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 anonymous classes and singleton objects, 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)
+
     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)
+
       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