diff --git a/include/gsl/gsl_algorithm b/include/gsl/gsl_algorithm
index c2ba31f..6813b6c 100644
--- a/include/gsl/gsl_algorithm
+++ b/include/gsl/gsl_algorithm
@@ -37,8 +37,8 @@ namespace gsl
 {
 // Note: this will generate faster code than std::copy using span iterator in older msvc+stl
 // not necessary for msvc since VS2017 15.8 (_MSC_VER >= 1915)
-template <class SrcElementType, std::ptrdiff_t SrcExtent, class DestElementType,
-          std::ptrdiff_t DestExtent>
+template <class SrcElementType, std::size_t SrcExtent, class DestElementType,
+          std::size_t DestExtent>
 void copy(span<SrcElementType, SrcExtent> src, span<DestElementType, DestExtent> dest)
 {
     static_assert(std::is_assignable<decltype(*dest.data()), decltype(*src.data())>::value,
diff --git a/include/gsl/span b/include/gsl/span
index b3e5349..0d8dcb0 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -25,17 +25,17 @@
 #include <array>     // for array
 #include <cstddef>   // for ptrdiff_t, size_t, nullptr_t
 #include <iterator>  // for reverse_iterator, distance, random_access_...
-#include <limits>
 #include <stdexcept>
 #include <type_traits> // for enable_if_t, declval, is_convertible, inte...
 #include <utility>
-#include <memory> // for std::addressof
 
 #if defined(_MSC_VER) && !defined(__clang__)
 #pragma warning(push)
 
 // turn off some warnings that are noisy about our Expects statements
 #pragma warning(disable : 4127) // conditional expression is constant
+#pragma warning(                                                                                   \
+    disable : 4146) // unary minus operator applied to unsigned type, result still unsigned
 #pragma warning(disable : 4702) // unreachable code
 
 // Turn MSVC /analyze rules that generate too much noise. TODO: fix in the tool.
@@ -70,9 +70,9 @@ namespace gsl
 {
 
 // [views.constants], constants
-constexpr const std::ptrdiff_t dynamic_extent = -1;
+constexpr const std::size_t dynamic_extent = static_cast<std::size_t>(-1);
 
-template <class ElementType, std::ptrdiff_t Extent = dynamic_extent>
+template <class ElementType, std::size_t Extent = dynamic_extent>
 class span;
 
 // implementation details
@@ -83,7 +83,7 @@ namespace details
     {
     };
 
-    template <class ElementType, std::ptrdiff_t Extent>
+    template <class ElementType, std::size_t Extent>
     struct is_span_oracle<gsl::span<ElementType, Extent>> : std::true_type
     {
     };
@@ -104,156 +104,189 @@ namespace details
     };
 
     template <class T>
-    struct is_std_array : public is_std_array_oracle<std::remove_cv_t<T>>
+    struct is_std_array : is_std_array_oracle<std::remove_cv_t<T>>
     {
     };
 
-    template <std::ptrdiff_t From, std::ptrdiff_t To>
+    template <std::size_t From, std::size_t To>
     struct is_allowed_extent_conversion
-        : public std::integral_constant<bool, From == To || From == gsl::dynamic_extent ||
-                                                  To == gsl::dynamic_extent>
+        : std::integral_constant<bool, From == To || To == gsl::dynamic_extent>
     {
     };
 
     template <class From, class To>
     struct is_allowed_element_type_conversion
-        : public std::integral_constant<bool, std::is_convertible<From (*)[], To (*)[]>::value>
+        : std::integral_constant<bool, std::is_convertible<From (*)[], To (*)[]>::value>
     {
     };
 
-    template <class Span, bool IsConst>
+    template <class Type>
     class span_iterator
     {
-        using element_type_ = typename Span::element_type;
-
     public:
-#ifdef _MSC_VER
-        // Tell Microsoft standard library that span_iterators are checked.
-        using _Unchecked_type = typename Span::pointer;
-#endif
-
         using iterator_category = std::random_access_iterator_tag;
-        using value_type = std::remove_cv_t<element_type_>;
-        using difference_type = typename Span::index_type;
+        using value_type = std::remove_cv_t<Type>;
+        using difference_type = std::ptrdiff_t;
+        using pointer = Type*;
+        using reference = Type&;
 
-        using reference = std::conditional_t<IsConst, const element_type_, element_type_>&;
-        using pointer = std::add_pointer_t<reference>;
+#ifdef _MSC_VER
+        using _Unchecked_type = pointer;
+#endif // _MSC_VER
+        constexpr span_iterator() = default;
 
-        span_iterator() = default;
-
-        constexpr span_iterator(const Span* span, difference_type idx) noexcept
-            : span_(span), index_(idx)
+        constexpr span_iterator(pointer begin, pointer end, pointer current)
+            : begin_(begin), end_(end), current_(current)
         {}
 
-        friend span_iterator<Span, true>;
-        template <bool B, std::enable_if_t<!B && IsConst>* = nullptr>
-        constexpr span_iterator(const span_iterator<Span, B>& other) noexcept
-            : span_iterator(other.span_, other.index_)
-        {}
-
-        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-        constexpr reference operator*() const
+        constexpr operator span_iterator<const Type>() const noexcept
         {
-            Expects(index_ != span_->size());
-            return *(span_->data() + index_);
+            return {begin_, end_, current_};
         }
 
-        constexpr pointer operator->() const
+        constexpr reference operator*() const noexcept
         {
-            Expects(index_ != span_->size());
-            return span_->data() + index_;
+            Expects(begin_ && end_);
+            Expects(begin_ <= current_ && current_ < end_);
+            return *current_;
         }
 
+        constexpr pointer operator->() const noexcept
+        {
+            Expects(begin_ && end_);
+            Expects(begin_ <= current_ && current_ < end_);
+            return current_;
+        }
         constexpr span_iterator& operator++() noexcept
         {
-            Expects(0 <= index_ && index_ != span_->size());
-            ++index_;
+            Expects(begin_ && current_ && end_);
+            Expects(current_ < end_);
+            ++current_;
             return *this;
         }
 
         constexpr span_iterator operator++(int) noexcept
         {
-            auto ret = *this;
-            ++(*this);
+            span_iterator ret = *this;
+            ++*this;
             return ret;
         }
 
         constexpr span_iterator& operator--() noexcept
         {
-            Expects(index_ != 0 && index_ <= span_->size());
-            --index_;
+            Expects(begin_ && end_);
+            Expects(begin_ < current_);
+            --current_;
             return *this;
         }
 
         constexpr span_iterator operator--(int) noexcept
         {
-            auto ret = *this;
-            --(*this);
+            span_iterator ret = *this;
+            --*this;
             return ret;
         }
 
-        constexpr span_iterator operator+(difference_type n) const noexcept
+        constexpr span_iterator& operator+=(const difference_type n) noexcept
         {
-            auto ret = *this;
-            return ret += n;
+            if (n != 0) Expects(begin_ && current_ && end_);
+            if (n > 0) Expects(end_ - current_ >= n);
+            if (n < 0) Expects(current_ - begin_ >= -n);
+            current_ += n;
+            return *this;
         }
 
-        friend constexpr span_iterator operator+(difference_type n, span_iterator const& rhs) noexcept
+        constexpr span_iterator operator+(const difference_type n) const noexcept
+        {
+            span_iterator ret = *this;
+            ret += n;
+            return ret;
+        }
+
+        friend constexpr span_iterator operator+(const difference_type n,
+                                                 const span_iterator& rhs) noexcept
         {
             return rhs + n;
         }
 
-        constexpr span_iterator& operator+=(difference_type n) noexcept
+        constexpr span_iterator& operator-=(const difference_type n) noexcept
         {
-            Expects((index_ + n) >= 0 && (index_ + n) <= span_->size());
-            index_ += n;
+            if (n != 0) Expects(begin_ && current_ && end_);
+            if (n > 0) Expects(current_ - begin_ >= n);
+            if (n < 0) Expects(end_ - current_ >= -n);
+            current_ -= n;
             return *this;
         }
 
-        constexpr span_iterator operator-(difference_type n) const noexcept
+        constexpr span_iterator operator-(const difference_type n) const noexcept
         {
-            auto ret = *this;
-            return ret -= n;
+            span_iterator ret = *this;
+            ret -= n;
+            return ret;
         }
 
-        constexpr span_iterator& operator-=(difference_type n) noexcept { return *this += -n; }
-
-        constexpr difference_type operator-(span_iterator rhs) const noexcept
+        template <
+            class Type2,
+            std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value, int> = 0>
+        constexpr difference_type operator-(const span_iterator<Type2>& rhs) const noexcept
         {
-            Expects(span_ == rhs.span_);
-            return index_ - rhs.index_;
+            Expects(begin_ == rhs.begin_ && end_ == rhs.end_);
+            return current_ - rhs.current_;
         }
 
-        constexpr reference operator[](difference_type n) const noexcept { return *(*this + n); }
-
-        constexpr friend bool operator==(span_iterator lhs, span_iterator rhs) noexcept
+        constexpr reference operator[](const difference_type n) const noexcept
         {
-            return lhs.span_ == rhs.span_ && lhs.index_ == rhs.index_;
+            return *(*this + n);
         }
 
-        constexpr friend bool operator!=(span_iterator lhs, span_iterator rhs) noexcept
+        template <
+            class Type2,
+            std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value, int> = 0>
+        constexpr bool operator==(const span_iterator<Type2>& rhs) const noexcept
         {
-            return !(lhs == rhs);
+            Expects(begin_ == rhs.begin_ && end_ == rhs.end_);
+            return current_ == rhs.current_;
         }
 
-        constexpr friend bool operator<(span_iterator lhs, span_iterator rhs) noexcept
+        template <
+            class Type2,
+            std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value, int> = 0>
+        constexpr bool operator!=(const span_iterator<Type2>& rhs) const noexcept
         {
-            return lhs.index_ < rhs.index_;
+            return !(*this == rhs);
         }
 
-        constexpr friend bool operator<=(span_iterator lhs, span_iterator rhs) noexcept
+        template <
+            class Type2,
+            std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value, int> = 0>
+        constexpr bool operator<(const span_iterator<Type2>& rhs) const noexcept
         {
-            return !(rhs < lhs);
+            Expects(begin_ == rhs.begin_ && end_ == rhs.end_);
+            return current_ < rhs.current_;
         }
 
-        constexpr friend bool operator>(span_iterator lhs, span_iterator rhs) noexcept
+        template <
+            class Type2,
+            std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value, int> = 0>
+        constexpr bool operator>(const span_iterator<Type2>& rhs) const noexcept
         {
-            return rhs < lhs;
+            return rhs < *this;
         }
 
-        constexpr friend bool operator>=(span_iterator lhs, span_iterator rhs) noexcept
+        template <
+            class Type2,
+            std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value, int> = 0>
+        constexpr bool operator<=(const span_iterator<Type2>& rhs) const noexcept
         {
-            return !(rhs > lhs);
+            return !(rhs < *this);
+        }
+
+        template <
+            class Type2,
+            std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value, int> = 0>
+        constexpr bool operator>=(const span_iterator<Type2>& rhs) const noexcept
+        {
+            return !(*this < rhs);
         }
 
 #ifdef _MSC_VER
@@ -262,22 +295,26 @@ namespace details
         // algorithm calls
         friend constexpr void _Verify_range(span_iterator lhs, span_iterator rhs) noexcept
         { // test that [lhs, rhs) forms a valid range inside an STL algorithm
-            Expects(lhs.span_ == rhs.span_        // range spans have to match
-                    && lhs.index_ <= rhs.index_); // range must not be transposed
+            Expects(lhs.begin_ == rhs.begin_ // range spans have to match
+                    && lhs.end_ == rhs.end_ &&
+                    lhs.current_ <= rhs.current_); // range must not be transposed
         }
 
         constexpr void _Verify_offset(const difference_type n) const noexcept
-        { // test that the iterator *this + n is a valid range in an STL
-            // algorithm call
-            Expects((index_ + n) >= 0 && (index_ + n) <= span_->size());
+        { // test that *this + n is within the range of this call
+            if (n != 0) Expects(begin_ && current_ && end_);
+            if (n > 0) Expects(end_ - current_ >= n);
+            if (n < 0) Expects(current_ - begin_ >= -n);
         }
 
+        // clang-format off
         GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
+        // clang-format on
         constexpr pointer _Unwrapped() const noexcept
         { // after seeking *this to a high water mark, or using one of the
             // _Verify_xxx functions above, unwrap this span_iterator to a raw
             // pointer
-            return span_->data() + index_;
+            return current_;
         }
 
         // Tell the STL that span_iterator should not be unwrapped if it can't
@@ -287,61 +324,64 @@ namespace details
 #else
         static constexpr bool _Unwrap_when_unverified = false;
 #endif
+        // clang-format off
         GSL_SUPPRESS(con.3) // NO-FORMAT: attribute // TODO: false positive
+        // clang-format on
         constexpr void _Seek_to(const pointer p) noexcept
         { // adjust the position of *this to previously verified location p
             // after _Unwrapped
-            index_ = p - span_->data();
+            current_ = p;
         }
 #endif
 
-    protected:
-        const Span* span_ = nullptr;
-        std::ptrdiff_t index_ = 0;
+        pointer begin_ = nullptr;
+        pointer end_ = nullptr;
+        pointer current_ = nullptr;
     };
 
-    template <std::ptrdiff_t Ext>
+    template <std::size_t Ext>
     class extent_type
     {
     public:
-        using index_type = std::ptrdiff_t;
+        using size_type = std::size_t;
 
-        static_assert(Ext >= 0, "A fixed-size span must be >= 0 in size.");
+        constexpr extent_type() noexcept = default;
 
-        constexpr extent_type() noexcept {}
-
-        template <index_type Other>
+        template <size_type Other>
         constexpr extent_type(extent_type<Other> ext)
         {
-            static_assert(Other == Ext || Other == dynamic_extent,
+            static_assert(Other == Ext,
                           "Mismatch between fixed-size extent and size of initializing data.");
             Expects(ext.size() == Ext);
         }
 
-        constexpr extent_type(index_type size) { Expects(size == Ext); }
+        constexpr extent_type(size_type size) { Expects(size == Ext); }
 
