check for dynamic_extent

Author: Werner Henze

include/gsl/span

@@ -582,7 +582,7 @@ public:
     template <std::size_t Count>
     constexpr span<element_type, Count> first() const noexcept
     {
-        static_assert(Count <= Extent,
+        static_assert(Extent == dynamic_extent || Count <= Extent,
                       "first() cannot extract more elements from a span than it contains.");
         Expects(Count <= size());
         return span<element_type, Count>{data(), Count};
@@ -594,7 +594,7 @@ public:
         // clang-format on
         constexpr span<element_type, Count> last() const noexcept
     {
-        static_assert(Count <= Extent,
+        static_assert(Extent == dynamic_extent || Count <= Extent,
                       "last() cannot extract more elements from a span than it contains.");
         Expects(Count <= size());
         return span<element_type, Count>{data() + (size() - Count), Count};
@@ -607,7 +607,8 @@ public:
         constexpr auto subspan() const noexcept ->
         typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
     {
-        static_assert(Extent >= Offset && (Count == dynamic_extent || Count <= Extent - Offset),
+        static_assert(Extent == dynamic_extent || (Extent >= Offset && (Count == dynamic_extent ||
+                                                                        Count <= Extent - Offset)),
                       "subspan() cannot extract more elements from a span than it contains.");
         Expects((size() >= Offset) && (Count == dynamic_extent || (Count <= size() - Offset)));
         using type =