Improve Class#superclass to ISO

src/class.c

@@ -822,10 +822,16 @@ mrb_class_new_class(mrb_state *mrb, mrb_value cv)
 mrb_value
 mrb_class_superclass(mrb_state *mrb, mrb_value klass)
 {
-  struct RClass *c, *s;
+  struct RClass *c;
+  mrb_value superclass;
+
   c = mrb_class_ptr(klass);
-  s = mrb_class_real(c->super);
-  return mrb_obj_value(s);
+  if (c->super)
+    superclass = mrb_obj_value(mrb_class_real(c->super));
+  else
+    superclass = mrb_nil_value();
+
+  return superclass;
 }
 
 static mrb_value

test/t/class.rb

@@ -5,9 +5,8 @@ assert('Class', '15.2.3') do
   Class.class == Class
 end
 
-assert('Class#superclass', '15.2.3.3.4') do
-  class SubClass < String; end
-  SubClass.superclass == String
+assert('Class superclass', '15.2.3.2') do
+  Class.superclass == Module
 end
 
 assert('Class#new', '15.2.3.3.3') do
@@ -40,6 +39,11 @@ assert('Class#new', '15.2.3.3.3') do
   # with block doesn't work yet
 end
 
+assert('Class#superclass', '15.2.3.3.4') do
+  class SubClass < String; end
+  SubClass.superclass == String
+end
+
 # Not ISO specified
 
 assert('Class 1') do