-        constexpr index_type size() const noexcept { return Ext; }
+        constexpr size_type size() const noexcept { return Ext; }
     };
 
     template <>
     class extent_type<dynamic_extent>
     {
     public:
-        using index_type = std::ptrdiff_t;
+        using size_type = std::size_t;
 
-        template <index_type Other>
+        template <size_type Other>
         explicit constexpr extent_type(extent_type<Other> ext) : size_(ext.size())
         {}
 
-        explicit constexpr extent_type(index_type size) : size_(size) { Expects(size >= 0); }
+        explicit constexpr extent_type(size_type size) : size_(size)
+        {
+            Expects(size != dynamic_extent);
+        }
 
-        constexpr index_type size() const noexcept { return size_; }
+        constexpr size_type size() const noexcept { return size_; }
 
     private:
-        index_type size_;
+        size_type size_;
     };
 
-    template <class ElementType, std::ptrdiff_t Extent, std::ptrdiff_t Offset, std::ptrdiff_t Count>
+    template <class ElementType, std::size_t Extent, std::size_t Offset, std::size_t Count>
     struct calculate_subspan_type
     {
         using type = span<ElementType, Count != dynamic_extent
@@ -351,95 +391,97 @@ namespace details
 } // namespace details
 
 // [span], class template span
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 class span
 {
 public:
     // constants and types
     using element_type = ElementType;
     using value_type = std::remove_cv_t<ElementType>;
-    using index_type = std::ptrdiff_t;
+    using size_type = std::size_t;
     using pointer = element_type*;
+    using const_pointer = const element_type*;
     using reference = element_type&;
+    using const_reference = const element_type&;
+    using difference_type = std::ptrdiff_t;
 
-    using iterator = details::span_iterator<span<ElementType, Extent>, false>;
-    using const_iterator = details::span_iterator<span<ElementType, Extent>, true>;
+    using iterator = details::span_iterator<ElementType>;
+    using const_iterator = details::span_iterator<const ElementType>;
     using reverse_iterator = std::reverse_iterator<iterator>;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
-    using size_type = index_type;
-
 #if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
-    static constexpr const index_type extent{Extent};
+    static constexpr const size_type extent{Extent};
 #else
-    static constexpr index_type extent{Extent};
+    static constexpr size_type extent{Extent};
 #endif
 
     // [span.cons], span constructors, copy, assignment, and destructor
     template <bool Dependent = false,
-              // "Dependent" is needed to make "std::enable_if_t<Dependent || Extent <= 0>" SFINAE,
-              // since "std::enable_if_t<Extent <= 0>" is ill-formed when Extent is greater than 0.
-              class = std::enable_if_t<(Dependent || Extent <= 0)>>
+              // "Dependent" is needed to make "std::enable_if_t<Dependent || Extent == 0 || Extent
+              // == dynamic_extent>" SFINAE, since "std::enable_if_t<Extent == 0 || Extent ==
+              // dynamic_extent>" is ill-formed when Extent is greater than 0.
+              class = std::enable_if_t<(Dependent ||
+                                        details::is_allowed_extent_conversion<0, Extent>::value)>>
     constexpr span() noexcept : storage_(nullptr, details::extent_type<0>())
     {}
 
-    constexpr span(pointer ptr, index_type count) noexcept: storage_(ptr, count) {}
+    constexpr span(pointer ptr, size_type count) noexcept : storage_(ptr, count)
+    {
+        if (Extent != dynamic_extent) Expects(count == Extent);
+    }
 
     constexpr span(pointer firstElem, pointer lastElem) noexcept
-        : storage_(firstElem, std::distance(firstElem, lastElem))
-    {}
+        : storage_(firstElem, static_cast<std::size_t>(lastElem - firstElem))
+    {
+        if (Extent != dynamic_extent)
+        { Expects(lastElem - firstElem == static_cast<difference_type>(Extent)); }
+    }
 
-    template <std::size_t N>
+    template <std::size_t N,
+              std::enable_if_t<details::is_allowed_extent_conversion<N, Extent>::value, int> = 0>
     constexpr span(element_type (&arr)[N]) noexcept
-        : storage_(KnownNotNull{std::addressof(arr[0])}, details::extent_type<N>())
+        : storage_(KnownNotNull{arr + 0}, details::extent_type<N>())
     {}
 
-    template <std::size_t N, class = std::enable_if_t<(N > 0)>>
-    constexpr span(std::array<std::remove_const_t<element_type>, N>& arr) noexcept
+    template <std::size_t N,
+              std::enable_if_t<details::is_allowed_extent_conversion<N, Extent>::value, int> = 0>
+    constexpr span(std::array<element_type, N>& arr) noexcept
         : storage_(KnownNotNull{arr.data()}, details::extent_type<N>())
-    {
-    }
+    {}
 
-    constexpr span(std::array<std::remove_const_t<element_type>, 0>&) noexcept
-        : storage_(static_cast<pointer>(nullptr), details::extent_type<0>())
-    {
-    }
-
-    template <std::size_t N, class = std::enable_if_t<(N > 0)>>
-    constexpr span(const std::array<std::remove_const_t<element_type>, N>& arr) noexcept
+    template <std::size_t N,
+              std::enable_if_t<(details::is_allowed_extent_conversion<N, Extent>::value &&
+                                details::is_allowed_element_type_conversion<const value_type,
+                                                                            element_type>::value),
+                               int> = 0>
+    constexpr span(const std::array<value_type, N>& arr) noexcept
         : storage_(KnownNotNull{arr.data()}, details::extent_type<N>())
-    {
-    }
+    {}
 
-    constexpr span(const std::array<std::remove_const_t<element_type>, 0>&) noexcept
-        : storage_(static_cast<pointer>(nullptr), details::extent_type<0>())
-    {
-    }
-
-    // NB: the SFINAE here uses .data() as a incomplete/imperfect proxy for the requirement
+    // NB: the SFINAE here uses .data() as an incomplete/imperfect proxy for the requirement
     // on Container to be a contiguous sequence container.
     template <class Container,
               class = std::enable_if_t<
                   !details::is_span<Container>::value && !details::is_std_array<Container>::value &&
-                  std::is_convertible<typename Container::pointer, pointer>::value &&
-                  std::is_convertible<typename Container::pointer,
-                                      decltype(std::declval<Container>().data())>::value>>
-    constexpr span(Container& cont) noexcept : span(cont.data(), narrow<index_type>(cont.size()))
+                  std::is_pointer<decltype(std::declval<Container&>().data())>::value &&
+                  std::is_convertible<std::remove_pointer_t<decltype(std::declval<Container&>().data())>(*)[], element_type(*)[]>::value>>
+    constexpr span(Container& cont) noexcept : span(cont.data(), cont.size())
     {}
 
     template <class Container,
               class = std::enable_if_t<
                   std::is_const<element_type>::value && !details::is_span<Container>::value &&
-                  std::is_convertible<typename Container::pointer, pointer>::value &&
-                  std::is_convertible<typename Container::pointer,
-                                      decltype(std::declval<Container>().data())>::value>>
-    constexpr span(const Container& cont) noexcept : span(cont.data(), narrow<index_type>(cont.size()))
+                  !details::is_std_array<Container>::value &&
+                  std::is_pointer<decltype(std::declval<const Container&>().data())>::value &&
+                  std::is_convertible<std::remove_pointer_t<decltype(std::declval<const Container&>().data())>(*)[], element_type(*)[]>::value>>
+    constexpr span(const Container& cont) noexcept : span(cont.data(), cont.size())
     {}
 
     constexpr span(const span& other) noexcept = default;
 
     template <
-        class OtherElementType, std::ptrdiff_t OtherExtent,
+        class OtherElementType, std::size_t OtherExtent,
         class = std::enable_if_t<
             details::is_allowed_extent_conversion<OtherExtent, Extent>::value &&
             details::is_allowed_element_type_conversion<OtherElementType, element_type>::value>>
@@ -451,62 +493,72 @@ public:
     constexpr span& operator=(const span& other) noexcept = default;
 
     // [span.sub], span subviews
-    template <std::ptrdiff_t Count>
+    template <std::size_t Count>
     constexpr span<element_type, Count> first() const noexcept
     {
-        Expects(Count >= 0 && Count <= size());
+        Expects(Count <= size());
         return {data(), Count};
     }
 
-    template <std::ptrdiff_t Count>
+    template <std::size_t Count>
+    // clang-format off
     GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
+    // clang-format on
     constexpr span<element_type, Count> last() const noexcept
     {
-        Expects(Count >= 0 && size() - Count >= 0);
+        Expects(Count <= size());
         return {data() + (size() - Count), Count};
     }
 
-    template <std::ptrdiff_t Offset, std::ptrdiff_t Count = dynamic_extent>
+    template <std::size_t Offset, std::size_t Count = dynamic_extent>
+    // clang-format off
     GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
+    // clang-format on
     constexpr auto subspan() const noexcept ->
-        typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
+    typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
     {
-        Expects((Offset >= 0 && size() - Offset >= 0) &&
-                (Count == dynamic_extent || (Count >= 0 && Offset + Count <= size())));
+        Expects((size() >= Offset) && (Count == dynamic_extent || (Count <= size() - Offset)));
 
         return {data() + Offset, Count == dynamic_extent ? size() - Offset : Count};
     }
 
-    constexpr span<element_type, dynamic_extent> first(index_type count) const noexcept
+    constexpr span<element_type, dynamic_extent> first(size_type count) const noexcept
     {
-        Expects(count >= 0 && count <= size());
+        Expects(count <= size());
         return {data(), count};
     }
 
-    constexpr span<element_type, dynamic_extent> last(index_type count) const noexcept
+    constexpr span<element_type, dynamic_extent> last(size_type count) const noexcept
     {
+        Expects(count <= size());
         return make_subspan(size() - count, dynamic_extent, subspan_selector<Extent>{});
     }
 
-    constexpr span<element_type, dynamic_extent> subspan(index_type offset,
-                                                         index_type count = dynamic_extent) const noexcept
+    constexpr span<element_type, dynamic_extent> subspan(size_type offset,
+                                                         size_type count = dynamic_extent) const
+        noexcept
     {
         return make_subspan(offset, count, subspan_selector<Extent>{});
     }
 
     // [span.obs], span observers
-    constexpr index_type size() const noexcept { return storage_.size(); }
-    constexpr index_type size_bytes() const noexcept
+    constexpr size_type size() const noexcept { return storage_.size(); }
+
+    constexpr size_type size_bytes() const noexcept
     {
-        return size() * narrow_cast<index_type>(sizeof(element_type));
+        Expects(size() < dynamic_extent / sizeof(element_type));
+        return size() * sizeof(element_type);
     }
+
     constexpr bool empty() const noexcept { return size() == 0; }
 
     // [span.elem], span element access
+    // clang-format off
     GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-    constexpr reference operator[](index_type idx) const noexcept
+    // clang-format on
+    constexpr reference operator[](size_type idx) const noexcept
     {
-        Expects(CheckRange(idx, storage_.size()));
+        Expects(idx < size());
         return data()[idx];
     }
 
@@ -521,23 +573,35 @@ public:
         Expects(size() > 0);
         return data()[size() - 1];
     }
-    
-    // at and operator() are deprecated to align to the public member functions of std::span
-    [[deprecated("Use operator[]")]]
-    constexpr reference at(index_type idx) const noexcept{ return this->operator[](idx); }
-    [[deprecated("Use operator[]")]]
-    constexpr reference operator()(index_type idx) const noexcept{ return this->operator[](idx); }
-
-
 
     constexpr pointer data() const noexcept { return storage_.data(); }
 
     // [span.iter], span iterator support
-    constexpr iterator begin() const noexcept { return {this, 0}; }
-    constexpr iterator end() const noexcept { return {this, size()}; }
+    constexpr iterator begin() const noexcept
+    {
+        const auto data = storage_.data();
+        return {data, data + size(), data};
+    }
 
-    constexpr const_iterator cbegin() const noexcept { return {this, 0}; }
-    constexpr const_iterator cend() const noexcept { return {this, size()}; }
+    constexpr iterator end() const noexcept
+    {
+        const auto data = storage_.data();
+        const auto endData = data + storage_.size();
+        return {data, endData, endData};
+    }
+
+    constexpr const_iterator cbegin() const noexcept
+    {
+        const auto data = storage_.data();
+        return {data, data + size(), data};
+    }
+
+    constexpr const_iterator cend() const noexcept
+    {
+        const auto data = storage_.data();
+        const auto endData = data + storage_.size();
+        return {data, endData, endData};
+    }
 
     constexpr reverse_iterator rbegin() const noexcept { return reverse_iterator{end()}; }
     constexpr reverse_iterator rend() const noexcept { return reverse_iterator{begin()}; }
@@ -556,38 +620,14 @@ public:
     constexpr pointer _Unchecked_begin() const noexcept { return data(); }
     constexpr pointer _Unchecked_end() const noexcept
     {
+        // clang-format off
         GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
+        // clang-format on
         return data() + size();
     }
 #endif // _MSC_VER
 
 private:
-    static constexpr bool CheckRange(index_type idx, index_type size) noexcept
-    {
-        // Optimization:
-        //
-        // idx >= 0 && idx < size
-        // =>
-        // static_cast<size_t>(idx) < static_cast<size_t>(size)
-        //
-        // because size >=0 by span construction, and negative idx will
-        // wrap around to a value always greater than size when casted.
-
-        // check if we have enough space to wrap around
-#if defined(__cpp_if_constexpr)
-        if constexpr (sizeof(index_type) <= sizeof(size_t))
-#else
-        if (sizeof(index_type) <= sizeof(size_t))
-#endif
-        {
-            return narrow_cast<size_t>(idx) < narrow_cast<size_t>(size);
-        }
-        else
-        {
-            return idx >= 0 && idx < size;
-        }
-    }
-
     // Needed to remove unnecessary null check in subspans
     struct KnownNotNull
     {
@@ -607,13 +647,13 @@ private:
         constexpr storage_type(KnownNotNull data, OtherExtentType ext)
             : ExtentType(ext), data_(data.p)
         {
-            Expects(ExtentType::size() >= 0);
+            Expects(ExtentType::size() != dynamic_extent);
         }
 
         template <class OtherExtentType>
         constexpr storage_type(pointer data, OtherExtentType ext) : ExtentType(ext), data_(data)
         {
-            Expects(ExtentType::size() >= 0);
+            Expects(ExtentType::size() != dynamic_extent);
             Expects(data || ExtentType::size() == 0);
         }
 
