Allow adding a formatter class that extends from an already added formatter superclass

Author: Sebastián Tello

lib/rspec/core/formatters.rb

@@ -182,7 +182,7 @@ def register(formatter, notifications)
 
     def duplicate_formatter_exists?(new_formatter)
       @formatters.any? do |formatter|
-        formatter.class === new_formatter && formatter.output == new_formatter.output
+        formatter.class == new_formatter.class && formatter.output == new_formatter.output
       end
     end
 

spec/rspec/core/formatters_spec.rb

@@ -1,4 +1,5 @@
 require 'pathname'
+require 'rspec/core/resources/a_custom_formatter'
 
 module RSpec::Core::Formatters
   RSpec.describe Loader do
@@ -137,6 +138,16 @@ module RSpec::Core::Formatters
           }.to change { loader.formatters.length }
         end
       end
+
+      context "When a custom formatter exists" do
+        before { loader.add Custom::AGeneralFormatter, output }
+
+        it "adds a subclass of that formatter for the same output target" do
+          expect {
+            loader.add Custom::AMoreSpecificFormatter, output
+          }.to change { loader.formatters.length }
+        end
+      end
     end
 
     describe "#setup_default" do

spec/rspec/core/resources/a_custom_formatter.rb

@@ -0,0 +1,22 @@
+module Custom
+  class AGeneralFormatter < RSpec::Core::Formatters::BaseFormatter
+    RSpec::Core::Formatters.register self, :message
+    attr_reader :call_times
+
+    def message(_notification)
+      puts message_builder
+    end
+
+    def message_builder
+      'Important Message'
+    end
+  end
+
+  class AMoreSpecificFormatter < AGeneralFormatter
+    RSpec::Core::Formatters.register self, :message
+
+    def message_builder
+      'Very ' + super
+    end
+  end
+end