Defined in header
1) Same as ranges::iterator_t when
Rmodels exposition-only concept __ForwardingRange, otherwise yields ranges::dangling instead.
2) Similar to (1), but it yields a specialization of ranges::subrange when the the same condition is met.
These two alias templates are used by some constrained algorithms to avoid returning potentially dangling iterators or views.
 See also
| a placeholder type indicating that an iterator or a |