@@ -627,71 +667,87 @@ private:
 
     // The rest is needed to remove unnecessary null check
     // in subspans and constructors from arrays
-    constexpr span(KnownNotNull ptr, index_type count) : storage_(ptr, count) {}
+    constexpr span(KnownNotNull ptr, size_type count) noexcept : storage_(ptr, count) {}
 
-    template <std::ptrdiff_t CallerExtent>
+    template <std::size_t CallerExtent>
     class subspan_selector
     {
     };
 
-    template <std::ptrdiff_t CallerExtent>
-    span<element_type, dynamic_extent> make_subspan(index_type offset, index_type count,
-                                                    subspan_selector<CallerExtent>) const
+    template <std::size_t CallerExtent>
+    constexpr span<element_type, dynamic_extent> make_subspan(size_type offset, size_type count,
+                                                              subspan_selector<CallerExtent>) const noexcept
     {
         const span<element_type, dynamic_extent> tmp(*this);
         return tmp.subspan(offset, count);
     }
 
+    // clang-format off
     GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
-    span<element_type, dynamic_extent> make_subspan(index_type offset, index_type count,
-                                                    subspan_selector<dynamic_extent>) const
+    // clang-format on
+    constexpr span<element_type, dynamic_extent>
+    make_subspan(size_type offset, size_type count, subspan_selector<dynamic_extent>) const noexcept
     {
-        Expects(offset >= 0 && size() - offset >= 0);
+        Expects(size() >= offset);
 
         if (count == dynamic_extent) { return {KnownNotNull{data() + offset}, size() - offset}; }
 
-        Expects(count >= 0 && size() - offset >= count);
+        Expects(size() - offset >= count);
         return {KnownNotNull{data() + offset}, count};
     }
 };
 
+#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+
+// Deduction Guides
+template <class Type, std::size_t Extent>
+span(Type (&)[Extent])->span<Type, Extent>;
+
+template <class Type, std::size_t Size>
+span(std::array<Type, Size>&)->span<Type, Size>;
+
+template <class Type, std::size_t Size>
+span(const std::array<Type, Size>&)->span<const Type, Size>;
+
+#endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L) )
+
 #if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr const typename span<ElementType, Extent>::index_type span<ElementType, Extent>::extent;
+template <class ElementType, std::size_t Extent>
+constexpr const typename span<ElementType, Extent>::size_type span<ElementType, Extent>::extent;
 #endif
 
 // [span.comparison], span comparison operators
-template <class ElementType, std::ptrdiff_t FirstExtent, std::ptrdiff_t SecondExtent>
+template <class ElementType, std::size_t FirstExtent, std::size_t SecondExtent>
 constexpr bool operator==(span<ElementType, FirstExtent> l, span<ElementType, SecondExtent> r)
 {
     return std::equal(l.begin(), l.end(), r.begin(), r.end());
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 constexpr bool operator!=(span<ElementType, Extent> l, span<ElementType, Extent> r)
 {
     return !(l == r);
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 constexpr bool operator<(span<ElementType, Extent> l, span<ElementType, Extent> r)
 {
     return std::lexicographical_compare(l.begin(), l.end(), r.begin(), r.end());
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 constexpr bool operator<=(span<ElementType, Extent> l, span<ElementType, Extent> r)
 {
     return !(l > r);
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 constexpr bool operator>(span<ElementType, Extent> l, span<ElementType, Extent> r)
 {
     return r < l;
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 constexpr bool operator>=(span<ElementType, Extent> l, span<ElementType, Extent> r)
 {
     return !(l < r);
@@ -705,36 +761,38 @@ namespace details
     // we should use a narrow_cast<> to go to std::size_t, but older compilers may not see it as
     // constexpr
     // and so will fail compilation of the template
-    template <class ElementType, std::ptrdiff_t Extent>
-    struct calculate_byte_size
-        : std::integral_constant<std::ptrdiff_t,
-                                 static_cast<std::ptrdiff_t>(sizeof(ElementType) *
-                                                             static_cast<std::size_t>(Extent))>
+    template <class ElementType, std::size_t Extent>
+    struct calculate_byte_size : std::integral_constant<std::size_t, sizeof(ElementType) * Extent>
     {
+        static_assert(Extent < dynamic_extent / sizeof(ElementType), "Size is too big.");
     };
 
     template <class ElementType>
     struct calculate_byte_size<ElementType, dynamic_extent>
-        : std::integral_constant<std::ptrdiff_t, dynamic_extent>
+        : std::integral_constant<std::size_t, dynamic_extent>
     {
     };
 } // namespace details
 
 // [span.objectrep], views of object representation
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
 as_bytes(span<ElementType, Extent> s) noexcept
 {
+    // clang-format off
     GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
+    // clang-format on
     return {reinterpret_cast<const byte*>(s.data()), s.size_bytes()};
 }
 
-template <class ElementType, std::ptrdiff_t Extent,
-          class = std::enable_if_t<!std::is_const<ElementType>::value>>
+template <class ElementType, std::size_t Extent,
+          std::enable_if_t<!std::is_const<ElementType>::value, int> = 0>
 span<byte, details::calculate_byte_size<ElementType, Extent>::value>
-as_writeable_bytes(span<ElementType, Extent> s) noexcept
+as_writable_bytes(span<ElementType, Extent> s) noexcept
 {
+    // clang-format off
     GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
+    // clang-format on
     return {reinterpret_cast<byte*>(s.data()), s.size_bytes()};
 }
 
@@ -742,8 +800,7 @@ as_writeable_bytes(span<ElementType, Extent> s) noexcept
 // make_span() - Utility functions for creating spans
 //
 template <class ElementType>
-constexpr span<ElementType> make_span(ElementType* ptr,
-                                      typename span<ElementType>::index_type count)
+constexpr span<ElementType> make_span(ElementType* ptr, typename span<ElementType>::size_type count)
 {
     return span<ElementType>(ptr, count);
 }
@@ -773,7 +830,7 @@ constexpr span<const typename Container::value_type> make_span(const Container&
 }
 
 template <class Ptr>
-constexpr span<typename Ptr::element_type> make_span(Ptr& cont, std::ptrdiff_t count)
+constexpr span<typename Ptr::element_type> make_span(Ptr& cont, std::size_t count)
 {
     return span<typename Ptr::element_type>(cont, count);
 }
@@ -784,68 +841,76 @@ constexpr span<typename Ptr::element_type> make_span(Ptr& cont)
     return span<typename Ptr::element_type>(cont);
 }
 
-// Specialization of gsl::at for span
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 constexpr ElementType& at(span<ElementType, Extent> s, index i)
 {
     // No bounds checking here because it is done in span::operator[] called below
-    return s[i];
+    Expects(i >= 0);
+    return s[static_cast<std::size_t>(i)];
 }
 
 // [span.obs] Free observer functions
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr typename span<ElementType, Extent>::index_type ssize(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent>
+constexpr std::ptrdiff_t ssize(const span<ElementType, Extent>& s) noexcept
 {
-    return span.size();
+    return static_cast<std::ptrdiff_t>(s.size());
 }
 
 // [span.iter] Free functions for begin/end functions
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr typename span<ElementType, Extent>::iterator begin(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::iterator
+begin(const span<ElementType, Extent>& s) noexcept
 {
-    return span.begin();
+    return s.begin();
 }
 
-template <class ElementType, std::ptrdiff_t Extent = dynamic_extent>
-constexpr typename span<ElementType, Extent>::iterator end(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent = dynamic_extent>
+constexpr typename span<ElementType, Extent>::iterator
+end(const span<ElementType, Extent>& s) noexcept
 {
-    return span.end();
+    return s.end();
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr typename span<ElementType, Extent>::const_iterator cbegin(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::const_iterator
+cbegin(const span<ElementType, Extent>& s) noexcept
 {
-    return span.cbegin();
+    return s.cbegin();
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr typename span<ElementType, Extent>::const_iterator cend(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::const_iterator
+cend(const span<ElementType, Extent>& s) noexcept
 {
-    return span.cend();
+    return s.cend();
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr typename span<ElementType, Extent>::reverse_iterator rbegin(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::reverse_iterator
+rbegin(const span<ElementType, Extent>& s) noexcept
 {
-    return span.rbegin();
+    return s.rbegin();
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr typename span<ElementType, Extent>::reverse_iterator rend(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::reverse_iterator
+rend(const span<ElementType, Extent>& s) noexcept
 {
-    return span.rend();
+    return s.rend();
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr typename span<ElementType, Extent>::const_reverse_iterator crbegin(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::const_reverse_iterator
+crbegin(const span<ElementType, Extent>& s) noexcept
 {
-    return span.crbegin();
+    return s.crbegin();
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
-constexpr typename span<ElementType, Extent>::const_reverse_iterator crend(const span<ElementType, Extent> &span) noexcept
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::const_reverse_iterator
+crend(const span<ElementType, Extent>& s) noexcept
 {
-    return span.crend();
+    return s.crend();
 }
 
 } // namespace gsl
diff --git a/include/gsl/string_span b/include/gsl/string_span
index 37cbe15..cc0588e 100644
--- a/include/gsl/string_span
+++ b/include/gsl/string_span
@@ -23,10 +23,10 @@
 
 #include <algorithm> // for equal, lexicographical_compare
 #include <array>     // for array
-#include <cstddef>   // for ptrdiff_t, size_t, nullptr_t
+#include <cstddef>   // for size_t, nullptr_t
 #include <cstdint>   // for PTRDIFF_MAX
 #include <cstring>
-#include <string>      // for basic_string, allocator, char_traits
+#include <string>    // for basic_string, allocator, char_traits
 #include <type_traits> // for declval, is_convertible, enable_if_t, add_...
 
 #if defined(_MSC_VER) && !defined(__clang__)
@@ -56,43 +56,43 @@ namespace gsl
 // (sometimes needlessly) break existing programs when introduced.
 //
 
-template <typename CharT, std::ptrdiff_t Extent = dynamic_extent>
+template <typename CharT, std::size_t Extent = dynamic_extent>
 using basic_zstring = CharT*;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using czstring = basic_zstring<const char, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using cwzstring = basic_zstring<const wchar_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using cu16zstring = basic_zstring<const char16_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using cu32zstring = basic_zstring<const char32_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using zstring = basic_zstring<char, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using wzstring = basic_zstring<wchar_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using u16zstring = basic_zstring<char16_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using u32zstring = basic_zstring<char32_t, Extent>;
 
 namespace details
 {
     template <class CharT>
-    std::ptrdiff_t string_length(const CharT* str, std::ptrdiff_t n)
+    std::size_t string_length(const CharT* str, std::size_t n)
     {
-        if (str == nullptr || n <= 0) return 0;
+        if (str == nullptr || n == dynamic_extent) return 0;
 
         const span<const CharT> str_span{str, n};
 
-        std::ptrdiff_t len = 0;
+        std::size_t len = 0;
         while (len < n && str_span[len]) len++;
 
         return len;
@@ -108,18 +108,20 @@ namespace details
 // Will fail-fast if sentinel cannot be found before max elements are examined.
 //
 template <typename T, const T Sentinel>
-span<T, dynamic_extent> ensure_sentinel(T* seq, std::ptrdiff_t max = PTRDIFF_MAX)
+span<T, dynamic_extent> ensure_sentinel(T* seq,
+                                        std::size_t max = static_cast<std::size_t>(-1))
 {
     Ensures(seq != nullptr);
 
-    GSL_SUPPRESS(f.23) // NO-FORMAT: attribute // TODO: false positive // TODO: suppress does not work
+    GSL_SUPPRESS(
+        f.23) // NO-FORMAT: attribute // TODO: false positive // TODO: suppress does not work
     auto cur = seq;
     Ensures(cur != nullptr); // workaround for removing the warning
 
     GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute // TODO: suppress does not work
-    while ((cur - seq) < max && *cur != Sentinel) ++cur;
+    while (static_cast<std::size_t>(cur - seq) < max && *cur != Sentinel) ++cur;
     Ensures(*cur == Sentinel);
-    return {seq, cur - seq};
+    return {seq, static_cast<std::size_t>(cur - seq)};
 }
 
 //
@@ -128,7 +130,8 @@ span<T, dynamic_extent> ensure_sentinel(T* seq, std::ptrdiff_t max = PTRDIFF_MAX
 // the limit of size_type.
 //
 template <typename CharT>
-span<CharT, dynamic_extent> ensure_z(CharT* const& sz, std::ptrdiff_t max = PTRDIFF_MAX)
+span<CharT, dynamic_extent> ensure_z(CharT* const& sz,
+                                     std::size_t max = static_cast<std::size_t>(-1))
 {
     return ensure_sentinel<CharT, CharT(0)>(sz, max);
 }
@@ -136,26 +139,27 @@ span<CharT, dynamic_extent> ensure_z(CharT* const& sz, std::ptrdiff_t max = PTRD
 template <typename CharT, std::size_t N>
 span<CharT, dynamic_extent> ensure_z(CharT (&sz)[N])
 {
-    return ensure_z(&sz[0], narrow_cast<std::ptrdiff_t>(N));
+    return ensure_z(&sz[0], N);
 }
 
 template <class Cont>
 span<typename std::remove_pointer<typename Cont::pointer>::type, dynamic_extent>
 ensure_z(Cont& cont)
 {
-    return ensure_z(cont.data(), narrow_cast<std::ptrdiff_t>(cont.size()));
+    return ensure_z(cont.data(), cont.size());
 }
 
-template <typename CharT, std::ptrdiff_t>
+template <typename CharT, std::size_t>
 class basic_string_span;
 
-namespace details {
+namespace details
+{
     template <typename T>
     struct is_basic_string_span_oracle : std::false_type
     {
     };
 
-    template <typename CharT, std::ptrdiff_t Extent>
+    template <typename CharT, std::size_t Extent>
     struct is_basic_string_span_oracle<basic_string_span<CharT, Extent>> : std::true_type
     {
     };
@@ -169,7 +173,7 @@ namespace details {
 //
 // string_span and relatives
 //
-template <typename CharT, std::ptrdiff_t Extent = dynamic_extent>
+template <typename CharT, std::size_t Extent = dynamic_extent>
 class basic_string_span
 {
 public:
@@ -180,14 +184,12 @@ public:
     using const_reference = std::add_lvalue_reference_t<std::add_const_t<element_type>>;
     using impl_type = span<element_type, Extent>;
 
