Implement P2821R5: Adding span::at()

Author: heckerpowered

stl/inc/span

@@ -495,6 +495,16 @@ public:
 #endif // ^^^ _ITERATOR_DEBUG_LEVEL == 0 ^^^
     }
 
+#if _HAS_CXX26
+    _NODISCARD constexpr reference at(size_type _Off) const {
+        if (_Off < _Mysize) {
+            _Xrange();
+        }
+
+        return _Mydata[_Off];
+    }
+#endif
+
 #if _HAS_CXX23
     _NODISCARD constexpr const_iterator cbegin() const noexcept {
         return begin();