Return nil when argument to ObjectSpace.internal_class_of is T_IMEMO

The added test case crashes the interpreter because it makes
ObjectSpace.internal_class_of return the second VALUE slot of an AST
imemo object. The second VALUE slot of `struct rb_ast_struct` is
not a VALUE and not a pointer to a Ruby object.

1 files changed, 5 insertions, 0 deletions

diff --git a/test/objspace/test_objspace.rb b/test/objspace/test_objspace.rb
index f9e6e82e8f..b3f7a56e51 100644
--- a/test/objspace/test_objspace.rb
+++ b/test/objspace/test_objspace.rb
@@ -516,6 +516,11 @@ class TestObjSpace < Test::Unit::TestCase
     assert_operator i, :>, 0
   end
 
+  def test_internal_class_of_on_ast
+    children = ObjectSpace.reachable_objects_from(RubyVM::AbstractSyntaxTree.parse("kadomatsu"))
+    children.each {|child| ObjectSpace.internal_class_of(child).itself} # this used to crash
+  end
+
   def traverse_super_classes klass
     while klass
       klass = ObjectSpace.internal_super_of(klass)