-    using index_type = typename impl_type::index_type;
+    using size_type = typename impl_type::size_type;
     using iterator = typename impl_type::iterator;
     using const_iterator = typename impl_type::const_iterator;
     using reverse_iterator = typename impl_type::reverse_iterator;
     using const_reverse_iterator = typename impl_type::const_reverse_iterator;
 
-    using size_type = index_type;
-
     // default (empty)
     constexpr basic_string_span() noexcept = default;
 
@@ -197,7 +199,7 @@ public:
     // assign
     constexpr basic_string_span& operator=(const basic_string_span& other) noexcept = default;
 
-    constexpr basic_string_span(pointer ptr, index_type length) : span_(ptr, length) {}
+    constexpr basic_string_span(pointer ptr, size_type length) : span_(ptr, length) {}
     constexpr basic_string_span(pointer firstElem, pointer lastElem) : span_(firstElem, lastElem) {}
 
     // From static arrays - if 0-terminated, remove 0 from the view
@@ -218,7 +220,7 @@ public:
     template <class Traits, class Allocator>
     // GSL_SUPPRESS(bounds.4) // NO-FORMAT: attribute // TODO: parser bug
     constexpr basic_string_span(std::basic_string<element_type, Traits, Allocator>& str)
-        : span_(&str[0], narrow_cast<std::ptrdiff_t>(str.length()))
+        : span_(&str[0], str.length())
     {}
 
     template <class Traits, class Allocator>
@@ -247,56 +249,56 @@ public:
 
     // from string_span
     template <
-        class OtherValueType, std::ptrdiff_t OtherExtent,
+        class OtherValueType, std::size_t OtherExtent,
         class = std::enable_if_t<std::is_convertible<
             typename basic_string_span<OtherValueType, OtherExtent>::impl_type, impl_type>::value>>
     constexpr basic_string_span(basic_string_span<OtherValueType, OtherExtent> other)
         : span_(other.data(), other.length())
     {}
 
-    template <index_type Count>
+    template <size_type Count>
     constexpr basic_string_span<element_type, Count> first() const
     {
         return {span_.template first<Count>()};
     }
 
-    constexpr basic_string_span<element_type, dynamic_extent> first(index_type count) const
+    constexpr basic_string_span<element_type, dynamic_extent> first(size_type count) const
     {
         return {span_.first(count)};
     }
 
-    template <index_type Count>
+    template <size_type Count>
     constexpr basic_string_span<element_type, Count> last() const
     {
         return {span_.template last<Count>()};
     }
 
-    constexpr basic_string_span<element_type, dynamic_extent> last(index_type count) const
+    constexpr basic_string_span<element_type, dynamic_extent> last(size_type count) const
     {
         return {span_.last(count)};
     }
 
-    template <index_type Offset, index_type Count>
+    template <size_type Offset, size_type Count>
     constexpr basic_string_span<element_type, Count> subspan() const
     {
         return {span_.template subspan<Offset, Count>()};
     }
 
     constexpr basic_string_span<element_type, dynamic_extent>
-    subspan(index_type offset, index_type count = dynamic_extent) const
+    subspan(size_type offset, size_type count = dynamic_extent) const
     {
         return {span_.subspan(offset, count)};
     }
 
-    constexpr reference operator[](index_type idx) const { return span_[idx]; }
-    constexpr reference operator()(index_type idx) const { return span_[idx]; }
+    constexpr reference operator[](size_type idx) const { return span_[idx]; }
+    constexpr reference operator()(size_type idx) const { return span_[idx]; }
 
     constexpr pointer data() const { return span_.data(); }
 
-    constexpr index_type length() const noexcept { return span_.size(); }
-    constexpr index_type size() const noexcept { return span_.size(); }
-    constexpr index_type size_bytes() const noexcept { return span_.size_bytes(); }
-    constexpr index_type length_bytes() const noexcept { return span_.length_bytes(); }
+    constexpr size_type length() const noexcept { return span_.size(); }
+    constexpr size_type size() const noexcept { return span_.size(); }
+    constexpr size_type size_bytes() const noexcept { return span_.size_bytes(); }
+    constexpr size_type length_bytes() const noexcept { return span_.length_bytes(); }
     constexpr bool empty() const noexcept { return size() == 0; }
 
     constexpr iterator begin() const noexcept { return span_.begin(); }
@@ -312,7 +314,7 @@ public:
     constexpr const_reverse_iterator crend() const noexcept { return span_.crend(); }
 
 private:
-    static impl_type remove_z(pointer const& sz, std::ptrdiff_t max)
+    static impl_type remove_z(pointer const& sz, std::size_t max)
     {
         return {sz, details::string_length(sz, max)};
     }
@@ -320,41 +322,41 @@ private:
     template <std::size_t N>
     static impl_type remove_z(element_type (&sz)[N])
     {
-        return remove_z(&sz[0], narrow_cast<std::ptrdiff_t>(N));
+        return remove_z(&sz[0], N);
     }
 
     impl_type span_;
 };
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using string_span = basic_string_span<char, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using cstring_span = basic_string_span<const char, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using wstring_span = basic_string_span<wchar_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using cwstring_span = basic_string_span<const wchar_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using u16string_span = basic_string_span<char16_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using cu16string_span = basic_string_span<const char16_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using u32string_span = basic_string_span<char32_t, Extent>;
 
-template <std::ptrdiff_t Extent = dynamic_extent>
+template <std::size_t Extent = dynamic_extent>
 using cu32string_span = basic_string_span<const char32_t, Extent>;
 
 //
 // to_string() allow (explicit) conversions from string_span to string
 //
 
-template <typename CharT, std::ptrdiff_t Extent>
+template <typename CharT, std::size_t Extent>
 std::basic_string<typename std::remove_const<CharT>::type>
 to_string(basic_string_span<CharT, Extent> view)
 {
@@ -362,13 +364,13 @@ to_string(basic_string_span<CharT, Extent> view)
 }
 
 template <typename CharT, typename Traits = typename std::char_traits<CharT>,
-          typename Allocator = std::allocator<CharT>, typename gCharT, std::ptrdiff_t Extent>
+          typename Allocator = std::allocator<CharT>, typename gCharT, std::size_t Extent>
 std::basic_string<CharT, Traits, Allocator> to_basic_string(basic_string_span<gCharT, Extent> view)
 {
     return {view.data(), narrow_cast<std::size_t>(view.length())};
 }
 
-template <class ElementType, std::ptrdiff_t Extent>
+template <class ElementType, std::size_t Extent>
 basic_string_span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
 as_bytes(basic_string_span<ElementType, Extent> s) noexcept
 {
@@ -376,10 +378,10 @@ as_bytes(basic_string_span<ElementType, Extent> s) noexcept
     return {reinterpret_cast<const byte*>(s.data()), s.size_bytes()};
 }
 
-template <class ElementType, std::ptrdiff_t Extent,
+template <class ElementType, std::size_t Extent,
           class = std::enable_if_t<!std::is_const<ElementType>::value>>
 basic_string_span<byte, details::calculate_byte_size<ElementType, Extent>::value>
-as_writeable_bytes(basic_string_span<ElementType, Extent> s) noexcept
+as_writable_bytes(basic_string_span<ElementType, Extent> s) noexcept
 {
     GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
     return {reinterpret_cast<byte*>(s.data()), s.size_bytes()};
@@ -387,8 +389,9 @@ as_writeable_bytes(basic_string_span<ElementType, Extent> s) noexcept
 
 // zero-terminated string span, used to convert
 // zero-terminated spans to legacy strings
-template <typename CharT, std::ptrdiff_t Extent = dynamic_extent>
-class basic_zstring_span {
+template <typename CharT, std::size_t Extent = dynamic_extent>
+class basic_zstring_span
+{
 public:
     using value_type = CharT;
     using const_value_type = std::add_const_t<CharT>;
@@ -435,32 +438,32 @@ private:
     impl_type span_;
 };
 
-template <std::ptrdiff_t Max = dynamic_extent>
+template <std::size_t Max = dynamic_extent>
 using zstring_span = basic_zstring_span<char, Max>;
 
-template <std::ptrdiff_t Max = dynamic_extent>
+template <std::size_t Max = dynamic_extent>
 using wzstring_span = basic_zstring_span<wchar_t, Max>;
 
-template <std::ptrdiff_t Max = dynamic_extent>
+template <std::size_t Max = dynamic_extent>
 using u16zstring_span = basic_zstring_span<char16_t, Max>;
 
-template <std::ptrdiff_t Max = dynamic_extent>
+template <std::size_t Max = dynamic_extent>
 using u32zstring_span = basic_zstring_span<char32_t, Max>;
 
-template <std::ptrdiff_t Max = dynamic_extent>
+template <std::size_t Max = dynamic_extent>
 using czstring_span = basic_zstring_span<const char, Max>;
 
-template <std::ptrdiff_t Max = dynamic_extent>
+template <std::size_t Max = dynamic_extent>
 using cwzstring_span = basic_zstring_span<const wchar_t, Max>;
 
-template <std::ptrdiff_t Max = dynamic_extent>
+template <std::size_t Max = dynamic_extent>
 using cu16zstring_span = basic_zstring_span<const char16_t, Max>;
 
-template <std::ptrdiff_t Max = dynamic_extent>
+template <std::size_t Max = dynamic_extent>
 using cu32zstring_span = basic_zstring_span<const char32_t, Max>;
 
 // operator ==
-template <class CharT, std::ptrdiff_t Extent, class T,
+template <class CharT, std::size_t Extent, class T,
           class = std::enable_if_t<
               details::is_basic_string_span<T>::value ||
               std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>>>::value>>
@@ -470,7 +473,7 @@ bool operator==(const gsl::basic_string_span<CharT, Extent>& one, const T& other
     return std::equal(one.begin(), one.end(), tmp.begin(), tmp.end());
 }
 
-template <class CharT, std::ptrdiff_t Extent, class T,
+template <class CharT, std::size_t Extent, class T,
           class = std::enable_if_t<
               !details::is_basic_string_span<T>::value &&
               std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>>>::value>>
@@ -481,7 +484,7 @@ bool operator==(const T& one, const gsl::basic_string_span<CharT, Extent>& other
 }
 
 // operator !=
-template <typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+template <typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
           typename = std::enable_if_t<std::is_convertible<
               T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
 bool operator!=(gsl::basic_string_span<CharT, Extent> one, const T& other)
@@ -490,7 +493,7 @@ bool operator!=(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename = std::enable_if_t<
         std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
         !gsl::details::is_basic_string_span<T>::value>>
@@ -500,7 +503,7 @@ bool operator!=(const T& one, gsl::basic_string_span<CharT, Extent> other)
 }
 
 // operator<
-template <typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+template <typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
           typename = std::enable_if_t<std::is_convertible<
               T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
 bool operator<(gsl::basic_string_span<CharT, Extent> one, const T& other)
@@ -510,7 +513,7 @@ bool operator<(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename = std::enable_if_t<
         std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
         !gsl::details::is_basic_string_span<T>::value>>
@@ -526,7 +529,7 @@ bool operator<(const T& one, gsl::basic_string_span<CharT, Extent> other)
 // so the cases below are already covered by the previous operators
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename DataType = typename T::value_type,
     typename = std::enable_if_t<
         !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
@@ -540,7 +543,7 @@ bool operator<(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename DataType = typename T::value_type,
     typename = std::enable_if_t<
         !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
@@ -555,7 +558,7 @@ bool operator<(const T& one, gsl::basic_string_span<CharT, Extent> other)
 #endif
 
 // operator <=
-template <typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+template <typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
           typename = std::enable_if_t<std::is_convertible<
               T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
 bool operator<=(gsl::basic_string_span<CharT, Extent> one, const T& other)
@@ -564,7 +567,7 @@ bool operator<=(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename = std::enable_if_t<
         std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
         !gsl::details::is_basic_string_span<T>::value>>
@@ -579,7 +582,7 @@ bool operator<=(const T& one, gsl::basic_string_span<CharT, Extent> other)
 // so the cases below are already covered by the previous operators
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename DataType = typename T::value_type,
     typename = std::enable_if_t<
         !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
@@ -592,7 +595,7 @@ bool operator<=(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename DataType = typename T::value_type,
     typename = std::enable_if_t<
         !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
@@ -606,7 +609,7 @@ bool operator<=(const T& one, gsl::basic_string_span<CharT, Extent> other)
 #endif
 
 // operator>
-template <typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+template <typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
           typename = std::enable_if_t<std::is_convertible<
               T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
 bool operator>(gsl::basic_string_span<CharT, Extent> one, const T& other)
@@ -615,7 +618,7 @@ bool operator>(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename = std::enable_if_t<
         std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
         !gsl::details::is_basic_string_span<T>::value>>
@@ -630,7 +633,7 @@ bool operator>(const T& one, gsl::basic_string_span<CharT, Extent> other)
 // so the cases below are already covered by the previous operators
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename DataType = typename T::value_type,
     typename = std::enable_if_t<
         !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
@@ -643,7 +646,7 @@ bool operator>(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename DataType = typename T::value_type,
     typename = std::enable_if_t<
         !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
@@ -657,7 +660,7 @@ bool operator>(const T& one, gsl::basic_string_span<CharT, Extent> other)
 #endif
 
