libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221]

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.

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index 251fed9..b336332 100644 (file)
--- 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)
       { }