String#succ will return subclass instance.

Test Script:
{{{
require 'test/unit/assertions.rb'
include Test::Unit::Assertions

class SubStr < String
  def value
    "value : " + self
  end
end

str = SubStr.new("abcd")
assert_equal(SubStr, str.succ.class)

puts :ok
}}}

string.c

@@ -5071,6 +5071,7 @@ rstr_succ(VALUE str, SEL sel)
     VALUE newstr = rb_enc_str_new(chars_ptr, len, RSTR(str)->encoding);
     free(chars_buf);
     OBJ_INFECT(newstr, str);
+    SET_CLASS(newstr, str);
 
     return newstr;
 }