 // operator >=
-template <typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+template <typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
           typename = std::enable_if_t<std::is_convertible<
               T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
 bool operator>=(gsl::basic_string_span<CharT, Extent> one, const T& other)
@@ -666,7 +669,7 @@ bool operator>=(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename = std::enable_if_t<
         std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
         !gsl::details::is_basic_string_span<T>::value>>
@@ -681,7 +684,7 @@ bool operator>=(const T& one, gsl::basic_string_span<CharT, Extent> other)
 // so the cases below are already covered by the previous operators
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename DataType = typename T::value_type,
     typename = std::enable_if_t<
         !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
@@ -694,7 +697,7 @@ bool operator>=(gsl::basic_string_span<CharT, Extent> one, const T& other)
 }
 
 template <
-    typename CharT, std::ptrdiff_t Extent = gsl::dynamic_extent, typename T,
+    typename CharT, std::size_t Extent = gsl::dynamic_extent, typename T,
     typename DataType = typename T::value_type,
     typename = std::enable_if_t<
         !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 3a6163c..38dbb85 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -153,6 +153,7 @@ function(add_gsl_test name)
 endfunction()
 
 add_gsl_test(span_tests)
+add_gsl_test(span_compatibility_tests)
 add_gsl_test(multi_span_tests)
 add_gsl_test(strided_span_tests)
 add_gsl_test(string_span_tests)
diff --git a/tests/CMakeLists.txt.in b/tests/CMakeLists.txt.in
index 50c041d..f380b8f 100644
--- a/tests/CMakeLists.txt.in
+++ b/tests/CMakeLists.txt.in
@@ -4,7 +4,7 @@ project(googletest-download NONE)
 include(ExternalProject)
 ExternalProject_Add(googletest
   GIT_REPOSITORY    https://github.com/google/googletest.git
-  GIT_TAG           master
+  GIT_TAG           703bd9caab50b139428cea1aaff9974ebee5742e
   SOURCE_DIR        "${CMAKE_CURRENT_BINARY_DIR}/googletest-src"
   BINARY_DIR        "${CMAKE_CURRENT_BINARY_DIR}/googletest-build"
   CONFIGURE_COMMAND ""
diff --git a/tests/span_compatibility_tests.cpp b/tests/span_compatibility_tests.cpp
new file mode 100644
index 0000000..0301ace
--- /dev/null
+++ b/tests/span_compatibility_tests.cpp
@@ -0,0 +1,1071 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#include <gtest/gtest.h>
+
+#include <gsl/gsl_byte> // for byte
+#include <gsl/span>     // for span, span_iterator, operator==, operator!=
+
+#include <array>       // for array
+#include <cstddef>     // for ptrdiff_t
+#include <iterator>    // for reverse_iterator, operator-, operator==
+#include <type_traits> // for integral_constant<>::value, is_default_co...
+#include <utility>
+
+using namespace std;
+using namespace gsl;
+
+// Below are tests that verify the gsl interface support the same things as the std
+// Ranges and Concepts support need to be added later.
+
+struct Base
+{
+};
+struct Derived : Base
+{
+};
+static_assert(std::is_convertible<Derived*, Base*>::value, "std::is_convertible<Derived*, Base*>");
+static_assert(!std::is_convertible<Derived (*)[], Base (*)[]>::value,
+              "!std::is_convertible<Derived(*)[], Base(*)[]>");
+
+TEST(span_compatibility_tests, assertion_tests)
+{
+    int arr[3]{10, 20, 30};
+    std::array<int, 3> stl{{100, 200, 300}};
+
+    {
+        gsl::span<int> sp_dyn;
+        EXPECT_TRUE(sp_dyn.data() == nullptr);
+        EXPECT_TRUE(sp_dyn.size() == 0);
+        EXPECT_TRUE(sp_dyn.empty());
+    }
+    {
+        gsl::span<int, 0> sp_zero;
+        EXPECT_TRUE(sp_zero.data() == nullptr);
+        EXPECT_TRUE(sp_zero.size() == 0);
+        EXPECT_TRUE(sp_zero.empty());
+
+        gsl::span<int> sp_dyn_a(arr, 3);
+        gsl::span<int> sp_dyn_b(begin(arr), 3);
+        EXPECT_TRUE(sp_dyn_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_dyn_b.data() == std::begin(arr));
+        EXPECT_TRUE(sp_dyn_a.size() == 3);
+        EXPECT_TRUE(sp_dyn_b.size() == 3);
+
+        gsl::span<int, 3> sp_three_a(arr, 3);
+        gsl::span<int, 3> sp_three_b(begin(arr), 3);
+        EXPECT_TRUE(sp_three_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_three_b.data() == std::begin(arr));
+        EXPECT_TRUE(sp_three_a.size() == 3);
+        EXPECT_TRUE(sp_three_b.size() == 3);
+
+        gsl::span<const int> sp_const_a(arr, 3);
+        gsl::span<const int> sp_const_b(begin(arr), 3);
+        EXPECT_TRUE(sp_const_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_b.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_a.size() == 3);
+        EXPECT_TRUE(sp_const_b.size() == 3);
+
+#if __cplusplus >= 201703l
+        gsl::span<const int> sp_const_c(std::as_const(arr), 3);
+        EXPECT_TRUE(sp_const_c.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_c.size() == 3);
+#endif // __cplusplus >= 201703l
+
+        gsl::span<const int> sp_const_d(cbegin(arr), 3);
+        EXPECT_TRUE(sp_const_d.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_d.size() == 3);
+    }
+    {
+        gsl::span<int> sp_dyn_a(begin(arr), std::end(arr));
+        EXPECT_TRUE(sp_dyn_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_dyn_a.size() == 3);
+
+        gsl::span<int, 3> sp_three_a(begin(arr), std::end(arr));
+        EXPECT_TRUE(sp_three_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_three_a.size() == 3);
+
+        gsl::span<const int> sp_const_a(begin(arr), std::end(arr));
+        gsl::span<const int> sp_const_b(begin(arr), std::cend(arr));
+        gsl::span<const int> sp_const_c(cbegin(arr), std::end(arr));
+        gsl::span<const int> sp_const_d(cbegin(arr), std::cend(arr));
+        EXPECT_TRUE(sp_const_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_b.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_c.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_d.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_a.size() == 3);
+        EXPECT_TRUE(sp_const_b.size() == 3);
+        EXPECT_TRUE(sp_const_c.size() == 3);
+        EXPECT_TRUE(sp_const_d.size() == 3);
+    }
+    {
+        gsl::span<int> sp_dyn_a(arr);
+        gsl::span<int> sp_dyn_b(stl);
+        gsl::span<int> sp_dyn_c{stl};
+        gsl::span<const int> sp_dyn_d{stl};
+        EXPECT_TRUE(sp_dyn_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_dyn_b.data() == stl.data());
+        EXPECT_TRUE(sp_dyn_a.size() == 3);
+        EXPECT_TRUE(sp_dyn_b.size() == 3);
+
+        gsl::span<int, 3> sp_three_a(arr);
+        gsl::span<int, 3> sp_three_b(stl);
+        EXPECT_TRUE(sp_three_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_three_b.data() == stl.data());
+        EXPECT_TRUE(sp_three_a.size() == 3);
+        EXPECT_TRUE(sp_three_b.size() == 3);
+
+        gsl::span<const int> sp_const_w(arr);
+        gsl::span<const int> sp_const_y(stl);
+        EXPECT_TRUE(sp_const_w.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_y.data() == stl.data());
+        EXPECT_TRUE(sp_const_w.size() == 3);
+        EXPECT_TRUE(sp_const_y.size() == 3);
+
+#if __cplusplus >= 201703l
+        gsl::span<const int> sp_const_x(std::as_const(arr));
+        EXPECT_TRUE(sp_const_x.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_x.size() == 3);
+
+        gsl::span<const int> sp_const_z(std::as_const(stl));
+        EXPECT_TRUE(sp_const_z.data() == stl.data());
+        EXPECT_TRUE(sp_const_z.size() == 3);
+#endif // __cplusplus >= 201703l
+    }
+    {
+        const gsl::span<int> orig_dyn(arr);
+        const gsl::span<int, 3> orig_three(arr);
+        const gsl::span<const int> orig_const_dyn(arr);
+        const gsl::span<const int, 3> orig_const_three(arr);
+
+        gsl::span<int> sp_a(orig_dyn);
+        gsl::span<int> sp_b(orig_three);
+
+        gsl::span<int, 3> sp_c(orig_three);
+
+        gsl::span<const int> sp_d(orig_dyn);
+        gsl::span<const int> sp_e(orig_three);
+        gsl::span<const int> sp_f(orig_const_dyn);
+        gsl::span<const int> sp_g(orig_const_three);
+
+        gsl::span<const int, 3> sp_h(orig_three);
+        gsl::span<const int, 3> sp_i(orig_const_three);
+
+        EXPECT_TRUE(sp_a.data() == std::begin(arr));
+        EXPECT_TRUE(sp_b.data() == std::begin(arr));
+        EXPECT_TRUE(sp_c.data() == std::begin(arr));
+        EXPECT_TRUE(sp_d.data() == std::begin(arr));
+        EXPECT_TRUE(sp_e.data() == std::begin(arr));
+        EXPECT_TRUE(sp_f.data() == std::begin(arr));
+        EXPECT_TRUE(sp_g.data() == std::begin(arr));
+        EXPECT_TRUE(sp_h.data() == std::begin(arr));
+        EXPECT_TRUE(sp_i.data() == std::begin(arr));
+        EXPECT_TRUE(sp_a.size() == 3);
+        EXPECT_TRUE(sp_b.size() == 3);
+        EXPECT_TRUE(sp_c.size() == 3);
+        EXPECT_TRUE(sp_d.size() == 3);
+        EXPECT_TRUE(sp_e.size() == 3);
+        EXPECT_TRUE(sp_f.size() == 3);
+        EXPECT_TRUE(sp_g.size() == 3);
+        EXPECT_TRUE(sp_h.size() == 3);
+        EXPECT_TRUE(sp_i.size() == 3);
+    }
+    {
+        gsl::span<int> sp_dyn(arr);
+        gsl::span<int, 3> sp_three(arr);
+        gsl::span<const int> sp_const_dyn(arr);
+        gsl::span<const int, 3> sp_const_three(arr);
+
+        EXPECT_TRUE(sp_dyn.data() == std::begin(arr));
+        EXPECT_TRUE(sp_three.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_dyn.data() == std::begin(arr));
+        EXPECT_TRUE(sp_const_three.data() == std::begin(arr));
+        EXPECT_TRUE(sp_dyn.size() == 3);
+        EXPECT_TRUE(sp_three.size() == 3);
+        EXPECT_TRUE(sp_const_dyn.size() == 3);
+        EXPECT_TRUE(sp_const_three.size() == 3);
+
+        int other[4]{12, 34, 56, 78};
+
+        sp_dyn = gsl::span<int>{other};
+        sp_three = gsl::span<int, 3>{stl};
+        sp_const_dyn = gsl::span<const int>{other};
+        sp_const_three = gsl::span<const int, 3>{stl};
+
+        EXPECT_TRUE(sp_dyn.data() == std::begin(other));
+        EXPECT_TRUE(sp_three.data() == stl.data());
+        EXPECT_TRUE(sp_const_dyn.data() == std::begin(other));
+        EXPECT_TRUE(sp_const_three.data() == stl.data());
+        EXPECT_TRUE(sp_dyn.size() == 4);
+        EXPECT_TRUE(sp_three.size() == 3);
+        EXPECT_TRUE(sp_const_dyn.size() == 4);
+        EXPECT_TRUE(sp_const_three.size() == 3);
+    }
+    {
+        gsl::span<int>::iterator it_dyn{};
+
+        {
+            gsl::span<int> sp_dyn(arr);
+            it_dyn = sp_dyn.begin();
+        }
+
+        EXPECT_TRUE(*it_dyn == arr[0]);
+        EXPECT_TRUE(it_dyn[2] == arr[2]);
+
+        gsl::span<int, 3>::iterator it_three{};
+
+        {
+            gsl::span<int, 3> sp_three(stl);
+            it_three = sp_three.begin();
+        }
+
+        EXPECT_TRUE(*it_three == stl[0]);
+        EXPECT_TRUE(it_three[2] == stl[2]);
+    }
+
+    {
+        int sequence[9]{10, 20, 30, 40, 50, 60, 70, 80, 90};
+
+        const gsl::span<int> sp_dyn(sequence);
+        const gsl::span<int, 9> sp_nine(sequence);
+
+        auto first_3 = sp_dyn.first<3>();
+        auto first_4 = sp_nine.first<4>();
+        auto first_5 = sp_dyn.first(5);
+        auto first_6 = sp_nine.first(6);
+        static_assert(noexcept(sp_dyn.first<3>()), "noexcept(sp_dyn.first<3>())");   // strengthened
+        static_assert(noexcept(sp_nine.first<4>()), "noexcept(sp_nine.first<4>())"); // strengthened
+        static_assert(noexcept(sp_dyn.first(5)), "noexcept(sp_dyn.first(5))");       // strengthened
+        static_assert(noexcept(sp_nine.first(6)), "noexcept(sp_nine.first(6))");     // strengthened
+        static_assert(is_same<decltype(first_3), gsl::span<int, 3>>::value,
+                      "is_same<decltype(first_3), gsl::span<int, 3>>::value");
+        static_assert(is_same<decltype(first_4), gsl::span<int, 4>>::value,
+                      "is_same<decltype(first_4), gsl::span<int, 4>>::value");
+        static_assert(is_same<decltype(first_5), gsl::span<int>>::value,
+                      "is_same<decltype(first_5), gsl::span<int>>::value");
+        static_assert(is_same<decltype(first_6), gsl::span<int>>::value,
+                      "is_same<decltype(first_6), gsl::span<int>>::value");
+        EXPECT_TRUE(first_3.data() == std::begin(sequence));
+        EXPECT_TRUE(first_4.data() == std::begin(sequence));
+        EXPECT_TRUE(first_5.data() == std::begin(sequence));
+        EXPECT_TRUE(first_6.data() == std::begin(sequence));
+        EXPECT_TRUE(first_3.size() == 3);
+        EXPECT_TRUE(first_4.size() == 4);
+        EXPECT_TRUE(first_5.size() == 5);
+        EXPECT_TRUE(first_6.size() == 6);
+
+        auto last_3 = sp_dyn.last<3>();
+        auto last_4 = sp_nine.last<4>();
+        auto last_5 = sp_dyn.last(5);
+        auto last_6 = sp_nine.last(6);
+        static_assert(noexcept(sp_dyn.last<3>()), "noexcept(sp_dyn.last<3>())");   // strengthened
+        static_assert(noexcept(sp_nine.last<4>()), "noexcept(sp_nine.last<4>())"); // strengthened
+        static_assert(noexcept(sp_dyn.last(5)), "noexcept(sp_dyn.last(5))");       // strengthened
+        static_assert(noexcept(sp_nine.last(6)), "noexcept(sp_nine.last(6))");     // strengthened
+        static_assert(is_same<decltype(last_3), gsl::span<int, 3>>::value,
+                      "is_same<decltype(last_3), gsl::span<int, 3>>::value");
+        static_assert(is_same<decltype(last_4), gsl::span<int, 4>>::value,
+                      "is_same<decltype(last_4), gsl::span<int, 4>>::value");
+        static_assert(is_same<decltype(last_5), gsl::span<int>>::value,
+                      "is_same<decltype(last_5), gsl::span<int>>::value");
+        static_assert(is_same<decltype(last_6), gsl::span<int>>::value,
+                      "is_same<decltype(last_6), gsl::span<int>>::value");
+        EXPECT_TRUE(last_3.data() == std::begin(sequence) + 6);
+        EXPECT_TRUE(last_4.data() == std::begin(sequence) + 5);
+        EXPECT_TRUE(last_5.data() == std::begin(sequence) + 4);
+        EXPECT_TRUE(last_6.data() == std::begin(sequence) + 3);
+        EXPECT_TRUE(last_3.size() == 3);
+        EXPECT_TRUE(last_4.size() == 4);
+        EXPECT_TRUE(last_5.size() == 5);
+        EXPECT_TRUE(last_6.size() == 6);
+
