std::ranges:: borrowed_iterator_t, std::ranges:: borrowed_subrange_t

From cppreference.com
C++
Ranges library
Range adaptors
Defined in header <ranges>
template < ranges:: range R >
using borrowed_iterator_t = /* see below */ ;
(1) (since C++20)
template < ranges:: range R >
using borrowed_subrange_t = /* see below */ ;
(2) (since C++20)
1) std :: ranges:: iterator_t < R > if R models borrowed_range , std::ranges::dangling otherwise.
2) std :: ranges:: subrange < std :: ranges:: iterator_t < R >> if R models borrowed_range , std::ranges::dangling otherwise.

These two alias templates are used by some constrained algorithms to avoid returning potentially dangling iterators or views.

Possible implementation

borrowed_iterator_t 
template< std::ranges::range R >
using borrowed_iterator_t = std::conditional_t<std::ranges::borrowed_range<R>,
    std::ranges::iterator_t<R>, std::ranges::dangling>;
borrowed_subrange_t 
template< std::ranges::range R >
using borrowed_subrange_t = std::conditional_t<std::ranges::borrowed_range<R>,
    std::ranges::subrange<std::ranges::iterator_t<R>>, std::ranges::dangling>;

See also

(C++20)
a placeholder type indicating that an iterator or a subrange should not be returned since it would be dangling
(class)
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