Fix false negative for RSpec/DescribedClass with constant base

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+- Fix a false negative for constants with constant base in `RSpec/DescribedClass`. ([@lovro-bikic])
+
 ## 3.8.0 (2025-11-12)
 
 - Add new cop `RSpec/LeakyLocalVariable`. ([@lovro-bikic])

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -1031,6 +1031,11 @@ end
 describe MyClass do
   subject { MyClass::CONSTANT }
 end
+
+# good
+describe MyClass do
+  subject { described_class::CONSTANT }
+end
 ----
 
 [#_enforcedstyle_-explicit_-rspecdescribedclass]

lib/rubocop/cop/rspec/described_class.rb

@@ -50,6 +50,11 @@ module RSpec
       #     subject { MyClass::CONSTANT }
       #   end
       #
+      #   # good
+      #   describe MyClass do
+      #     subject { described_class::CONSTANT }
+      #   end
+      #
       # @example `EnforcedStyle: explicit`
       #   # bad
       #   describe MyClass do
@@ -205,13 +210,15 @@ def full_const_name(node)
         # @return [Array<Symbol>]
         # @example
         #   # nil represents base constant
-        #   collapse_namespace([], [:C])                # => [:C]
+        #   collapse_namespace([], [:C])                # => [nil, :C]
         #   collapse_namespace([:A, :B], [:C])          # => [:A, :B, :C]
         #   collapse_namespace([:A, :B], [:B, :C])      # => [:A, :B, :C]
         #   collapse_namespace([:A, :B], [nil, :C])     # => [nil, :C]
         #   collapse_namespace([:A, :B], [nil, :B, :C]) # => [nil, :B, :C]
         def collapse_namespace(namespace, const)
-          return const if namespace.empty? || const.first.nil?
+          return const if const.nil?
+
+          namespace = [nil] if namespace.empty?
 
           start = [0, (namespace.length - const.length)].max
           max = namespace.length

spec/rubocop/cop/rspec/described_class_spec.rb

@@ -130,6 +130,56 @@
       RUBY
     end
 
+    it 'flags when references use constant base and described class does not' do
+      expect_offense(<<~RUBY)
+        describe MyClass do
+          include ::MyClass
+                  ^^^^^^^^^ Use `described_class` instead of `MyClass`.
+
+          subject { ::MyClass.do_something }
+                    ^^^^^^^^^ Use `described_class` instead of `MyClass`.
+
+          before { ::MyClass.do_something }
+                   ^^^^^^^^^ Use `described_class` instead of `MyClass`.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        describe MyClass do
+          include described_class
+
+          subject { described_class.do_something }
+
+          before { described_class.do_something }
+        end
+      RUBY
+    end
+
+    it 'flags when described class uses constant base and references do not' do
+      expect_offense(<<~RUBY)
+        describe ::MyClass do
+          include MyClass
+                  ^^^^^^^ Use `described_class` instead of `MyClass`.
+
+          subject { MyClass.do_something }
+                    ^^^^^^^ Use `described_class` instead of `MyClass`.
+
+          before { MyClass.do_something }
+                   ^^^^^^^ Use `described_class` instead of `MyClass`.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        describe ::MyClass do
+          include described_class
+
+          subject { described_class.do_something }
+
+          before { described_class.do_something }
+        end
+      RUBY
+    end
+
     it 'allows accessing constants from variables when in a nested namespace' do
       expect_no_offenses(<<~RUBY)
         module Foo
@@ -438,6 +488,16 @@ module SomeGem
         end
       RUBY
     end
+
+    it 'ignores parenthesized namespaces within modules - begin_types' do
+      expect_no_offenses(<<~RUBY)
+        module Foo
+          describe MyClass do
+            subject { (MyNamespace)::MyClass }
+          end
+        end
+      RUBY
+    end
   end
 
   context 'when EnforcedStyle is :explicit' do