o allows converting indices to the proper type if required (such as
     variant, mantis #20873)
   o do not create temporary defs for this type checking anymore if not
     necessary
   o makes sure that errors are thrown in case of conversions considered
     as invalid by the compiler rather than that wrong code is silently
     generated (such as in mantis #20873 before this change)

git-svn-id: trunk@20108 -