default slot should use fallback content if it has only whitespace (fix #2723)

Author: yyx990803

src/compiler/resolve-slots.js

@@ -38,6 +38,14 @@ export function resolveSlots (vm, content) {
     contents[name] = extractFragment(contents[name], content)
   }
   if (content.hasChildNodes()) {
+    const nodes = content.childNodes
+    if (
+      nodes.length === 1 &&
+      nodes[0].nodeType === 3 &&
+      !nodes[0].data.trim()
+    ) {
+      return
+    }
     contents['default'] = extractFragment(content.childNodes, content)
   }
 }

test/unit/specs/directives/element/slot_spec.js

@@ -457,4 +457,23 @@ describe('Slot Distribution', function () {
     })
     expect('"slot" attribute must be static').toHaveBeenWarned()
   })
+
+  it('default slot should use fallback content if has only whitespace', () => {
+    new Vue({
+      el: el,
+      template: '<test><div slot="first">1</div> <div slot="second">2</div></test>',
+      components: {
+        test: {
+          replace: true,
+          template:
+            '<div class="wrapper">' +
+              '<slot name="first"><p>first slot</p></slot>' +
+              '<slot><p>this is the default slot</p></slot>' +
+              '<slot name="second"><p>second named slot</p></slot>' +
+            '</div>'
+        }
+      }
+    })
+    expect(el.children[0].innerHTML).toBe('<div slot="first">1</div><p>this is the default slot</p><div slot="second">2</div>')
+  })
 })