Merge pull request #485 from beoran/bdm_extend_main_crashes_fix

Bugfix for crash if main was extended with a module.

src/class.c

@@ -786,6 +786,8 @@ mrb_singleton_class(mrb_state *mrb, mrb_value v)
     return mrb_obj_value(mrb->false_class);
   case MRB_TT_TRUE:
     return mrb_obj_value(mrb->true_class);
+  case MRB_TT_MAIN:
+    return mrb_obj_value(mrb->object_class);
   case MRB_TT_SYMBOL:
   case MRB_TT_FIXNUM:
   case MRB_TT_FLOAT:

test/t/kernel.rb

@@ -204,6 +204,16 @@ assert('Kernel#extend', '15.3.1.3.13') do
   a.respond_to?(:test_method) == true && b.respond_to?(:test_method) == false
 end
 
+assert('Kernel#extend works on toplevel', '15.3.1.3.13') do
+  module Test4ExtendModule
+    def test_method; end
+  end
+  # This would crash... 
+  extend(Test4ExtendModule)
+
+  respond_to?(:test_method) == true
+end
+
 assert('Kernel#global_variables', '15.3.1.3.14') do
   global_variables.class == Array
 end