Disallow anonymous Data from being dumped

This feature leaked out without much discussion, but there's many
reasons why it should not be supported:

* Each instance of an anonymous Data in the stream will cause a new
  Data subtype to be defined.
* Multiple objects of the same anonymous Data cannot be round-
  tripped, since they will no longer be of the same type after
  loading.

As with Marshal's behavior for anonymous classes and structs, Psych
should reject such objects rather than attempt to fake a Data class
that appears to fit the original structure.

Author: headius

lib/psych/visitors/yaml_tree.rb

@@ -163,6 +163,7 @@ def visit_Object o
       alias :visit_Delegator :visit_Object
 
       def visit_Data o
+        raise TypeError, "can't dump anonymous Data: #{o}" unless o.class.name
         ivars = o.instance_variables
         if ivars.empty?
           tag = ['!ruby/data', o.class.name].compact.join(':')

test/psych/test_object_references.rb

@@ -31,9 +31,11 @@ def test_struct_has_references
       assert_reference_trip Struct.new(:foo).new(1)
     end
 
+    D = Data.define(:foo) unless RUBY_VERSION < "3.2"
+
     def test_data_has_references
       omit "Data requires ruby >= 3.2" if RUBY_VERSION < "3.2"
-      assert_reference_trip Data.define(:foo).new(1)
+      assert_reference_trip D.new(1)
     end
 
     def assert_reference_trip obj

test/psych/visitors/test_yaml_tree.rb

@@ -83,13 +83,14 @@ def test_data
       def test_data_anon
         omit "Data requires ruby >= 3.2" if RUBY_VERSION < "3.2"
         d = Data.define(:foo).new('bar')
-        obj =  Psych.unsafe_load(Psych.dump(d))
-        assert_equal d.foo, obj.foo
+        assert_raise(TypeError) { Psych.dump(d) }
       end
 
+      D2 = Data.define(:method) unless RUBY_VERSION < "3.2"
+
       def test_data_override_method
         omit "Data requires ruby >= 3.2" if RUBY_VERSION < "3.2"
-        d = Data.define(:method).new('override')
+        d = D2.new('override')
         obj =  Psych.unsafe_load(Psych.dump(d))
         assert_equal d.method, obj.method
       end