Keep hidden local variables when dumping and loading iseqs

Fixes [Bug #19975]

compile.c

@@ -11863,6 +11863,10 @@ ibf_load_id(const struct ibf_load *load, const ID id_index)
         return 0;
     }
     VALUE sym = ibf_load_object(load, id_index);
+    if (rb_type_p(sym, T_FIXNUM)) {
+        /* Load hidden local variables as indexes */
+        return FIX2INT(sym);
+    }
     return rb_sym2id(sym);
 }
 
@@ -12386,7 +12390,12 @@ ibf_dump_local_table(struct ibf_dump *dump, const rb_iseq_t *iseq)
     int i;
 
     for (i=0; i<size; i++) {
-        table[i] = ibf_dump_id(dump, body->local_table[i]);
+        VALUE v = ibf_dump_id(dump, body->local_table[i]);
+        if (v == 0) {
+            /* Dump hidden local variables as indexes, so load_from_binary will work with them */
+            v = ibf_dump_object(dump, ULONG2NUM(size-i+1));
+        }
+        table[i] = v;
     }
 
     IBF_W_ALIGN(ID);

test/ruby/test_iseq.rb

@@ -566,6 +566,23 @@ class TestISeq < Test::Unit::TestCase
     iseq2
   end
 
+  def test_to_binary_with_hidden_local_variables
+    assert_iseq_to_binary("for foo in bar; end")
+
+    bin = RubyVM::InstructionSequence.compile(<<-RUBY).to_binary
+      Object.new.instance_eval do
+        a = []
+        def self.bar; [1] end
+        for foo in bar
+          a << (foo * 2)
+        end
+        a
+      end
+    RUBY
+    v = RubyVM::InstructionSequence.load_from_binary(bin).eval
+    assert_equal([2], v)
+  end
+
   def test_to_binary_with_objects
     assert_iseq_to_binary("[]"+100.times.map{|i|"<</#{i}/"}.join)
     assert_iseq_to_binary("@x ||= (1..2)")