std::unreachable_sentinel_t,std::unreachable_sentinel(3)

NAME
std::unreachable_sentinel_t,std::unreachable_sentinel - std::unreachable_sentinel_t,std::unreachable_sentinel

Synopsis
   Defined in header <iterator>
   struct unreachable_sentinel_t; (1) (since C++20)
   inline constexpr unreachable_sentinel_t unreachable_sentinel{}; (2) (since C++20)
   1) unreachable_sentinel_t is an empty class type that can be used to denote the
   “upper bound” of an unbounded interval.
   2) unreachable_sentinel is a constant of type unreachable_sentinel_t.

Non-member functions
   operator== compares an unreachable_sentinel_t with a value of any
   (C++20) weakly_incrementable type
              (function template) operator==(std::unreachable_sentinel_t)
   template<std::weakly_incrementable I>
   friend constexpr bool operator==( unreachable_sentinel_t, const I& ) (since C++20)
   noexcept
   { return false; }
   unreachable_sentinel_t can be compared with any weakly_incrementable type and the
   result is always false.
   This function template is not visible to ordinary unqualified or qualified lookup,
   and can only be found by argument-dependent lookup when std::unreachable_sentinel_t
   is an associated class of the arguments.

Example
// Run this code
 #include <cstddef>
 #include <iterator>
 #include <algorithm>
 #include <iostream>
 template<class CharT>
 constexpr std::size_t strlen(const CharT *s)
 {
     return std::ranges::find(s, std::unreachable_sentinel, CharT{}) - s;
 }
 template<class CharT>
 constexpr std::size_t pos(const CharT *haystack, const CharT *needle)
 {
     // search(begin, unreachable_sentinel) is usually more efficient than
     // search(begin, end) due to one less comparison per cycle.
     // But "needle" MUST BE in the "haystack", otherwise the call is UB,
     // which is a compile-time error in constexpr context.
     return std::ranges::search(
                haystack, std::unreachable_sentinel,
                needle, needle + strlen(needle)
            ).begin() - haystack;
 }
 int main()
 {
     static_assert(
         strlen("The quick brown fox jumps over the lazy dog.") == 44
     );
     static_assert(
         pos("const short int", "short") == 6
     );
 // static_assert(pos("long int", "float")); // compile-time error
 }

See also
   ranges::iota_view a view consisting of a sequence generated by repeatedly
   views::iota incrementing an initial value
   (C++20) (class template) (customization point object)