Ruby 3.1: Handle `Class#subclasses` existing without `Class#descendants`

Since `#subclasses` was introduced a bit after `#descendants`, the feature testing
code assumed that if the former was present, the later would be too.

However it was decided to revert `#descendants` for now, but to keep `#subclasses`
https://bugs.ruby-lang.org/issues/14394#note-33

Since this was totally unexpected, the `#descendants` core_ext and `DescendantsTracker#descendants`
are broken on Active Support 7.0.0 / Ruby 3.1.0-dev.

We now test the existence of both methods to handle this new situation. Since
for now it's planned for `#descendants` to make it back in Ruby 3.2, we keep
checking for it.

activesupport/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Fix `Class#descendants` and `DescendantsTracker#descendants` compatibility with Ruby 3.1.
 
+    [The native `Class#descendants` was reverted prior to Ruby 3.1 release](https://bugs.ruby-lang.org/issues/14394#note-33),
+    but `Class#subclasses` was kept, breaking the feature detection.
+
+    *Jean Boussier*
 
 Please check [7-0-stable](https://github.com/rails/rails/blob/7-0-stable/activesupport/CHANGELOG.md) for previous changes.

activesupport/lib/active_support/core_ext/class/subclasses.rb

@@ -3,24 +3,32 @@
 require "active_support/ruby_features"
 
 class Class
-  # Returns an array with all classes that are < than its receiver.
-  #
-  #   class C; end
-  #   C.descendants # => []
-  #
-  #   class B < C; end
-  #   C.descendants # => [B]
-  #
-  #   class A < B; end
-  #   C.descendants # => [B, A]
-  #
-  #   class D < C; end
-  #   C.descendants # => [B, A, D]
-  def descendants
-    ObjectSpace.each_object(singleton_class).reject do |k|
-      k.singleton_class? || k == self
+  unless ActiveSupport::RubyFeatures::CLASS_DESCENDANTS
+    if ActiveSupport::RubyFeatures::CLASS_SUBCLASSES
+      def descendants
+        subclasses.concat(subclasses.flat_map(&:descendants))
+      end
+    else
+      # Returns an array with all classes that are < than its receiver.
+      #
+      #   class C; end
+      #   C.descendants # => []
+      #
+      #   class B < C; end
+      #   C.descendants # => [B]
+      #
+      #   class A < B; end
+      #   C.descendants # => [B, A]
+      #
+      #   class D < C; end
+      #   C.descendants # => [B, A, D]
+      def descendants
+        ObjectSpace.each_object(singleton_class).reject do |k|
+          k.singleton_class? || k == self
+        end
+      end
     end
-  end unless ActiveSupport::RubyFeatures::CLASS_SUBCLASSES
+  end
 
   # Returns an array with the direct children of +self+.
   #

activesupport/lib/active_support/descendants_tracker.rb

@@ -51,7 +51,7 @@ module ActiveSupport
           unless @clear_disabled
             @clear_disabled = true
             remove_method(:subclasses)
-            remove_method(:descendants)
+            remove_method(:descendants) if RubyFeatures::CLASS_DESCENDANTS
             @@excluded_descendants = nil
           end
         end
@@ -86,10 +86,16 @@ module ActiveSupport
         subclasses
       end
 
-      def descendants
-        descendants = super
-        descendants.reject! { |d| @@excluded_descendants[d] }
-        descendants
+      if RubyFeatures::CLASS_DESCENDANTS
+        def descendants
+          descendants = super
+          descendants.reject! { |d| @@excluded_descendants[d] }
+          descendants
+        end
+      else
+        def descendants
+          children.concat(children.flat_map(&:descendants))
+        end
       end
 
       def direct_descendants

activesupport/lib/active_support/ruby_features.rb

@@ -3,5 +3,6 @@
 module ActiveSupport
   module RubyFeatures # :nodoc:
     CLASS_SUBCLASSES = Class.method_defined?(:subclasses) # RUBY_VERSION >= "3.1"
+    CLASS_DESCENDANTS = Class.method_defined?(:descendants)
   end
 end