prevent nested thunk children from failing (vuejs#3367)

Author: chrisvfritz

src/core/vdom/helpers.js

@@ -7,12 +7,16 @@ export function normalizeChildren (
   children: any,
   ns: string | void
 ): Array<VNode> | void {
-  // invoke children thunks.
-  // components always receive their children as thunks so that they
-  // can perform the actual render inside their own dependency collection cycle.
-  if (typeof children === 'function') {
+  // Invoke children thunks. Components always receive their children
+  // as thunks so that they can perform the actual render inside their
+  // own dependency collection cycle. Also, since JSX automatically
+  // wraps component children in a thunk, we handle nested thunks to
+  // prevent situations such as <MyComponent>{ children }</MyComponent>
+  // from failing when it produces a double thunk.
+  while (typeof children === 'function') {
     children = children()
   }
+
   if (isPrimitive(children)) {
     return [createTextVNode(children)]
   }

test/unit/features/options/render.spec.js

@@ -36,4 +36,16 @@ describe('Options render', () => {
     new Vue().$mount()
     expect('Failed to mount component: template or render function not defined.').toHaveBeenWarned()
   })
+
+  // Since JSX automatically thunkifies children, this will
+  // prevent <MyComponent>{ children }</MyComponent> from
+  // failing when it produces a double thunk.
+  it('should support nested thunk children', () => {
+    const vm = new Vue({
+      render: h => h('div',
+        () => () => () => ['hello ', h('strong', 'world')]
+      )
+    }).$mount()
+    expect(vm.$el.innerHTML).toBe('hello <strong>world</strong>')
+  })
 })