test programmatic $slot access

src/core/instance/render.js

@@ -63,6 +63,12 @@ export function renderMixin (Vue: Class<Component>) {
     let vnode = render.call(vm._renderProxy)
     // return empty vnode in case the render function errored out
     if (!(vnode instanceof VNode)) {
+      if (process.env.NODE_ENV !== 'production' && Array.isArray(vnode)) {
+        warn(
+          'Multiple root nodes returned from render function. Render function ' +
+          'should return a single root node.'
+        )
+      }
       vnode = emptyVNode
     }
     // set parent

test/unit/features/component/component-slot.spec.js

@@ -268,4 +268,48 @@ describe('Component slot', () => {
     )
     Vue.config.preserveWhitespace = false
   })
+
+  it('programmatic access to $slots', () => {
+    const vm = new Vue({
+      template: '<test><p slot="a">A</p><div>C</div><p slot="b">B</div></p></test>',
+      components: {
+        test: {
+          render () {
+            expect(this.$slots.a.length).toBe(1)
+            expect(this.$slots.a[0].tag).toBe('p')
+            expect(this.$slots.a[0].children.length).toBe(1)
+            expect(this.$slots.a[0].children[0].text).toBe('A')
+
+            expect(this.$slots.b.length).toBe(1)
+            expect(this.$slots.b[0].tag).toBe('p')
+            expect(this.$slots.b[0].children.length).toBe(1)
+            expect(this.$slots.b[0].children[0].text).toBe('B')
+
+            expect(this.$slots.default.length).toBe(1)
+            expect(this.$slots.default[0].tag).toBe('div')
+            expect(this.$slots.default[0].children.length).toBe(1)
+            expect(this.$slots.default[0].children[0].text).toBe('C')
+
+            return this.$slots.default[0]
+          }
+        }
+      }
+    }).$mount()
+    expect(vm.$el.tagName).toBe('DIV')
+    expect(vm.$el.textContent).toBe('C')
+  })
+
+  it('warn if user directly returns array', () => {
+    new Vue({
+      template: '<test><div></div></test>',
+      components: {
+        test: {
+          render () {
+            return this.$slots.default
+          }
+        }
+      }
+    }).$mount()
+    expect('Render function should return a single root node').toHaveBeenWarned()
+  })
 })