Ensure the shortcut cached in the class

As well as the other places using RCLASS_IV_INDEX_TBL.
`IO#reopen` seems the only case that the class of an object can be
changed.

1 files changed, 11 insertions, 0 deletions

diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 8c7fe69128..82f122489a 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -2489,6 +2489,17 @@ class TestIO < Test::Unit::TestCase
     end
   end
 
+  def test_reopen_ivar
+    assert_ruby_status([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      f = File.open(IO::NULL)
+      f.instance_variable_set(:@foo, 42)
+      f.reopen(STDIN)
+      f.instance_variable_defined?(:@foo)
+      f.instance_variable_get(:@foo)
+    end;
+  end
+
   def test_foreach
     a = []
     IO.foreach("|" + EnvUtil.rubybin + " -e 'puts :foo; puts :bar; puts :baz'") {|x| a << x }