Fix NoMethodError on custom ActiveSupport::Deprecation behavior

with some additional changes made later:

1. Flip the condition for backward compatibility
   Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>

2. Improve custom behavior test
   Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>

3. Fix indentation

activesupport/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   Fix `NoMethodError` on custom `ActiveSupport::Deprecation` behavior.
+
+    `ActiveSupport::Deprecation.behavior=` was supposed to accept any object
+    that responds to `call`, but in fact its internal implementation assumed that
+    this object could respond to `arity`, so it was restricted to only `Proc` objects.
+
+    This change removes this `arity` restriction of custom behaviors.
+
+    *Ryo Nakamura*
+
 *   Support `:urlsafe` option for `MessageEncryptor`.
 
     The `MessageEncryptor` constructor now accepts a `:urlsafe` option, similar

activesupport/lib/active_support/deprecation/behaviors.rb

@@ -114,10 +114,10 @@ def arity_coerce(behavior)
             raise ArgumentError, "#{behavior.inspect} is not a valid deprecation behavior."
           end
 
-          if behavior.arity == 4 || behavior.arity == -1
-            behavior
-          else
+          if behavior.respond_to?(:arity) && behavior.arity == 2
             -> message, callstack, _, _ { behavior.call(message, callstack) }
+          else
+            behavior
           end
         end
     end

activesupport/test/deprecation_test.rb

@@ -210,6 +210,21 @@ def test_default_invalid_behavior
     assert_equal ":invalid is not a valid deprecation behavior.", e.message
   end
 
+  def test_custom_behavior
+    custom_behavior_class = Class.new do
+      def call(message, callstack, horizon, gem_name)
+        $stderr.puts message
+      end
+    end
+    ActiveSupport::Deprecation.behavior = custom_behavior_class.new
+
+    content = capture(:stderr) do
+      ActiveSupport::Deprecation.warn("foo")
+    end
+
+    assert_match(/foo/, content)
+  end
+
   def test_deprecated_instance_variable_proxy
     assert_not_deprecated { @dtc.request.size }