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

Author: peterzhu2118

gc.c

@@ -3441,6 +3441,8 @@ vm_weak_table_gen_ivar_foreach_too_complex_i(st_data_t _key, st_data_t value, st
 
     GC_ASSERT(!iter_data->weak_only);
 
+    if (SPECIAL_CONST_P((VALUE)value)) return ST_CONTINUE;
+
     return iter_data->callback((VALUE)value, iter_data->data);
 }
 

test/ruby/test_gc_compact.rb

@@ -452,4 +452,21 @@ def set_a
       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