ClassModule#superclass= accepts a ClassModule as an argument (#1222)

It is necessary for ClassModule's instance variable @superclass to
always be a String (or nil) so that the class can be saved with
`#marshal_dump` and loaded with `#marshal_load`.

However, there's no type checking being done, which allows a bug like
the one reported in #1221 (which was introduced in #1217) that sets
superclass to a ClassModule. That bug requires:

- setting a superclass to a NormalClass
- marshal_save
- marshal_load (which raises an exception)

With this change, passing a ClassModule to ClassModule#superclass= is
explicitly allowed by saving the full name of the ClassModule in the
@superclass instance variable.

lib/rdoc/code_object/class_module.rb

@@ -705,10 +705,23 @@ def superclass
 
   ##
   # Set the superclass of this class to +superclass+
+  #
+  # where +superclass+ is one of:
+  #
+  # - +nil+
+  # - a String containing the full name of the superclass
+  # - the RDoc::ClassModule representing the superclass
 
   def superclass=(superclass)
     raise NoMethodError, "#{full_name} is a module" if module?
-    @superclass = superclass
+    case superclass
+    when RDoc::ClassModule
+      @superclass = superclass.full_name
+    when nil, String
+      @superclass = superclass
+    else
+      raise TypeError, "superclass must be a String or RDoc::ClassModule, not #{superclass.class}"
+    end
   end
 
   ##

test/rdoc/test_rdoc_class_module.rb

@@ -1279,6 +1279,26 @@ def test_superclass
     assert_equal @c3_h1, @c3_h2.superclass
   end
 
+  def test_setting_superclass
+    @c1.superclass = nil
+    assert_nil(@c1.superclass)
+    assert_nil(@c1.instance_variable_get("@superclass")) # proxy to test marshalling
+
+    @c1.superclass = @c4_c4.full_name
+    assert_equal(@c1.superclass, @c4_c4)
+    assert_equal(@c4_c4.full_name, @c1.instance_variable_get("@superclass"))
+
+    @c1.superclass = @c4_c4
+    assert_equal(@c1.superclass, @c4_c4)
+    assert_equal(@c4_c4.full_name, @c1.instance_variable_get("@superclass"))
+
+    # we could support this if we find we need to in the future.
+    assert_raise(TypeError) { @c1.superclass = Object }
+
+    # but this doesn't make sense.
+    assert_raise(TypeError) { @c1.superclass = Object.new }
+  end
+
   def test_super_classes
     rdoc_c3_h1 = @xref_data.find_module_named('C3::H1')
     rdoc_object = @xref_data.find_module_named('Object')