1 files changed, 45 insertions, 0 deletions
diff --git a/rubocop/cop/gitlab/delegate_predicate_methods.rb b/rubocop/cop/gitlab/delegate_predicate_methods.rb
new file mode 100644
index 00000000000..43b5184faab
--- /dev/null
+++ b/rubocop/cop/gitlab/delegate_predicate_methods.rb
@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Gitlab
+      # This cop looks for delegations to predicate methods with `allow_nil: true` option.
+      # This construct results in three possible results: true, false and nil.
+      # In other words, it does not preserve the strict Boolean nature of predicate method return value.
+      # This cop suggests creating a method to handle `nil` delegator and ensure only Boolean type is returned.
+      #
+      # @example
+      #   # bad
+      #   delegate :is_foo?, to: :bar, allow_nil: true
+      #
+      #   # good
+      #   def is_foo?
+      #     return false unless bar
+      #     bar.is_foo?
+      #   end
+      #
+      #   def is_foo?
+      #     !!bar&.is_foo?
+      #   end
+      class DelegatePredicateMethods < RuboCop::Cop::Cop
+        MSG = "Using `delegate` with `allow_nil` on the following predicate methods is discouraged: %s."
+        RESTRICT_ON_SEND = %i[delegate].freeze
+        def_node_matcher :predicate_allow_nil_option, <<~PATTERN
+          (send nil? :delegate
+            (sym $_)*
+            (hash <$(pair (sym :allow_nil) true) ...>)
+          )
+        PATTERN
+
+        def on_send(node)
+          predicate_allow_nil_option(node) do |delegated_methods, _options|
+            offensive_methods = delegated_methods.select { |method| method.end_with?('?') }
+            next if offensive_methods.empty?
+
+            add_offense(node, message: format(MSG, offensive_methods.join(', ')))
+          end
+        end
+      end
+    end
+  end
+end