Revert from v-slot to :slot syntax

see https://gitlab.com/gitlab-org/gitlab/merge_requests/17608#note_241493589

app/assets/javascripts/vue_shared/components/slot_switch.vue

+<script>
+/**
+ * Allows to toggle slots based on an array of slot names.
+ */
+export default {
+  name: 'SlotSwitch',
+
+  props: {
+    activeSlotNames: {
+      type: Array,
+      required: true,
+    },
+
+    tagName: {
+      type: String,
+      required: false,
+      default: 'div',
+    },
+  },
+
+  computed: {
+    allSlotNames() {
+      return Object.keys(this.$slots);
+    },
+  },
+};
+</script>
+
+<template>
+  <component :is="tagName">
+    <template v-for="slotName in allSlotNames">
+      <slot v-if="activeSlotNames.includes(slotName)" :name="slotName"></slot>
+    </template>
+  </component>
+</template>

ee/app/assets/javascripts/related_items_tree/components/related_items_tree_app.vue

@@ -5,6 +5,7 @@ import { GlLoadingIcon } from '@gitlab/ui';
 
 import { issuableTypesMap } from 'ee/related_issues/constants';
 
+import SlotSwitch from '~/vue_shared/components/slot_switch.vue';
 import AddItemForm from 'ee/related_issues/components/add_issuable_form.vue';
 import CreateEpicForm from './create_epic_form.vue';
 import CreateIssueForm from './create_issue_form.vue';
@@ -16,8 +17,15 @@ import RelatedItemsTreeBody from './related_items_tree_body.vue';
 
 import { OVERFLOW_AFTER } from '../constants';
 
+const FORM_SLOTS = {
+  addItem: 'addItem',
+  createEpic: 'createEpic',
+  createIssue: 'createIssue',
+};
+
 export default {
   OVERFLOW_AFTER,
+  FORM_SLOTS,
   components: {
     GlLoadingIcon,
     RelatedItemsTreeHeader,
@@ -27,6 +35,7 @@ export default {
     TreeItemRemoveModal,
     CreateIssueForm,
     IssueActionsSplitButton,
+    SlotSwitch,
   },
   data() {
     return {
@@ -57,6 +66,21 @@ export default {
     createIssueEnabled() {
       return gon.features && gon.features.epicNewIssue;
     },
+    visibleForm() {
+      if (this.showAddItemForm) {
+        return FORM_SLOTS.addItem;
+      }
+
+      if (this.showCreateEpicForm) {
+        return FORM_SLOTS.createEpic;
+      }
+
+      if (this.isCreateIssueFormVisible) {
+        return FORM_SLOTS.createIssue;
+      }
+
+      return null;
+    },
   },
   mounted() {
     this.fetchItems({
@@ -144,13 +168,14 @@ export default {
           @showCreateIssueForm="showCreateIssueForm"
         />
       </related-items-tree-header>
-      <div
-        v-if="showAddItemForm || showCreateEpicForm || isCreateIssueFormVisible"
+      <slot-switch
+        v-if="visibleForm"
+        :active-slot-names="[visibleForm]"
         class="card-body add-item-form-container"
         :class="{ 'border-bottom-0': itemsFetchResultEmpty }"
       >
         <add-item-form
-          v-if="showAddItemForm"
+          :slot="$options.FORM_SLOTS.addItem"
           :issuable-type="issuableType"
           :input-value="itemInputValue"
           :is-submitting="itemAddInProgress"
@@ -164,16 +189,16 @@ export default {
           @addIssuableFormCancel="handleAddItemFormCancel"
         />
         <create-epic-form
-          v-if="showCreateEpicForm"
+          :slot="$options.FORM_SLOTS.createEpic"
           :is-submitting="itemCreateInProgress"
           @createEpicFormSubmit="handleCreateEpicFormSubmit"
           @createEpicFormCancel="handleCreateEpicFormCancel"
         />
         <create-issue-form
-          v-if="isCreateIssueFormVisible && !showAddItemForm && !showCreateEpicForm"
+          :slot="$options.FORM_SLOTS.createIssue"
           @cancel="isCreateIssueFormVisible = false"
         />
-      </div>
+      </slot-switch>
       <related-items-tree-body
         v-if="!itemsFetchResultEmpty"
         :parent-item="parentItem"

spec/frontend/vue_shared/components/slot_switch_spec.js

+import { shallowMount } from '@vue/test-utils';
+
+import SlotSwitch from '~/vue_shared/components/slot_switch';
+
+describe('SlotSwitch', () => {
+  const slots = {
+    first: '<a>AGP</a>',
+    second: '<p>PCI</p>',
+  };
+
+  let wrapper;
+
+  const createComponent = propsData => {
+    wrapper = shallowMount(SlotSwitch, {
+      propsData,
+      slots,
+      sync: false,
+    });
+  };
+
+  const getChildrenHtml = () => wrapper.findAll('* *').wrappers.map(c => c.html());
+
+  afterEach(() => {
+    if (wrapper) {
+      wrapper.destroy();
+    }
+  });
+
+  it('throws an error if activeSlotNames is missing', () => {
+    expect(createComponent).toThrow('[Vue warn]: Missing required prop: "activeSlotNames"');
+  });
+
+  it('renders no slots if activeSlotNames is empty', () => {
+    createComponent({
+      activeSlotNames: [],
+    });
+
+    expect(getChildrenHtml().length).toBe(0);
+  });
+
+  it('renders one slot if activeSlotNames contains single slot name', () => {
+    createComponent({
+      activeSlotNames: ['first'],
+    });
+
+    expect(getChildrenHtml()).toEqual([slots.first]);
+  });
+
+  it('renders multiple slots if activeSlotNames contains multiple slot names', () => {
+    createComponent({
+      activeSlotNames: Object.keys(slots),
+    });
+
+    expect(getChildrenHtml()).toEqual(Object.values(slots));
+  });
+});