Modify to get constant of parent module in singleton class; fix #3568

src/variable.c

@@ -949,7 +949,7 @@ mrb_vm_const_get(mrb_state *mrb, mrb_sym sym)
       klass = mrb_obj_iv_get(mrb, (struct RObject *)c,
                              mrb_intern_lit(mrb, "__attached__"));
       c2 = mrb_class_ptr(klass);
-      if (c2->tt == MRB_TT_CLASS)
+      if (c2->tt == MRB_TT_CLASS || c2->tt == MRB_TT_MODULE)
         c = c2;
     }
     c2 = c;

test/t/module.rb

@@ -849,3 +849,14 @@ assert('module with non-class/module outer raises TypeError') do
   assert_raise(TypeError) { module 0::M1 end }
   assert_raise(TypeError) { module []::M2 end }
 end
+
+assert('get constant of parent module in singleton class; issue #3568') do
+  actual = module GetConstantInSingletonTest
+    EXPECTED = "value"
+    class << self
+      EXPECTED
+    end
+  end
+
+  assert_equal("value", actual)
+end