[Bug #20389] Chilled string cannot be a shared root

string.c

@@ -1340,7 +1340,7 @@ rb_str_new_shared(VALUE str)
 VALUE
 rb_str_new_frozen(VALUE orig)
 {
-    if (OBJ_FROZEN(orig)) return orig;
+    if (RB_FL_TEST_RAW(orig, FL_FREEZE | STR_CHILLED) == FL_FREEZE) return orig;
     return str_new_frozen(rb_obj_class(orig), orig);
 }
 

test/ruby/test_string.rb

@@ -3648,6 +3648,18 @@ CODE
     Warning[:deprecated] = deprecated
   end
 
+  def test_chilled_string_substring
+    deprecated = Warning[:deprecated]
+    Warning[:deprecated] = false
+    chilled_string = eval('"a chilled string."')
+    substring = chilled_string[0..-1]
+    assert_equal("a chilled string.", substring)
+    chilled_string[0..-1] = "This string is defrosted."
+    assert_equal("a chilled string.", substring)
+  ensure
+    Warning[:deprecated] = deprecated
+  end
+
   private
 
   def assert_bytesplice_result(expected, s, *args)