Currently subscription api on notifications is only supported for String, Regex or nil. If we pass anything different, this causes issues because, it gets added to "other subscribers" silently.

We now raise an argument error instead if something invalid is passed.

Partially accesses original class implementation from 4f2a04cc08 which used the case/when

activesupport/lib/active_support/notifications.rb

@@ -231,6 +231,12 @@ module ActiveSupport
       #   ActiveSupport::Notifications.subscribe(/render/) do |event|
       #     @event = event
       #   end
+      #
+      # Raises an error if invalid event name type is passed:
+      #
+      #  ActiveSupport::Notifications.subscribe(:render) {|*args| ...}
+      #  #=> ArgumentError (pattern must be specified as a String, Regexp or empty)
+      #
       def subscribe(pattern = nil, callback = nil, &block)
         notifier.subscribe(pattern, callback, monotonic: false, &block)
       end

activesupport/lib/active_support/notifications/fanout.rb

@@ -24,12 +24,15 @@ module ActiveSupport
       def subscribe(pattern = nil, callable = nil, monotonic: false, &block)
         subscriber = Subscribers.new(pattern, callable || block, monotonic)
         synchronize do
-          if String === pattern
+          case pattern
+          when String
             @string_subscribers[pattern] << subscriber
             @listeners_for.delete(pattern)
-          else
+          when NilClass, Regexp
             @other_subscribers << subscriber
             @listeners_for.clear
+          else
+            raise ArgumentError,  "pattern must be specified as a String, Regexp or empty"
           end
         end
         subscriber

activesupport/test/notifications_test.rb

@@ -443,6 +443,17 @@ module Notifications
       assert_not not_child.parent_of?(parent)
     end
 
+    def test_subscribe_raises_error_on_non_supported_arguments
+      notifier = ActiveSupport::Notifications::Fanout.new
+
+      assert_raises ArgumentError do
+        notifier.subscribe(:symbol) { |*_| }
+      end
+      assert_raises ArgumentError do
+        notifier.subscribe(Object.new) { |*_| }
+      end
+    end
+
     private
       def random_id
         @random_id ||= SecureRandom.hex(10)