| Issue |
91952
|
| Summary |
`<algorithm>`: `ranges::ends_with`'s constraint check for bidirectional case is not quite right
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
hewillk
|
`ranges::ends_with` only checks whether the two sentinel types satisfy `bidirectional_iterator` then unconditionally warps all iterators and sentinels into `reverse_iterator`s.
https://github.com/llvm/llvm-project/blob/19a62fbe00930d7eaa9f948c8dd26d58f5422c00/libcxx/include/__algorithm/ranges_ends_with.h#L67-L69
There are two implicit issues here. First, theoretically, the iterator can still not satisfy `bidirectional_iterator` in such case, which makes the construction of `reverse_iterator` ill-formed. Second, the reference of sentinels and iterators may not be the same, which makes the predicate and projection functions not necessarily applicable to the `reverse_iterator`.
The following demos the issue:
```cpp
#include <algorithm>
#include <list>
struct Proj {
int operator()(auto&);
int operator()(const auto&) = delete;
};
int main() {
std::list<int> l;
std::ranges::subrange s{l.begin(), l.cend()};
return std::ranges::ends_with(s, s, {}, Proj{}, Proj{}); // hard error in libc++
}
```
https://godbolt.org/z/xrMdreWEv
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