Display merge conflicts message when comparing against head

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/230991

app/assets/javascripts/diffs/components/app.vue

 <script>
 import { mapState, mapGetters, mapActions } from 'vuex';
-import { GlLoadingIcon, GlButtonGroup, GlButton } from '@gitlab/ui';
+import { GlLoadingIcon, GlButtonGroup, GlButton, GlAlert } from '@gitlab/ui';
 import Mousetrap from 'mousetrap';
 import { __ } from '~/locale';
 import createFlash from '~/flash';
+import { getParameterByName, parseBoolean } from '~/lib/utils/common_utils';
 import PanelResizer from '~/vue_shared/components/panel_resizer.vue';
 import glFeatureFlagsMixin from '~/vue_shared/mixins/gl_feature_flags_mixin';
 import { isSingleViewStyle } from '~/helpers/diffs_helper';
@@ -38,6 +39,7 @@ export default {
     PanelResizer,
     GlButtonGroup,
     GlButton,
+    GlAlert,
   },
   mixins: [glFeatureFlagsMixin()],
   props: {
@@ -133,6 +135,9 @@ export default {
       'startVersion',
       'currentDiffFileId',
       'isTreeLoaded',
+      'conflictResolutionPath',
+      'canMerge',
+      'hasConflicts',
     ]),
     ...mapGetters('diffs', ['isParallelView', 'currentDiffIndex']),
     ...mapGetters(['isNotesFetched', 'getNoteableData']),
@@ -161,6 +166,9 @@ export default {
     isLimitedContainer() {
       return !this.showTreeList && !this.isParallelView && !this.isFluidLayout;
     },
+    isDiffHead() {
+      return parseBoolean(getParameterByName('diff_head'));
+    },
   },
   watch: {
     commit(newCommit, oldCommit) {
@@ -422,6 +430,49 @@ export default {
         :email-patch-path="emailPatchPath"
       />
 
+      <div
+        v-if="isDiffHead && hasConflicts"
+        :class="{
+          [CENTERED_LIMITED_CONTAINER_CLASSES]: isLimitedContainer,
+        }"
+      >
+        <gl-alert
+          :dismissible="false"
+          :title="__('There are merge conflicts')"
+          variant="warning"
+          class="w-100 mb-3"
+        >
+          <p class="mb-1">
+            {{ __('The comparison view may be inaccurate due to merge conflicts.') }}
+          </p>
+          <p class="mb-0">
+            {{
+              __(
+                'Resolve these conflicts or ask someone with write access to this repository to merge it locally.',
+              )
+            }}
+          </p>
+          <template #actions>
+            <gl-button
+              v-if="conflictResolutionPath"
+              :href="conflictResolutionPath"
+              variant="info"
+              class="mr-3 gl-alert-action"
+            >
+              {{ __('Resolve conflicts') }}
+            </gl-button>
+            <gl-button
+              v-if="canMerge"
+              class="gl-alert-action"
+              data-toggle="modal"
+              data-target="#modal_merge_info"
+            >
+              {{ __('Merge locally') }}
+            </gl-button>
+          </template>
+        </gl-alert>
+      </div>
+
       <div
         :data-can-create-note="getNoteableData.current_user.can_create_note"
         class="files d-flex"

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -41,7 +41,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
   def diffs_metadata
     diffs = @compare.diffs(diff_options)
 
-    render json: DiffsMetadataSerializer.new(project: @merge_request.project)
+    render json: DiffsMetadataSerializer.new(project: @merge_request.project, current_user: current_user)
                    .represent(diffs, additional_attributes)
   end
 

app/serializers/diffs_metadata_entity.rb

@@ -3,4 +3,23 @@
 class DiffsMetadataEntity < DiffsEntity
   unexpose :diff_files
   expose :raw_diff_files, as: :diff_files, using: DiffFileMetadataEntity
+
+  expose :conflict_resolution_path do |_, options|
+    presenter(options[:merge_request]).conflict_resolution_path
+  end
+
+  expose :has_conflicts do |_, options|
+    options[:merge_request].cannot_be_merged?
+  end
+
+  expose :can_merge do |_, options|
+    options[:merge_request].can_be_merged_by?(request.current_user)
+  end
+
+  private
+
+  def presenter(merge_request)
+    @presenters ||= {}
+    @presenters[merge_request] ||= MergeRequestPresenter.new(merge_request, current_user: request.current_user) # rubocop: disable CodeReuse/Presenter
+  end
 end

app/views/projects/merge_requests/_widget.html.haml

-- if @merge_request.source_branch_exists?
-  = render "projects/merge_requests/how_to_merge"
-
 = javascript_tag nonce: true do
   :plain
     window.gl = window.gl || {};

app/views/projects/merge_requests/show.html.haml

@@ -12,6 +12,9 @@
 .merge-request{ data: { mr_action: mr_action, url: merge_request_path(@merge_request, format: :json), project_path: project_path(@merge_request.project), lock_version: @merge_request.lock_version } }
   = render "projects/merge_requests/mr_title"
 
+  - if @merge_request.source_branch_exists?
+    = render "projects/merge_requests/how_to_merge"
+
   .merge-request-details.issuable-details{ data: { id: @merge_request.project.id } }
     = render "projects/merge_requests/mr_box"
 

locale/gitlab.pot

@@ -14906,6 +14906,9 @@ msgstr ""
 msgid "Merge in progress"
 msgstr ""
 
+msgid "Merge locally"
+msgstr ""
+
 msgid "Merge options"
 msgstr ""
 
@@ -20729,9 +20732,15 @@ msgstr ""
 msgid "Resolve all threads in new issue"
 msgstr ""
 
+msgid "Resolve conflicts"
+msgstr ""
+
 msgid "Resolve conflicts on source branch"
 msgstr ""
 
+msgid "Resolve these conflicts or ask someone with write access to this repository to merge it locally."
+msgstr ""
+
 msgid "Resolve thread"
 msgstr ""
 
@@ -24060,6 +24069,9 @@ msgstr ""
 msgid "The commit does not exist"
 msgstr ""
 
+msgid "The comparison view may be inaccurate due to merge conflicts."
+msgstr ""
+
 msgid "The connection will time out after %{timeout}. For repositories that take longer, use a clone/push combination."
 msgstr ""
 
@@ -24377,6 +24389,9 @@ msgstr ""
 msgid "There are currently no events."
 msgstr ""
 
+msgid "There are merge conflicts"
+msgstr ""
+
 msgid "There are no %{replicableTypeName} to show"
 msgstr ""
 

spec/serializers/diffs_metadata_entity_spec.rb

@@ -30,6 +30,7 @@ RSpec.describe DiffsMetadataEntity do
         :email_patch_path, :plain_diff_path,
         :merge_request_diffs, :context_commits,
         :definition_path_prefix, :source_branch_exists,
+        :can_merge, :conflict_resolution_path, :has_conflicts,
         # Attributes
         :diff_files
       )