On Tue, Jan 10, 2023 at 11:46:55AM +0000, Jonathan Wakely via Gcc-patches wrote: > Tested x86_64-linux. Pushed to trunk. > > -- >8 -- > > The default constructor has a constraint that is always false if > arithmetic on size_t values promotes to int. Rewrite the constraint > exactly as written in the standard, which works correctly. > > libstdc++-v3/ChangeLog: > > PR libstdc++/108221 > * include/std/span (span::span()): Un-simplify constraint to > work for size_t of lesser rank than int. > --- > libstdc++-v3/include/std/span | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span > index 251fed91abf..b336332b190 100644 > --- a/libstdc++-v3/include/std/span > +++ b/libstdc++-v3/include/std/span > @@ -145,7 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > > constexpr > span() noexcept > - requires ((_Extent + 1u) <= 1u) > + requires (_Extent == dynamic_extent || _Extent == 0) > : _M_ptr(nullptr), _M_extent(0) > { }
If it would be C++23 only, you could use ((_Extent + 1uz) <= 1uz). As this is evaluated at compile time only, it is unfortunate it is 3 operations compared to former 2, but not a big deal. If this was in code that would be emitted at runtime, GCC already optimizes (x == -1uz || x == 0) or (x == 0 || x == -1uz) to ((x + 1uz) <= 1uz) Jakub