[unreachable.sentinel] Incorrect usage example of std::unreachable_sentinel

[xmcgcg]

An incorrect description of an example was spotted on Stack Overflow (the comment under the question), [unreachable.sentinel] p2 says:

[Example 1 :

char* p;
// set p to point to a character buffer containing newlines
char* nl = find(p, unreachable_sentinel, '\n');

Provided a newline character really exists in the buffer, the use of unreachable_sentinel above potentially makes
the call to find more efficient since the loop test against the sentinel does not require a conditional branch. — end
example]

The description says that the example is well-defined if a null character is reachable from p, but std::ranges::find requires the input range to be valid, as [iterator.requirements.general] p12 says:

The result of the application of library functions to invalid ranges is undefined.

[jwakely]

This is a known issue (e.g. it's mentioned in the comments of LWG4019). The reality is that such infinite ranges do work, there's nothing in the library that actually exhibits undefined behaviour if you write this code. The rule requiring a valid range should probably be relaxed to only require a valid range for at least as many elements as will actually be iterated over. Otherwise there are almost no valid uses of unreachable_sentinel.

[jwakely]

I would be opposed to removing the example, because it does show how the unreachable sentinel is intended to be used. The fact that [iterator.requirements.general] is too strict to allow this is a defect, and that's what should be fixed. So not editorial.