directive v-model text', () => {
     expect('You are binding v-model directly to a v-for iteration alias').toHaveBeenWarned()
   })
 
+  it('warn if v-model and v-bind:value conflict', () => {
+    new Vue({
+      data: {
+        test: 'foo'
+      },
+      template: '<input type="text" v-model="test" v-bind:value="test"/>'
+    }).$mount()
+    expect(
+      'v-bind:value conflicts with v-model on the same element because the latter already ' +
+      'expands to a value binding internally'
+    ).toHaveBeenWarned()
+  })
+
+  it('warn if v-model and :value conflict', () => {
+    new Vue({
+      data: {
+        test: 'foo'
+      },
+      template: '<input type="text" v-model="test" :value="test"/>'
+    }).$mount()
+    expect(
+      ':value conflicts with v-model on the same element because the latter already ' +
+      'expands to a value binding internally'
+    ).toHaveBeenWarned()
+  })
+
   if (!isAndroid) {
     it('does not trigger extra input events with single compositionend', () => {
       const spy = jasmine.createSpy()