Don't allow creating a new app with certain confusing names

lib/hanami/cli/commands/gem/new.rb

@@ -46,6 +46,9 @@ class New < Command
           # @api private
           SUPPORTED_DATABASES = [DATABASE_SQLITE, DATABASE_POSTGRES, DATABASE_MYSQL].freeze
 
+          # @api private
+          FORBIDDEN_APP_NAMES = %w[app slice].freeze
+
           desc "Generate a new Hanami app"
 
           # @since 2.0.0
@@ -126,6 +129,7 @@ def call(
             app = inflector.underscore(app)
 
             raise PathAlreadyExistsError.new(app) if fs.exist?(app)
+            raise ForbiddenAppNameError.new(app) if FORBIDDEN_APP_NAMES.include?(app)
 
             normalized_database ||= normalize_database(database)
 

lib/hanami/cli/errors.rb

@@ -44,6 +44,13 @@ def initialize(path)
       end
     end
 
+    # @api public
+    class ForbiddenAppNameError < Error
+      def initialize(name)
+        super("Cannot create new Hanami app with the name: `#{name}'")
+      end
+    end
+
     # @since 2.0.0
     # @api public
     class MissingSliceError < Error

spec/unit/hanami/cli/commands/gem/new_spec.rb

@@ -68,6 +68,17 @@
     expect(fs.read("#{app}/config/app.rb")).to include("module #{module_name}")
   end
 
+  it "forbids certain confusing app names" do
+    %w[app slice].each do |forbidden_name|
+      expect {
+        subject.call(app: forbidden_name)
+      }.to raise_error(
+        Hanami::CLI::ForbiddenAppNameError,
+        "Cannot create new Hanami app with the name: `#{forbidden_name}'"
+      )
+    end
+  end
+
   it "generates an app" do
     expect(bundler).to receive(:install!)
       .and_return(true)