Disallow switch to basic search on some tabs

Only some tabs support basic search and others need Elasticsearch

ee/app/helpers/ee/search_helper.rb

@@ -3,6 +3,8 @@ module EE
   module SearchHelper
     extend ::Gitlab::Utils::Override
 
+    SWITCH_TO_BASIC_SEARCHABLE_TABS = %w[projects issues merge_requests milestones users].freeze
+
     override :search_filter_input_options
     def search_filter_input_options(type)
       options = super
@@ -68,6 +70,16 @@ module EE
       search_path(search_params)
     end
 
+    # Switching to basic search should only be possible for a subset of
+    # scopes. This method assumes you're already allowed to search within
+    # this scope but it's just here to determine if you can switch to basic
+    # search.
+    def switch_to_basic_searchable_tab?(scope)
+      return true if @project
+
+      scope.in?(SWITCH_TO_BASIC_SEARCHABLE_TABS)
+    end
+
     private
 
     def search_multiple_assignees?(type)

ee/app/views/search/_form_revert_to_basic.html.haml

-- if search_service.use_elasticsearch?
+- if search_service.use_elasticsearch? && switch_to_basic_searchable_tab?(search_service.scope)
   - begin_link = '<a href="%{url}">'.html_safe % { url: revert_to_basic_search_filter_url }
   - end_link = '</a>'.html_safe
   %p.text-center

ee/spec/features/search/elastic/global_search_spec.rb

@@ -220,10 +220,8 @@ describe 'Global elastic search', :elastic do
     end
   end
 
-  describe 'When requesting basic search' do
-    it 'does not use Elasticsearch' do
-      create(:issue, project: project, title: 'project issue')
-
+  context 'when no results are returned' do
+    it 'allows basic search without Elasticsearch' do
       visit dashboard_projects_path
 
       submit_search('project')
@@ -238,5 +236,16 @@ describe 'Global elastic search', :elastic do
       # Project is found now that we are using basic search
       expect(page).to have_content('Projects 1')
     end
+
+    context 'when performing Commits search' do
+      it 'does not allow basic search' do
+        visit dashboard_projects_path
+
+        submit_search('project')
+        select_search_scope('Commits')
+
+        expect(page).not_to have_link('basic search')
+      end
+    end
   end
 end

ee/spec/helpers/search_helper_spec.rb

@@ -175,4 +175,27 @@ describe SearchHelper do
       it_behaves_like 'returns old message'
     end
   end
+
+  describe '#switch_to_basic_searchable_tab?' do
+    let(:scope) { 'commits' }
+    subject { switch_to_basic_searchable_tab?(scope) }
+
+    context 'when project scope' do
+      before do
+        @project = create(:project)
+      end
+
+      it { is_expected.to eq(true) }
+    end
+
+    context 'when commits tab' do
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when issues tab' do
+      let(:scope) { 'issues' }
+
+      it { is_expected.to eq(true) }
+    end
+  end
 end