fix(v-slot): fix scoped slot normalization combined with v-if (#12104)

src/core/vdom/helpers/normalize-scoped-slots.js

@@ -61,10 +61,10 @@ function normalizeScopedSlot(normalSlots, key, fn) {
     res = res && typeof res === 'object' && !Array.isArray(res)
       ? [res] // single vnode
       : normalizeChildren(res)
-    let vnode: VNode = res && res[0]
+    let vnode: ?VNode = res && res[0]
     return res && (
       !vnode ||
-      (vnode.isComment && !isAsyncPlaceholder(vnode)) // #9658, #10391
+      (res.length === 1 && vnode.isComment && !isAsyncPlaceholder(vnode)) // #9658, #10391
     ) ? undefined
       : res
   }

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

@@ -986,4 +986,18 @@ describe('Component slot', () => {
       expect(vm.$el.firstChild.innerHTML).toBe('<span><b>2</b></span>')
     }).then(done)
   })
+
+  // #12102
+  it('v-if inside scoped slot', () => {
+    const vm = new Vue({
+      template: `<test><template #custom><span v-if="false">a</span><span>b</span></template></test>`,
+      components: {
+        test: {
+          template: `<div><slot name="custom"/></div>`
+        }
+      }
+    }).$mount()
+
+    expect(vm.$el.innerHTML).toBe(`<!----><span>b</span>`)
+  })
 })