support slot default content

Author: yyx990803

Diff for: src/compiler/codegen/index.js

@@ -121,7 +121,7 @@ function genSlot (el) {
   const name = el.name
     ? `"${el.name}"`
     : (el.dynamicName || '"default"')
-  return `$slots[${name}]`
+  return `($slots[${name}] || ${genChildren(el)})`
 }
 
 function genProps (props) {

Diff for: src/runtime/instance/render.js

@@ -106,14 +106,18 @@ export function renderMixin (Vue) {
     this.$options._renderKey = key
     this.$options._renderData = data
     this.$options._renderChildren = children
-    if (children) {
-      this.$forceUpdate()
-      return
-    }
     // set props - this will trigger update if any of them changed
+    // but not guaranteed
     if (data) {
       updateProps(this, data)
     }
+    // for now, if the component has content it always updates
+    // because we don't know whether the children have changed.
+    // need to optimize in the future.
+    if (children) {
+      this.$forceUpdate()
+      return
+    }
   }
 
   Vue.prototype._render = function () {