Fix crash for special constants in too complex generic ivars

We should skip reference updating for entries in too complex generic ivars
that are special constants. This fixes the following crash:

    MAX_SHAPES = 0x80000

    MAX_SHAPES.times do |i|
      o = []
      o.instance_variable_set(:"@foo#{i}", 1)
    end

    o = []

    o.instance_variable_set(:"@a", 123)

    GC.compact

1 files changed, 17 insertions, 0 deletions

diff --git a/test/ruby/test_gc_compact.rb b/test/ruby/test_gc_compact.rb
index 4c8aa20215..2a72ecfc5f 100644
--- a/test/ruby/test_gc_compact.rb
+++ b/test/ruby/test_gc_compact.rb
@@ -452,4 +452,21 @@ class TestGCCompact < Test::Unit::TestCase
       assert_raise(FrozenError) { a.set_a }
     end;
   end
+
+  def test_moving_too_compex_generic_ivar
+    omit "not compiled with SHAPE_DEBUG" unless defined?(RubyVM::Shape)
+
+    assert_separately([], <<~RUBY)
+      RubyVM::Shape.exhaust_shapes
+
+      obj = []
+      obj.instance_variable_set(:@fixnum, 123)
+      obj.instance_variable_set(:@str, "hello")
+
+      GC.verify_compaction_references(expand_heap: true, toward: :empty)
+
+      assert_equal(123, obj.instance_variable_get(:@fixnum))
+      assert_equal("hello", obj.instance_variable_get(:@str))
+    RUBY
+  end
 end