the group is if the region is empty.
* Literals, variables, custom directives, and type directives are the only
valid elements within the group.
- - Any attribute variable may be used, but only optional attributes can be
- marked as the anchor.
+ - Any attribute variable may be used, but only optional or default-valued
+ attributes can be marked as the anchor. A default-valued anchor is
+ considered present if it holds a value other than the default.
- Only variadic or optional results and operand arguments and can be used.
- All region variables can be used. When a non-variable length region is
used, if the group is not present the region is empty.