+        auto offset_3 = sp_dyn.subspan<3>();
+        auto offset_4 = sp_nine.subspan<4>();
+        auto offset_5 = sp_dyn.subspan(5);
+        auto offset_6 = sp_nine.subspan(6);
+        static_assert(noexcept(sp_dyn.subspan<3>()),
+                      "noexcept(sp_dyn.subspan<3>())"); // strengthened
+        static_assert(noexcept(sp_nine.subspan<4>()),
+                      "noexcept(sp_nine.subspan<4>())");                             // strengthened
+        static_assert(noexcept(sp_dyn.subspan(5)), "noexcept(sp_dyn.subspan(5))");   // strengthened
+        static_assert(noexcept(sp_nine.subspan(6)), "noexcept(sp_nine.subspan(6))"); // strengthened
+        static_assert(is_same<decltype(offset_3), gsl::span<int>>::value,
+                      "is_same<decltype(offset_3), gsl::span<int>>::value");
+        static_assert(is_same<decltype(offset_4), gsl::span<int, 5>>::value,
+                      "is_same<decltype(offset_4), gsl::span<int, 5>>::value");
+        static_assert(is_same<decltype(offset_5), gsl::span<int>>::value,
+                      "is_same<decltype(offset_5), gsl::span<int>>::value");
+        static_assert(is_same<decltype(offset_6), gsl::span<int>>::value,
+                      "is_same<decltype(offset_6), gsl::span<int>>::value");
+        EXPECT_TRUE(offset_3.data() == std::begin(sequence) + 3);
+        EXPECT_TRUE(offset_4.data() == std::begin(sequence) + 4);
+        EXPECT_TRUE(offset_5.data() == std::begin(sequence) + 5);
+        EXPECT_TRUE(offset_6.data() == std::begin(sequence) + 6);
+        EXPECT_TRUE(offset_3.size() == 6);
+        EXPECT_TRUE(offset_4.size() == 5);
+        EXPECT_TRUE(offset_5.size() == 4);
+        EXPECT_TRUE(offset_6.size() == 3);
+
+        auto subspan_3 = sp_dyn.subspan<3, 2>();
+        auto subspan_4 = sp_nine.subspan<4, 2>();
+        auto subspan_5 = sp_dyn.subspan(5, 2);
+        auto subspan_6 = sp_nine.subspan(6, 2);
+        static_assert(noexcept(sp_dyn.subspan<3, 2>()),
+                      "noexcept(sp_dyn.subspan<3, 2>())"); // strengthened
+        static_assert(noexcept(sp_nine.subspan<4, 2>()),
+                      "noexcept(sp_nine.subspan<4, 2>())"); // strengthened
+        static_assert(noexcept(sp_dyn.subspan(5, 2)),
+                      "noexcept(sp_dyn.subspan(5, 2))"); // strengthened
+        static_assert(noexcept(sp_nine.subspan(6, 2)),
+                      "noexcept(sp_nine.subspan(6, 2))"); // strengthened
+        static_assert(is_same<decltype(subspan_3), gsl::span<int, 2>>::value,
+                      "is_same<decltype(subspan_3), gsl::span<int, 2>>::value");
+        static_assert(is_same<decltype(subspan_4), gsl::span<int, 2>>::value,
+                      "is_same<decltype(subspan_4), gsl::span<int, 2>>::value");
+        static_assert(is_same<decltype(subspan_5), gsl::span<int>>::value,
+                      "is_same<decltype(subspan_5), gsl::span<int>>::value");
+        static_assert(is_same<decltype(subspan_6), gsl::span<int>>::value,
+                      "is_same<decltype(subspan_6), gsl::span<int>>::value");
+        EXPECT_TRUE(subspan_3.data() == std::begin(sequence) + 3);
+        EXPECT_TRUE(subspan_4.data() == std::begin(sequence) + 4);
+        EXPECT_TRUE(subspan_5.data() == std::begin(sequence) + 5);
+        EXPECT_TRUE(subspan_6.data() == std::begin(sequence) + 6);
+        EXPECT_TRUE(subspan_3.size() == 2);
+        EXPECT_TRUE(subspan_4.size() == 2);
+        EXPECT_TRUE(subspan_5.size() == 2);
+        EXPECT_TRUE(subspan_6.size() == 2);
+
+        static_assert(noexcept(sp_dyn.size()), "noexcept(sp_dyn.size())");
+        static_assert(noexcept(sp_dyn.size_bytes()), "noexcept(sp_dyn.size_bytes())");
+        static_assert(noexcept(sp_dyn.empty()), "noexcept(sp_dyn.empty())");
+        static_assert(noexcept(sp_dyn[0]), "noexcept(sp_dyn[0])");           // strengthened
+        static_assert(noexcept(sp_dyn.front()), "noexcept(sp_dyn.front())"); // strengthened
+        static_assert(noexcept(sp_dyn.back()), "noexcept(sp_dyn.back())");   // strengthened
+        static_assert(noexcept(sp_dyn.data()), "noexcept(sp_dyn.data())");
+        static_assert(noexcept(sp_dyn.begin()), "noexcept(sp_dyn.begin())");
+        static_assert(noexcept(sp_dyn.end()), "noexcept(sp_dyn.end())");
+        static_assert(noexcept(sp_dyn.cbegin()), "noexcept(sp_dyn.cbegin())");
+        static_assert(noexcept(sp_dyn.cend()), "noexcept(sp_dyn.cend())");
+        static_assert(noexcept(sp_dyn.rbegin()), "noexcept(sp_dyn.rbegin())");
+        static_assert(noexcept(sp_dyn.rend()), "noexcept(sp_dyn.rend())");
+        static_assert(noexcept(sp_dyn.crbegin()), "noexcept(sp_dyn.crbegin())");
+        static_assert(noexcept(sp_dyn.crend()), "noexcept(sp_dyn.crend())");
+
+        static_assert(noexcept(sp_nine.size()), "noexcept(sp_nine.size())");
+        static_assert(noexcept(sp_nine.size_bytes()), "noexcept(sp_nine.size_bytes())");
+        static_assert(noexcept(sp_nine.empty()), "noexcept(sp_nine.empty())");
+        static_assert(noexcept(sp_nine[0]), "noexcept(sp_nine[0])");           // strengthened
+        static_assert(noexcept(sp_nine.front()), "noexcept(sp_nine.front())"); // strengthened
+        static_assert(noexcept(sp_nine.back()), "noexcept(sp_nine.back())");   // strengthened
+        static_assert(noexcept(sp_nine.data()), "noexcept(sp_nine.data())");
+        static_assert(noexcept(sp_nine.begin()), "noexcept(sp_nine.begin())");
+        static_assert(noexcept(sp_nine.end()), "noexcept(sp_nine.end())");
+        static_assert(noexcept(sp_nine.cbegin()), "noexcept(sp_nine.cbegin())");
+        static_assert(noexcept(sp_nine.cend()), "noexcept(sp_nine.cend())");
+        static_assert(noexcept(sp_nine.rbegin()), "noexcept(sp_nine.rbegin())");
+        static_assert(noexcept(sp_nine.rend()), "noexcept(sp_nine.rend())");
+        static_assert(noexcept(sp_nine.crbegin()), "noexcept(sp_nine.crbegin())");
+        static_assert(noexcept(sp_nine.crend()), "noexcept(sp_nine.crend())");
+
+        EXPECT_TRUE(sp_dyn.size() == 9);
+        EXPECT_TRUE(sp_nine.size() == 9);
+
+        EXPECT_TRUE(sp_dyn.size_bytes() == 9 * sizeof(int));
+        EXPECT_TRUE(sp_nine.size_bytes() == 9 * sizeof(int));
+
+        EXPECT_TRUE(!sp_dyn.empty());
+        EXPECT_TRUE(!sp_nine.empty());
+
+        EXPECT_TRUE(sp_dyn[0] == 10);
+        EXPECT_TRUE(sp_nine[0] == 10);
+        EXPECT_TRUE(sp_dyn[8] == 90);
+        EXPECT_TRUE(sp_nine[8] == 90);
+
+        EXPECT_TRUE(sp_dyn.front() == 10);
+        EXPECT_TRUE(sp_nine.front() == 10);
+
+        EXPECT_TRUE(sp_dyn.back() == 90);
+        EXPECT_TRUE(sp_nine.back() == 90);
+
+        EXPECT_TRUE(&sp_dyn.front() == std::begin(sequence));
+        EXPECT_TRUE(&sp_nine.front() == std::begin(sequence));
+        EXPECT_TRUE(&sp_dyn[4] == std::begin(sequence) + 4);
+        EXPECT_TRUE(&sp_nine[4] == std::begin(sequence) + 4);
+        EXPECT_TRUE(&sp_dyn.back() == std::begin(sequence) + 8);
+        EXPECT_TRUE(&sp_nine.back() == std::begin(sequence) + 8);
+
+        EXPECT_TRUE(sp_dyn.data() == std::begin(sequence));
+        EXPECT_TRUE(sp_nine.data() == std::begin(sequence));
+
+        EXPECT_TRUE(*sp_dyn.begin() == 10);
+        EXPECT_TRUE(*sp_nine.begin() == 10);
+
+        EXPECT_TRUE(sp_dyn.end()[-2] == 80);
+        EXPECT_TRUE(sp_nine.end()[-2] == 80);
+
+        EXPECT_TRUE(*sp_dyn.cbegin() == 10);
+        EXPECT_TRUE(*sp_nine.cbegin() == 10);
+
+        EXPECT_TRUE(sp_dyn.cend()[-2] == 80);
+        EXPECT_TRUE(sp_nine.cend()[-2] == 80);
+
+        EXPECT_TRUE(*sp_dyn.rbegin() == 90);
+        EXPECT_TRUE(*sp_nine.rbegin() == 90);
+
+        EXPECT_TRUE(sp_dyn.rend()[-2] == 20);
+        EXPECT_TRUE(sp_nine.rend()[-2] == 20);
+
+        EXPECT_TRUE(*sp_dyn.crbegin() == 90);
+        EXPECT_TRUE(*sp_nine.crbegin() == 90);
+
+        EXPECT_TRUE(sp_dyn.crend()[-2] == 20);
+        EXPECT_TRUE(sp_nine.crend()[-2] == 20);
+
+        static_assert(is_same<decltype(sp_dyn.begin()), gsl::span<int>::iterator>::value,
+                      "is_same<decltype(sp_dyn.begin()), gsl::span<int>::iterator>::value");
+        static_assert(is_same<decltype(sp_nine.begin()), gsl::span<int, 9>::iterator>::value,
+                      "is_same<decltype(sp_nine.begin()), gsl::span<int, 9>::iterator>::value");
+        static_assert(is_same<decltype(sp_dyn.end()), gsl::span<int>::iterator>::value,
+                      "is_same<decltype(sp_dyn.end()), gsl::span<int>::iterator>::value");
+        static_assert(is_same<decltype(sp_nine.end()), gsl::span<int, 9>::iterator>::value,
+                      "is_same<decltype(sp_nine.end()), gsl::span<int, 9>::iterator>::value");
+        static_assert(is_same<decltype(sp_dyn.cbegin()), gsl::span<int>::const_iterator>::value,
+                      "is_same<decltype(sp_dyn.cbegin()), gsl::span<int>::const_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_nine.cbegin()), gsl::span<int, 9>::const_iterator>::value,
+            "is_same<decltype(sp_nine.cbegin()), gsl::span<int, 9>::const_iterator>::value");
+        static_assert(is_same<decltype(sp_dyn.cend()), gsl::span<int>::const_iterator>::value,
+                      "is_same<decltype(sp_dyn.cend()), gsl::span<int>::const_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_nine.cend()), gsl::span<int, 9>::const_iterator>::value,
+            "is_same<decltype(sp_nine.cend()), gsl::span<int, 9>::const_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_dyn.rbegin()), gsl::span<int>::reverse_iterator>::value,
+            "is_same<decltype(sp_dyn.rbegin()), gsl::span<int>::reverse_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_nine.rbegin()), gsl::span<int, 9>::reverse_iterator>::value,
+            "is_same<decltype(sp_nine.rbegin()), gsl::span<int, 9>::reverse_iterator>::value");
+        static_assert(is_same<decltype(sp_dyn.rend()), gsl::span<int>::reverse_iterator>::value,
+                      "is_same<decltype(sp_dyn.rend()), gsl::span<int>::reverse_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_nine.rend()), gsl::span<int, 9>::reverse_iterator>::value,
+            "is_same<decltype(sp_nine.rend()), gsl::span<int, 9>::reverse_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_dyn.crbegin()), gsl::span<int>::const_reverse_iterator>::value,
+            "is_same<decltype(sp_dyn.crbegin()), gsl::span<int>::const_reverse_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_nine.crbegin()), gsl::span<int, 9>::const_reverse_iterator>::value,
+            "is_same<decltype(sp_nine.crbegin()), gsl::span<int, "
+            "9>::const_reverse_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_dyn.crend()), gsl::span<int>::const_reverse_iterator>::value,
+            "is_same<decltype(sp_dyn.crend()), gsl::span<int>::const_reverse_iterator>::value");
+        static_assert(
+            is_same<decltype(sp_nine.crend()), gsl::span<int, 9>::const_reverse_iterator>::value,
+            "is_same<decltype(sp_nine.crend()), gsl::span<int, 9>::const_reverse_iterator>::value");
+    }
+    {
+        int sequence[9]{10, 20, 30, 40, 50, 60, 70, 80, 90};
+
+        constexpr size_t SizeBytes = sizeof(sequence);
+
+        const gsl::span<int> sp_dyn(sequence);
+        const gsl::span<int, 9> sp_nine(sequence);
+        const gsl::span<const int> sp_const_dyn(sequence);
+        const gsl::span<const int, 9> sp_const_nine(sequence);
+
+        static_assert(noexcept(as_bytes(sp_dyn)), "noexcept(as_bytes(sp_dyn))");
+        static_assert(noexcept(as_bytes(sp_nine)), "noexcept(as_bytes(sp_nine))");
+        static_assert(noexcept(as_bytes(sp_const_dyn)), "noexcept(as_bytes(sp_const_dyn))");
+        static_assert(noexcept(as_bytes(sp_const_nine)), "noexcept(as_bytes(sp_const_nine))");
+        static_assert(noexcept(as_writable_bytes(sp_dyn)), "noexcept(as_writable_bytes(sp_dyn))");
+        static_assert(noexcept(as_writable_bytes(sp_nine)), "noexcept(as_writable_bytes(sp_nine))");
+
+        auto sp_1 = as_bytes(sp_dyn);
+        auto sp_2 = as_bytes(sp_nine);
+        auto sp_3 = as_bytes(sp_const_dyn);
+        auto sp_4 = as_bytes(sp_const_nine);
+        auto sp_5 = as_writable_bytes(sp_dyn);
+        auto sp_6 = as_writable_bytes(sp_nine);
+
+        static_assert(is_same<decltype(sp_1), gsl::span<const byte>>::value,
+                      "is_same<decltype(sp_1), gsl::span<const byte>>::value");
+        static_assert(is_same<decltype(sp_2), gsl::span<const byte, SizeBytes>>::value,
+                      "is_same<decltype(sp_2), gsl::span<const byte, SizeBytes>>::value");
+        static_assert(is_same<decltype(sp_3), gsl::span<const byte>>::value,
+                      "is_same<decltype(sp_3), gsl::span<const byte>>::value");
+        static_assert(is_same<decltype(sp_4), gsl::span<const byte, SizeBytes>>::value,
+                      "is_same<decltype(sp_4), gsl::span<const byte, SizeBytes>>::value");
+        static_assert(is_same<decltype(sp_5), gsl::span<byte>>::value,
+                      "is_same<decltype(sp_5), gsl::span<byte>>::value");
+        static_assert(is_same<decltype(sp_6), gsl::span<byte, SizeBytes>>::value,
+                      "is_same<decltype(sp_6), gsl::span<byte, SizeBytes>>::value");
+
+        EXPECT_TRUE(sp_1.data() == reinterpret_cast<const byte*>(begin(sequence)));
+        EXPECT_TRUE(sp_2.data() == reinterpret_cast<const byte*>(begin(sequence)));
+        EXPECT_TRUE(sp_3.data() == reinterpret_cast<const byte*>(begin(sequence)));
+        EXPECT_TRUE(sp_4.data() == reinterpret_cast<const byte*>(begin(sequence)));
+        EXPECT_TRUE(sp_5.data() == reinterpret_cast<byte*>(begin(sequence)));
+        EXPECT_TRUE(sp_6.data() == reinterpret_cast<byte*>(begin(sequence)));
+
+        EXPECT_TRUE(sp_1.size() == SizeBytes);
+        EXPECT_TRUE(sp_2.size() == SizeBytes);
+        EXPECT_TRUE(sp_3.size() == SizeBytes);
+        EXPECT_TRUE(sp_4.size() == SizeBytes);
+        EXPECT_TRUE(sp_5.size() == SizeBytes);
+        EXPECT_TRUE(sp_6.size() == SizeBytes);
+    }
+}
+
+// assertions for span's definition
+static_assert(std::is_same<decltype(gsl::dynamic_extent), const std::size_t>::value,
+              "gsl::dynamic_extent must be respresented as std::size_t");
+static_assert(gsl::dynamic_extent == static_cast<std::size_t>(-1),
