Custom domain types don't deserialize properly from anchors

[josh-wise-proof]

I discovered this while trying to make YAML deserialization work with Sorbet enum classes, but this could happen with any custom domain type. I wrote a custom serializer using add_domain_type and a corresponding implementation of encode_with that I defined on T::Enum only to find that, if I serialize an object with the same enum value more than once, anything besides the first instance would actually just be a Hash with the properties I serialized on it. I have a simple example that repros the behavior:

require "yaml"
require "json"

::YAML.add_domain_type('test/custom_yaml', 'my_test_object') do |type, value|
  MyTestClass.new(prop1: value['prop1'], prop2: value['prop2'])
end

class MyTestClass
  attr_reader :prop1, :prop2

  def initialize(prop1:, prop2:)
    @prop1 = prop1
    @prop2 = prop2
  end

  def encode_with(coder)
    coder.tag = '!test/custom_yaml:my_test_object'

    coder.map = {
      'prop1' => @prop1,
      'prop2' => @prop2
    }
  end
end

test_obj = MyTestClass.new(prop1: 13, prop2: 1989)

generated = ::Psych.dump(
  {
    'object1' => test_obj,
    'object2' => test_obj,
    'object3' => test_obj
  }
)

puts "GENERATED YAML:"
puts generated
puts "END"

puts "PARSED YAML:"
puts ::Psych.safe_load(generated, permitted_classes: [MyTestClass], aliases: true)
puts "END"

Gives me the output:

GENERATED YAML:
---
object1: &1 !test/custom_yaml:my_test_object
  prop1: 13
  prop2: 1989
object2: *1
object3: *1
END
PARSED YAML:
{"object1" => #<MyTestClass:0x0000000120b4cfa0 @prop1=13, @prop2=1989>, "object2" => {"prop1" => 13, "prop2" => 1989}, "object3" => {"prop1" => 13, "prop2" => 1989}}
END

where the values of object2 and object3 in the resulting hash are actually hashes of the serialized properties, rather than an instance of MyTestClass

I believe the bug is that when objects are deserialized via domain types, the result isn't registered, such that subsequent alias references don't get the parsed version of the object. If I add in a register method inside of Psych::Visitors::ToRuby.accept, I get the same object for all 3 references in my example:

require "yaml"
require "json"

module Psych
  module Visitors
    class ToRuby
      def accept(target)
        result = super

        unless @domain_types.empty? || !target.tag
          key = target.tag.sub(/^[!\/]*/, '').sub(/(,\d+)\//, '\1:')
          key = "tag:#{key}" unless key.match?(/^(?:tag:|x-private)/)

          if @domain_types.key? key
            value, block = @domain_types[key]
            result = block.call value, result

            register(target, result)
          end
        end

        result = deduplicate(result).freeze if @freeze
        result
      end
    end
  end
end

::YAML.add_domain_type('test/custom_yaml', 'my_test_object') do |type, value|
  MyTestClass.new(prop1: value['prop1'], prop2: value['prop2'])
end

class MyTestClass
  attr_reader :prop1, :prop2

  def initialize(prop1:, prop2:)
    @prop1 = prop1
    @prop2 = prop2
  end

  def encode_with(coder)
    coder.tag = '!test/custom_yaml:my_test_object'

    coder.map = {
      'prop1' => @prop1,
      'prop2' => @prop2
    }
  end
end

test_obj = MyTestClass.new(prop1: 13, prop2: 1989)

generated = ::Psych.dump(
  {
    'object1' => test_obj,
    'object2' => test_obj,
    'object3' => test_obj
  }
)

puts "GENERATED YAML:"
puts generated
puts "END"

puts "PARSED YAML:"
puts ::Psych.safe_load(generated, permitted_classes: [MyTestClass], aliases: true)
puts "END"

Gives me the output:

GENERATED YAML:
---
object1: &1 !test/custom_yaml:my_test_object
  prop1: 13
  prop2: 1989
object2: *1
object3: *1
END
PARSED YAML:
{"object1" => #<MyTestClass:0x00000001015dc918 @prop1=13, @prop2=1989>, "object2" => #<MyTestClass:0x00000001015dc918 @prop1=13, @prop2=1989>, "object3" => #<MyTestClass:0x00000001015dc918 @prop1=13, @prop2=1989>}
END

Machine details

Apple M3 Macbook Air
Reproduced on: "ruby 3.4.9 (2026-03-11 revision 76cca827ab) +PRISM [arm64-darwin25]"