fix(vuejs#7913): Prevent erroneous warning when using <slot> inside slot-scope

Because slotNodes inside a slot-scope context are already set to _rendered = true after initial
render, the warning for duplicate slot presence always fires when a slot-scope prop change triggers
a re-render. With this change, the compiler tracks whether any slot-scoped elements have been
encountered at the point the slot is compiled. If so, the direct ancestors of the slot are checked
for slot-scope presence, and if found, the warning is supressed. This is admittedly not a perfect
solution, as within a slot-scope context the warning now does not fire even when there _are_
duplicate slots, but I couldn't find a good way to get around that.

fix vuejs#7913

Author: mpbarlow

src/compiler/codegen/index.js

@@ -18,6 +18,7 @@ export class CodegenState {
   maybeComponent: (el: ASTElement) => boolean;
   onceId: number;
   staticRenderFns: Array<string>;
+  slotScopeinAst: boolean;
 
   constructor (options: CompilerOptions) {
     this.options = options
@@ -29,6 +30,8 @@ export class CodegenState {
     this.maybeComponent = (el: ASTElement) => !isReservedTag(el.tag)
     this.onceId = 0
     this.staticRenderFns = []
+    // we can skip checks for slot-scope parents if we haven't seen any
+    this.slotScopeinAst = false
   }
 }
 
@@ -50,6 +53,7 @@ export function generate (
 }
 
 export function genElement (el: ASTElement, state: CodegenState): string {
+  if (el.slotScope) state.slotScopeinAst = true
   if (el.staticRoot && !el.staticProcessed) {
     return genStatic(el, state)
   } else if (el.once && !el.onceProcessed) {
@@ -456,19 +460,18 @@ export function genComment (comment: ASTText): string {
 function genSlot (el: ASTElement, state: CodegenState): string {
   const slotName = el.slotName || '"default"'
   const children = genChildren(el, state)
-  let res = `_t(${slotName}${children ? `,${children}` : ''}`
   const attrs = el.attrs && `{${el.attrs.map(a => `${camelize(a.name)}:${a.value}`).join(',')}}`
   const bind = el.attrsMap['v-bind']
-  if ((attrs || bind) && !children) {
-    res += `,null`
-  }
-  if (attrs) {
-    res += `,${attrs}`
-  }
-  if (bind) {
-    res += `${attrs ? '' : ',null'},${bind}`
+  let inScopedSlot = false
+  let ancestor = el
+  if (process.env.NODE_ENV !== 'production' && state.slotScopeinAst) {
+    while (!inScopedSlot && ancestor.parent) {
+      if (ancestor.slotScope) inScopedSlot = true
+      ancestor = ancestor.parent
+    }
   }
-  return res + ')'
+  return `_t(${slotName},${children || 'null'},${attrs || 'null'},` +
+    `${bind || 'null'},${inScopedSlot ? 'true' : 'false'})`
 }
 
 // componentName is el.component, take it as argument to shun flow's pessimistic refinement

src/core/instance/render-helpers/render-slot.js

@@ -9,7 +9,8 @@ export function renderSlot (
   name: string,
   fallback: ?Array<VNode>,
   props: ?Object,
-  bindObject: ?Object
+  bindObject: ?Object,
+  inSlotScope: ?boolean
 ): ?Array<VNode> {
   const scopedSlotFn = this.$scopedSlots[name]
   let nodes
@@ -29,7 +30,7 @@ export function renderSlot (
     const slotNodes = this.$slots[name]
     // warn duplicate slot usage
     if (slotNodes) {
-      if (process.env.NODE_ENV !== 'production' && slotNodes._rendered) {
+      if (process.env.NODE_ENV !== 'production' && !inSlotScope && slotNodes._rendered) {
         warn(
           `Duplicate presence of slot "${name}" found in the same render tree ` +
           `- this will likely cause render errors.`,

test/unit/modules/compiler/codegen.spec.js

@@ -161,21 +161,21 @@ describe('codegen', () => {
   it('generate single slot', () => {
     assertCodegen(
       '<div><slot></slot></div>',
-      `with(this){return _c('div',[_t("default")],2)}`
+      `with(this){return _c('div',[_t("default",null,null,null,false)],2)}`
     )
   })
 
   it('generate named slot', () => {
     assertCodegen(
       '<div><slot name="one"></slot></div>',
-      `with(this){return _c('div',[_t("one")],2)}`
+      `with(this){return _c('div',[_t("one",null,null,null,false)],2)}`
     )
   })
 
   it('generate slot fallback content', () => {
     assertCodegen(
       '<div><slot><div>hi</div></slot></div>',
-      `with(this){return _c('div',[_t("default",[_c('div',[_v("hi")])])],2)}`
+      `with(this){return _c('div',[_t("default",[_c('div',[_v("hi")])],null,null,false)],2)}`
     )
   })