+              "gsl::dynamic_extent must be defined as the max value of std::size_t");
+
+static_assert(std::is_same<decltype(gsl::span<int>::extent), const std::size_t>::value,
+              "Ensure that the type of  gsl::span::extent is std::size_t");
+static_assert(gsl::span<int>::extent == gsl::dynamic_extent,
+              "gsl::span<int>::extent should be equivalent to gsl::dynamic_extent");
+
+static_assert(std::is_same<decltype(gsl::span<int, 3>::extent), const std::size_t>::value,
+              "Ensure that the type of gsl::span::extent is std::size_t");
+static_assert(gsl::span<int, 3>::extent == 3, "Ensure that span<int, 3>::extent is equal to 3");
+
+static_assert(std::is_same<gsl::span<int>::element_type, int>::value,
+              "span<int>::element_type should be int");
+static_assert(std::is_same<gsl::span<int>::value_type, int>::value,
+              "span<int>::value_type should be int");
+static_assert(std::is_same<gsl::span<int>::size_type, std::size_t>::value,
+              "span<int>::size_type should be std::size_t");
+static_assert(std::is_same<gsl::span<int>::difference_type, ptrdiff_t>::value,
+              "span<int>::difference_type should be std::ptrdiff_t");
+static_assert(std::is_same<gsl::span<int>::pointer, int*>::value,
+              "span<int>::pointer should be int*");
+static_assert(std::is_same<gsl::span<int>::const_pointer, const int*>::value,
+              "span<int>::const_pointer should be const int*");
+static_assert(std::is_same<gsl::span<int>::reference, int&>::value,
+              "span<int>::reference should be int&");
+static_assert(std::is_same<gsl::span<int>::const_reference, const int&>::value,
+              "span<int>::const_reference should be const int&");
+
+static_assert(std::is_same<gsl::span<int, 3>::element_type, int>::value,
+              "span<int, 3>::element_type should be int");
+static_assert(std::is_same<gsl::span<int, 3>::value_type, int>::value,
+              "span<int, 3>::value_type should be int");
+static_assert(std::is_same<gsl::span<int, 3>::size_type, std::size_t>::value,
+              "span<int, 3>::size_type should be std::size_t");
+static_assert(std::is_same<gsl::span<int, 3>::difference_type, ptrdiff_t>::value,
+              "span<int, 3>::difference_type should be std::ptrdiff_t");
+static_assert(std::is_same<gsl::span<int, 3>::pointer, int*>::value,
+              "span<int, 3>::pointer should be int*");
+static_assert(std::is_same<gsl::span<int, 3>::const_pointer, const int*>::value,
+              "span<int, 3>::const_pointer should be const int*");
+static_assert(std::is_same<gsl::span<int, 3>::reference, int&>::value,
+              "span<int, 3>::reference should be int&");
+static_assert(std::is_same<gsl::span<int, 3>::const_reference, const int&>::value,
+              "span<int, 3>::const_reference should be const int&");
+
+static_assert(std::is_same<gsl::span<const int>::element_type, const int>::value,
+              "span<const int>::element_type should be const int");
+static_assert(std::is_same<gsl::span<const int>::value_type, int>::value,
+              "span<const int>::value_type should be int");
+static_assert(std::is_same<gsl::span<const int>::size_type, std::size_t>::value,
+              "span<const int>::size_type should be size_t");
+static_assert(std::is_same<gsl::span<const int>::difference_type, ptrdiff_t>::value,
+              "span<const int>::difference_type should be ptrdiff_t");
+static_assert(std::is_same<gsl::span<const int>::pointer, const int*>::value,
+              "span<const int>::pointer should be const int*");
+static_assert(std::is_same<gsl::span<const int>::const_pointer, const int*>::value,
+              "span<const int>::const_pointer should be const int*");
+static_assert(std::is_same<gsl::span<const int>::reference, const int&>::value,
+              "span<const int>::reference should be const int&");
+static_assert(std::is_same<gsl::span<const int>::const_reference, const int&>::value,
+              "span<const int>::const_reference should be const int&");
+
+static_assert(std::is_same<gsl::span<const int, 3>::element_type, const int>::value,
+              "span<const int, 3>::element_type should be const int");
+static_assert(std::is_same<gsl::span<const int, 3>::value_type, int>::value,
+              "span<const int, 3>::value_type should be int");
+static_assert(std::is_same<gsl::span<const int, 3>::size_type, std::size_t>::value,
+              "span<const int, 3>::size_type should be size_t");
+static_assert(std::is_same<gsl::span<const int, 3>::difference_type, ptrdiff_t>::value,
+              "span<const int, 3>::difference_type should be ptrdiff_t");
+static_assert(std::is_same<gsl::span<const int, 3>::pointer, const int*>::value,
+              "span<const int, 3>::pointer should be const int*");
+static_assert(std::is_same<gsl::span<const int, 3>::const_pointer, const int*>::value,
+              "span<const int, 3>::const_pointer should be const int*");
+static_assert(std::is_same<gsl::span<const int, 3>::reference, const int&>::value,
+              "span<const int, 3>::reference should be const int&");
+static_assert(std::is_same<gsl::span<const int, 3>::const_reference, const int&>::value,
+              "span<const int, 3>::const_reference should be const int&");
+
+// assertions for span_iterator
+static_assert(std::is_same<std::iterator_traits<gsl::span<int>::iterator>::pointer, int*>::value,
+              "span<int>::iterator's pointer should be int*");
+static_assert(
+    std::is_same<std::iterator_traits<gsl::span<int>::const_iterator>::pointer, const int*>::value,
+    "span<int>::const_iterator's pointer should be const int*");
+static_assert(
+    std::is_same<gsl::span<int>::reverse_iterator,
+                 std::reverse_iterator<gsl::span<int>::iterator>>::value,
+    "span<int>::reverse_iterator should equal std::reverse_iterator<span<int>::iterator>");
+static_assert(std::is_same<gsl::span<int>::const_reverse_iterator,
+                           std::reverse_iterator<gsl::span<int>::const_iterator>>::value,
+              "span<int>::const_reverse_iterator should equal "
+              "std::reverse_iterator<span<int>::const_iterator>");
+
+static_assert(std::is_same<std::iterator_traits<gsl::span<int, 3>::iterator>::pointer, int*>::value,
+              "span<int, 3>::iterator's pointer should be int*");
+static_assert(std::is_same<std::iterator_traits<gsl::span<int, 3>::const_iterator>::pointer,
+                           const int*>::value,
+              "span<int, 3>::const_iterator's pointer should be const int*");
+static_assert(
+    std::is_same<gsl::span<int, 3>::reverse_iterator,
+                 std::reverse_iterator<gsl::span<int, 3>::iterator>>::value,
+    "span<int, 3>::reverse_iterator should equal std::reverse_iterator<span<int, 3>::iterator>");
+static_assert(std::is_same<gsl::span<int, 3>::const_reverse_iterator,
+                           std::reverse_iterator<gsl::span<int, 3>::const_iterator>>::value,
+              "span<int, 3>::const_reverse_iterator should equal std::reverse_iterator<span<int, "
+              "3>::const_iterator>");
+
+static_assert(
+    std::is_same<std::iterator_traits<gsl::span<const int>::iterator>::pointer, const int*>::value,
+    "span<const int>::iterator's pointer should be int*");
+static_assert(std::is_same<std::iterator_traits<gsl::span<const int>::const_iterator>::pointer,
+                           const int*>::value,
+              "span<const int>::const_iterator's pointer should be const int*");
+static_assert(std::is_same<gsl::span<const int>::reverse_iterator,
+                           std::reverse_iterator<gsl::span<const int>::iterator>>::value,
+              "span<const int>::reverse_iterator should equal std::reverse_iterator<span<const "
+              "int>::iterator>");
+static_assert(std::is_same<gsl::span<const int>::const_reverse_iterator,
+                           std::reverse_iterator<gsl::span<const int>::const_iterator>>::value,
+              "span<const int>::const_reverse_iterator should equal "
+              "std::reverse_iterator<span<const int>::const_iterator>");
+
+static_assert(std::is_same<std::iterator_traits<gsl::span<const int, 3>::iterator>::pointer,
+                           const int*>::value,
+              "span<const int, 3>::iterator's pointer should be int*");
+static_assert(std::is_same<std::iterator_traits<gsl::span<const int, 3>::const_iterator>::pointer,
+                           const int*>::value,
+              "span<const int, 3>::const_iterator's pointer should be const int*");
+static_assert(std::is_same<gsl::span<const int, 3>::reverse_iterator,
+                           std::reverse_iterator<gsl::span<const int, 3>::iterator>>::value,
+              "span<const int, 3>::reverse_iterator should equal std::reverse_iterator<span<const "
+              "int, 3>::iterator>");
+static_assert(std::is_same<gsl::span<const int, 3>::const_reverse_iterator,
+                           std::reverse_iterator<gsl::span<const int, 3>::const_iterator>>::value,
+              "span<const int, 3>::const_reverse_iterator should equal "
+              "std::reverse_iterator<span<const int, 3>::const_iterator>");
+
+// copyability assertions
+static_assert(std::is_trivially_copyable<gsl::span<int>>::value,
+              "span<int> should be trivially copyable");
+static_assert(std::is_trivially_copyable<gsl::span<int>::iterator>::value,
+              "span<int>::iterator should be trivially copyable");
+static_assert(std::is_trivially_copyable<gsl::span<int>::const_iterator>::value,
+              "span<int>::const_iterator should be trivially copyable");
+
+static_assert(std::is_trivially_copyable<gsl::span<int, 3>>::value,
+              "span<int, 3> should be trivially copyable");
+static_assert(std::is_trivially_copyable<gsl::span<int, 3>::iterator>::value,
+              "span<int, 3>::iterator should be trivially copyable");
+static_assert(std::is_trivially_copyable<gsl::span<int, 3>::const_iterator>::value,
+              "span<int, 3>::const_iterator should be trivially copyable");
+
+static_assert(std::is_trivially_copyable<gsl::span<const int>>::value,
+              "span<const int> should be trivially copyable");
+static_assert(std::is_trivially_copyable<gsl::span<const int>::iterator>::value,
+              "span<const int>::iterator should be trivially copyable");
+static_assert(std::is_trivially_copyable<gsl::span<const int>::const_iterator>::value,
+              "span<const int>::const_iterator should be trivially copyable");
+
+static_assert(std::is_trivially_copyable<gsl::span<const int, 3>>::value,
+              "span<const int, 3> should be trivially copyable");
+static_assert(std::is_trivially_copyable<gsl::span<const int, 3>::iterator>::value,
+              "span<const int, 3>::iterator should be trivially copyable");
+static_assert(std::is_trivially_copyable<gsl::span<const int, 3>::const_iterator>::value,
+              "span<const int, 3>::const_iterator should be trivially copyable");
+
+// nothrow constructible assertions
+static_assert(std::is_nothrow_constructible<gsl::span<int>, int*, std::size_t>::value,
+              "std::is_nothrow_constructible<gsl::span<int>, int*, std::size_t>");
+static_assert(std::is_nothrow_constructible<gsl::span<int>, int*, std::uint16_t>::value,
+              "std::is_nothrow_constructible<gsl::span<int>, int*, std::uint16_t>");
+static_assert(std::is_nothrow_constructible<gsl::span<int>, int*, int*>::value,
+              "std::is_nothrow_constructible<gsl::span<int>, int*, int*>");
+static_assert(std::is_nothrow_constructible<gsl::span<int>, int (&)[3]>::value,
+              "std::is_nothrow_constructible<gsl::span<int>, int(&)[3]>");
+static_assert(std::is_nothrow_constructible<gsl::span<int>, const gsl::span<int>&>::value,
+              "std::is_nothrow_constructible<gsl::span<int>, const gsl::span<int>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<int>, const gsl::span<int, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<int>, const gsl::span<int, 3>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<int>, const gsl::span<int, 500>&>::value,
+              "std::is_nothrow_constructible<gsl::span<int>, const gsl::span<int, 500>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<int>, std::array<int, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<int>, std::array<int, 3>&>");
+
+static_assert(std::is_nothrow_constructible<gsl::span<int, 3>, int*, std::size_t>::value,
+              "std::is_nothrow_constructible<gsl::span<int, 3>, int*, std::size_t>");
+static_assert(std::is_nothrow_constructible<gsl::span<int, 3>, int*, std::uint16_t>::value,
+              "std::is_nothrow_constructible<gsl::span<int, 3>, int*, std::uint16_t>");
+static_assert(std::is_nothrow_constructible<gsl::span<int, 3>, int*, int*>::value,
+              "std::is_nothrow_constructible<gsl::span<int, 3>, int*, int*>");
+static_assert(std::is_nothrow_constructible<gsl::span<int, 3>, int (&)[3]>::value,
+              "std::is_nothrow_constructible<gsl::span<int, 3>, int(&)[3]>");
+static_assert(std::is_nothrow_constructible<gsl::span<int, 3>, const gsl::span<int, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<int, 3>, const gsl::span<int, 3>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<int, 3>, std::array<int, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<int, 3>, std::array<int, 3>&>");
+
