Copy non-inlined encoding index

string.c

@@ -1511,6 +1511,7 @@ str_duplicate(VALUE klass, VALUE str)
 	;
     VALUE flags = FL_TEST_RAW(str, flag_mask);
     VALUE dup = str_alloc(klass);
+    int encidx = 0;
     MEMCPY(RSTRING(dup)->as.ary, RSTRING(str)->as.ary,
 	   char, embed_size);
     if (flags & STR_NOEMBED) {
@@ -1530,7 +1531,12 @@ str_duplicate(VALUE klass, VALUE str)
 		   char, embed_size);
 	}
     }
+    if ((flags & ENCODING_MASK) == (ENCODING_INLINE_MAX<<ENCODING_SHIFT)) {
+	encidx = rb_enc_get_index(str);
+	flags &= ~ENCODING_MASK;
+    }
     FL_SET_RAW(dup, flags & ~FL_FREEZE);
+    if (encidx) rb_enc_associate_index(dup, encidx);
     return dup;
 }
 

test/ruby/test_encoding.rb

@@ -74,6 +74,8 @@ class TestEncoding < Test::Unit::TestCase
       e = Encoding.list.last
       format = "%d".force_encoding(e)
       assert_equal("0", format % 0)
+      assert_equal(e, format.dup.encoding)
+      assert_equal(e, (format*1).encoding)
     end;
   end