Make /reassign work in any case

The /reassign quick command should work even when no multiple assignees are
allowed of there isn't any assignee yet.

For consistency, it will also be backported to CE. The functionality will be the
same as the /assign quick action.

app/services/ee/slash_commands/interpret_service.rb

-module EE
-  module SlashCommands
-    module InterpretService
-      include ::Gitlab::SlashCommands::Dsl
-
-      desc 'Change assignee(s)'
-      explanation do
-        'Change assignee(s)'
-      end
-      params '@user1 @user2'
-      condition do
-        issuable.allows_multiple_assignees? &&
-          issuable.persisted? &&
-          issuable.assignees.any? &&
-          current_user.can?(:"admin_#{issuable.to_ability_name}", project)
-      end
-      command :reassign do |unassign_param|
-        @updates[:assignee_ids] = extract_users(unassign_param).map(&:id)
-      end
-    end
-  end
-end

app/services/quick_actions/interpret_service.rb

@@ -150,6 +150,24 @@ module QuickActions
       end
     end
 
+    desc do
+      "Change assignee#{'(s)' if issuable.allows_multiple_assignees?}"
+    end
+    explanation do |users|
+      users = issuable.allows_multiple_assignees? ? users : users.take(1)
+      "Change #{'assignee'.pluralize(users.size)} to #{users.map(&:to_reference).to_sentence}."
+    end
+    params do
+      issuable.allows_multiple_assignees? ? '@user1 @user2' : '@user'
+    end
+    condition do
+      issuable.persisted? &&
+        current_user.can?(:"admin_#{issuable.to_ability_name}", project)
+    end
+    command :reassign do |unassign_param|
+      @updates[:assignee_ids] = extract_users(unassign_param).map(&:id)
+    end
+
     desc 'Set milestone'
     explanation do |milestone|
       "Sets the milestone to #{milestone.to_reference}." if milestone

spec/services/quick_actions/interpret_service_spec.rb

@@ -411,9 +411,6 @@ describe QuickActions::InterpretService, services: true do
 
       context 'Merge Request' do
         it 'fetches assignee and populates assignee_id if content contains /assign' do
-          user = create(:user)
-          merge_request.update(assignee: user)
-
           _, updates = service.execute(content, merge_request)
 
           expect(updates).to eq(assignee_id: developer.id)
@@ -486,22 +483,6 @@ describe QuickActions::InterpretService, services: true do
         end
       end
 
-      context 'reassign command' do
-        let(:content) { '/reassign' }
-
-        context 'Issue' do
-          it 'reassigns user if content contains /reassign @user' do
-            user = create(:user)
-
-            issue.update(assignee_ids: [developer.id, developer2.id])
-
-            _, updates = service.execute("/reassign @#{user.username}", issue)
-
-            expect(updates).to eq(assignee_ids: [user.id])
-          end
-        end
-      end
-
       context 'Merge Request' do
         it 'populates assignee_id: nil if content contains /unassign' do
           merge_request.update(assignee_id: developer.id)
@@ -512,6 +493,22 @@ describe QuickActions::InterpretService, services: true do
       end
     end
 
+    context 'reassign command' do
+      let(:content) { '/reassign' }
+
+      context 'Issue' do
+        it 'reassigns user if content contains /reassign @user' do
+          user = create(:user)
+
+          issue.update(assignee_ids: [developer.id])
+
+          _, updates = service.execute("/reassign @#{user.username}", issue)
+
+          expect(updates).to eq(assignee_ids: [user.id])
+        end
+      end
+    end
+
     it_behaves_like 'milestone command' do
       let(:content) { "/milestone %#{milestone.title}" }
       let(:issuable) { issue }
@@ -1000,7 +997,7 @@ describe QuickActions::InterpretService, services: true do
       it 'includes current assignee reference' do
         _, explanations = service.explain(content, issue)
 
-        expect(explanations).to eq(["Removes assignee #{developer.to_reference}"])
+        expect(explanations).to eq(["Removes assignee #{developer.to_reference}."])
       end
     end