Index: include/span
===================================================================
--- include/span
+++ include/span
@@ -445,10 +445,10 @@
 
     template <size_t _Offset, size_t _Count = dynamic_extent>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span<_Tp, dynamic_extent> subspan() const noexcept
+    constexpr span<element_type, dynamic_extent> subspan() const noexcept
     {
         _LIBCPP_ASSERT(_Offset <= size(), "Offset out of range in span::subspan()");
-        _LIBCPP_ASSERT(_Count == dynamic_extent || _Offset + _Count <= size(), "Count out of range in span::subspan()");
+        _LIBCPP_ASSERT(_Count == dynamic_extent || _Offset <= size() - _Count, "Count out of range in span::subspan()");
         return {data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
     }