mrb_const_defined returns true if it is defined on a super class.

src/errno.c

@@ -60,14 +60,14 @@ mrb_sce_init(mrb_state *mrb, mrb_value self)
 static mrb_value
 mrb_sce_errno(mrb_state *mrb, mrb_value self)
 {
+  struct RClass *c;
   mrb_sym sym;
-  mrb_value klass;
 
-  klass = mrb_obj_value(mrb_class(mrb, self));
+  c = mrb_class(mrb, self);
   sym = mrb_intern_lit(mrb, "Errno");
-  if (mrb_const_defined(mrb, klass, sym))
-    return mrb_const_get(mrb, klass, sym);
-  else {
+  if (mrb_const_defined_at(mrb, c, sym)) {
+    return mrb_const_get(mrb, mrb_obj_value(c), sym);
+  } else {
     sym = mrb_intern_lit(mrb, "errno");
     return mrb_attr_get(mrb, self, sym);
   }

test/errno.rb

@@ -17,10 +17,10 @@ assert('SystemCallError#initialize') do
 end
 
 assert('SystemCallError#errno') do
-  SystemCallError.new("a", 1).errno == 1 and
-  SystemCallError.new(1).errno == 1 and
-  SystemCallError.new("a", 12345).errno == 12345 and
-  SystemCallError.new(23456).errno == 23456
+  assert_equal 1, SystemCallError.new("a", 1).errno
+  assert_equal 1, SystemCallError.new(1).errno
+  assert_equal 12345, SystemCallError.new("a", 12345).errno
+  assert_equal 23456, SystemCallError.new(23456).errno
 end
 
 assert('SystemCallError#inspect') do