I think removing the exiting comment ("For WITH RECURSIVE, we rearrange
the list elements...") is not appropriate as this explains subsequent
process, which is not changed after the patch.

Agreed -- the original comment still describes the work-array
and tree-walker setup that follows, which the patch does not
touch.  Restored it in place and moved the new ISO citation
comment to sit directly above the RPR rejection foreach so

ERRCODE_FEATURE_NOT_SUPPORTED should be ERRCODE_SYNTAX_ERROR instead?
IMO ERRCODE_FEATURE_NOT_SUPPORTED is used when the feature is defined
by the standard but PostgreSQL just has not implemented yet. In this
case the standard disllow RPR in recursive CTE.

Right, the standard explicitly prohibits this combination
rather than leaving it unimplemented.  Switched to
ERRCODE_SYNTAX_ERROR.  The .out files reference only the
ERROR message so the regress expected output is unchanged.

Thanks,
Henson