https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116549
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Zhao Dai from comment #0)
> Hi, the following code fail to compile:
>
> ```
> // std=c++23
>
> #include <iterator>
>
> int main() {
> static_assert(not std::sized_sentinel_for<int*, int**>);
> using S = std::move_iterator<int*>;
> using I = std::move_iterator<int**>;
> static_assert(true == std::disable_sized_sentinel_for<S, I>); // error
> }
> ```
>
> When S is not a sized sentinel of I, we should not make
> std::move_iterator<S> a sized sentinel for std::move_iterator<I>.
We don't make it a sized sentinel though:
#include <iterator>
using S = std::move_iterator<int*>;
using I = std::move_iterator<int**>;
static_assert(not std::sized_sentinel_for<S, I>);
static_assert(not std::sized_sentinel_for<I, S>);
The way to tell if something is a sized sentinel is to test
std::sized_sentinel_for.
Testing std::disable_sized_sentinel_for is wrong. You cannot rely on that to
tell you if something is a sized sentinel. In the example above,
std::sentinel_for<S, I> is false, so
std::disable_sized_sentinel_for is never needed.
The resolution to LWG issue 3736 says we should add that specialization anyway,
but I think it's redundant and not observable by any sensible code that uses
std::sized_sentinel_for correctly. (This was noted in the issue discussion.)
I'll add the disable_sized_sentinel_for specialization, because the standard
says we should define it. But your example code is not something that anybody
should ever write in real code.
>
> According to cppreference
> (https://en.cppreference.com/w/cpp/iterator/move_iterator), there should be
> a specialisation of std::disable_sized_sentinel_for:
>
> ```
> template< class Iterator1, class Iterator2 >
> requires (!std::sized_sentinel_for<Iterator1, Iterator2>)
> constexpr bool disable_sized_sentinel_for<
> std::move_iterator<Iterator1>,
> std::move_iterator<Iterator2>> = true;
> ```
>
> But I couldn't find it in any of the current GCC source, especially
> *libstdc++-v3/include/bits/stl_iterator.h* where std::move_iterator is
> defined.
>
> Please confirm if it's a miss?
>
> Thanks,
> Zhao