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, int*, std::size_t>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, int*, std::size_t>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, int*, int*>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, int*, int*>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, int*, const int*>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, int*, const int*>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, int (&)[3]>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, int(&)[3]>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, const int*, int*>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, const int*, int*>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, const int*, const int*>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, const int*, const int*>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, const int*, std::size_t>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, const int*, std::size_t>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, const int (&)[3]>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, const int(&)[3]>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<int>&>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<int>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<int, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<int, 3>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<int, 500>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<int, 500>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<const int>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<const int>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<const int, 3>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<const int, 3>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<const int, 500>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const int>, const gsl::span<const int, 500>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, std::array<int, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, std::array<int, 3>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<const int>, const std::array<int, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<const int>, const std::array<int, 3>&>");
+
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const int, 3>, const gsl::span<int, 3>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const int, 3>, const gsl::span<int, 3>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const int, 3>, const gsl::span<const int, 3>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const int, 3>, const gsl::span<const int, 3>&>");
+
+static_assert(std::is_nothrow_constructible<gsl::span<Base>, Base (&)[3]>::value,
+              "std::is_nothrow_constructible<gsl::span<Base>, Base(&)[3]>");
+static_assert(std::is_nothrow_constructible<gsl::span<Base>, const gsl::span<Base>&>::value,
+              "std::is_nothrow_constructible<gsl::span<Base>, const gsl::span<Base>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<Base>, const gsl::span<Base, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<Base>, const gsl::span<Base, 3>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<Base>, std::array<Base, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<Base>, std::array<Base, 3>&>");
+
+static_assert(std::is_nothrow_constructible<gsl::span<Base, 3>, Base (&)[3]>::value,
+              "std::is_nothrow_constructible<gsl::span<Base, 3>, Base(&)[3]>");
+static_assert(std::is_nothrow_constructible<gsl::span<Base, 3>, const gsl::span<Base, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<Base, 3>, const gsl::span<Base, 3>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<Base, 3>, std::array<Base, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<Base, 3>, std::array<Base, 3>&>");
+
+static_assert(std::is_nothrow_constructible<gsl::span<const Base>, Base (&)[3]>::value,
+              "std::is_nothrow_constructible<gsl::span<const Base>, Base(&)[3]>");
+static_assert(std::is_nothrow_constructible<gsl::span<const Base>, const Base (&)[3]>::value,
+              "std::is_nothrow_constructible<gsl::span<const Base>, const Base(&)[3]>");
+static_assert(std::is_nothrow_constructible<gsl::span<const Base>, const gsl::span<Base>&>::value,
+              "std::is_nothrow_constructible<gsl::span<const Base>, const gsl::span<Base>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const Base>, const gsl::span<Base, 3>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const Base>, const gsl::span<Base, 3>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const Base>, const gsl::span<const Base>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const Base>, const gsl::span<const Base>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const Base>, const gsl::span<const Base, 3>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const Base>, const gsl::span<const Base, 3>&>");
+static_assert(std::is_nothrow_constructible<gsl::span<const Base>, std::array<Base, 3>&>::value,
+              "std::is_nothrow_constructible<gsl::span<const Base>, std::array<Base, 3>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const Base>, const std::array<Base, 3>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const Base>, const std::array<Base, 3>&>");
+
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const Base, 3>, const gsl::span<Base, 3>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const Base, 3>, const gsl::span<Base, 3>&>");
+static_assert(
+    std::is_nothrow_constructible<gsl::span<const Base, 3>, const gsl::span<const Base, 3>&>::value,
+    "std::is_nothrow_constructible<gsl::span<const Base, 3>, const gsl::span<const Base, 3>&>");
+
+// non-constructible assertions
+static_assert(!std::is_constructible<gsl::span<int>, const int*, int*>::value,
+              "!std::is_constructible<gsl::span<int>, const int*, int*>");
+static_assert(!std::is_constructible<gsl::span<int>, const int*, const int*>::value,
+              "!std::is_constructible<gsl::span<int>, const int*, const int*>");
+static_assert(!std::is_constructible<gsl::span<int>, const int*, double*>::value,
+              "!std::is_constructible<gsl::span<int>, const int*, double*>");
+static_assert(!std::is_constructible<gsl::span<int>, const int*, std::size_t>::value,
+              "!std::is_constructible<gsl::span<int>, const int*, std::size_t>");
+static_assert(!std::is_constructible<gsl::span<int>, const int (&)[3]>::value,
+              "!std::is_constructible<gsl::span<int>, const int(&)[3]>");
+static_assert(!std::is_constructible<gsl::span<int>, double*, int*>::value,
+              "!std::is_constructible<gsl::span<int>, double*, int*>");
+static_assert(!std::is_constructible<gsl::span<int>, double*, const int*>::value,
+              "!std::is_constructible<gsl::span<int>, double*, const int*>");
+static_assert(!std::is_constructible<gsl::span<int>, double*, double*>::value,
+              "!std::is_constructible<gsl::span<int>, double*, double*>");
+static_assert(!std::is_constructible<gsl::span<int>, double*, std::size_t>::value,
+              "!std::is_constructible<gsl::span<int>, double*, std::size_t>");
+static_assert(!std::is_constructible<gsl::span<int>, double (&)[3]>::value,
+              "!std::is_constructible<gsl::span<int>, double(&)[3]>");
+static_assert(!std::is_constructible<gsl::span<int>, int*, double*>::value,
+              "!std::is_constructible<gsl::span<int>, int*, double*>");
+static_assert(!std::is_constructible<gsl::span<int>, std::size_t, int*>::value,
+              "!std::is_constructible<gsl::span<int>, std::size_t, int*>");
+static_assert(!std::is_constructible<gsl::span<int>, std::size_t, std::size_t>::value,
+              "!std::is_constructible<gsl::span<int>, std::size_t, std::size_t>");
+static_assert(!std::is_constructible<gsl::span<int>, const gsl::span<const int>&>::value,
+              "!std::is_constructible<gsl::span<int>, const gsl::span<const int>&>");
+static_assert(!std::is_constructible<gsl::span<int>, const gsl::span<const int, 3>&>::value,
+              "!std::is_constructible<gsl::span<int>, const gsl::span<const int, 3>&>");
+static_assert(!std::is_constructible<gsl::span<int>, const gsl::span<const int, 500>&>::value,
+              "!std::is_constructible<gsl::span<int>, const gsl::span<const int, 500>&>");
+static_assert(!std::is_constructible<gsl::span<int>, const gsl::span<double, 3>&>::value,
+              "!std::is_constructible<gsl::span<int>, const gsl::span<double, 3>&>");
+static_assert(!std::is_constructible<gsl::span<int>, std::array<double, 3>&>::value,
+              "!std::is_constructible<gsl::span<int>, std::array<double, 3>&>");
+static_assert(!std::is_constructible<gsl::span<int>, const std::array<int, 3>&>::value,
+              "!std::is_constructible<gsl::span<int>, const std::array<int, 3>&>");
+
+static_assert(!std::is_constructible<gsl::span<int, 3>, int*, double*>::value,
+              "!std::is_constructible<gsl::span<int, 3>, int*, double*>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, int (&)[500]>::value,
+              "!std::is_constructible<gsl::span<int, 3>, int(&)[500]>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const int*, int*>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const int*, int*>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const int*, const int*>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const int*, const int*>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const int*, std::size_t>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const int*, std::size_t>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const int*, double*>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const int*, double*>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const int (&)[3]>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const int(&)[3]>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, double*, std::size_t>::value,
+              "!std::is_constructible<gsl::span<int, 3>, double*, std::size_t>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, double*, int*>::value,
+              "!std::is_constructible<gsl::span<int, 3>, double*, int*>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, double*, const int*>::value,
+              "!std::is_constructible<gsl::span<int, 3>, double*, const int*>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, double*, double*>::value,
+              "!std::is_constructible<gsl::span<int, 3>, double*, double*>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, double (&)[3]>::value,
+              "!std::is_constructible<gsl::span<int, 3>, double(&)[3]>");
+
+static_assert(!std::is_constructible<gsl::span<int, 3>, std::size_t, int*>::value,
+              "!std::is_constructible<gsl::span<int, 3>, std::size_t, int*>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, std::size_t, std::size_t>::value,
+              "!std::is_constructible<gsl::span<int, 3>, std::size_t, std::size_t>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, std::array<double, 3>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, std::array<double, 3>&>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, std::array<int, 500>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, std::array<int, 500>&>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const std::array<int, 3>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const std::array<int, 3>&>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<int>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const gsl::span<int>&>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<int, 500>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const gsl::span<int, 500>&>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<const int>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const gsl::span<const int>&>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<const int, 3>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const gsl::span<const int, 3>&>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<const int, 500>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const gsl::span<const int, 500>&>");
+static_assert(!std::is_constructible<gsl::span<int, 3>, const gsl::span<double, 3>&>::value,
+              "!std::is_constructible<gsl::span<int, 3>, const gsl::span<double, 3>&>");
+
+static_assert(!std::is_constructible<gsl::span<const int>, double (&)[3]>::value,
+              "!std::is_constructible<gsl::span<const int>, double(&)[3]>");
+static_assert(!std::is_constructible<gsl::span<const int>, std::array<double, 3>&>::value,
+              "!std::is_constructible<gsl::span<const int>, std::array<double, 3>&>");
+static_assert(!std::is_constructible<gsl::span<const int>, const gsl::span<double, 3>&>::value,
+              "!std::is_constructible<gsl::span<const int>, const gsl::span<double, 3>&>");
+
+static_assert(!std::is_constructible<gsl::span<const int, 3>, const gsl::span<int>&>::value,
+              "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<int>&>");
+static_assert(!std::is_constructible<gsl::span<const int, 3>, const gsl::span<int, 500>&>::value,
+              "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<int, 500>&>");
+static_assert(!std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int>&>::value,
+              "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int>&>");
+static_assert(
+    !std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int, 500>&>::value,
+    "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<const int, 500>&>");
+static_assert(!std::is_constructible<gsl::span<const int, 3>, const gsl::span<double, 3>&>::value,
+              "!std::is_constructible<gsl::span<const int, 3>, const gsl::span<double, 3>&>");
+
+static_assert(!std::is_constructible<gsl::span<Base>, Derived (&)[3]>::value,
+              "!std::is_constructible<gsl::span<Base>, Derived(&)[3]>");
+static_assert(!std::is_constructible<gsl::span<Base>, std::array<Derived, 3>&>::value,
+              "!std::is_constructible<gsl::span<Base>, std::array<Derived, 3>&>");
+static_assert(!std::is_constructible<gsl::span<Base>, std::vector<Derived>&>::value,
+              "!std::is_constructible<gsl::span<Base>, std::vector<Derived>&>");
+static_assert(!std::is_constructible<gsl::span<Base>, const gsl::span<Derived>&>::value,
+              "!std::is_constructible<gsl::span<Base>, const gsl::span<Derived>&>");
+static_assert(!std::is_constructible<gsl::span<Base>, const gsl::span<Derived, 3>&>::value,
+              "!std::is_constructible<gsl::span<Base>, const gsl::span<Derived, 3>&>");
+
+static_assert(!std::is_constructible<gsl::span<Base, 3>, const gsl::span<Derived, 3>&>::value,
+              "!std::is_constructible<gsl::span<Base, 3>, const gsl::span<Derived, 3>&>");
+static_assert(!std::is_constructible<gsl::span<Base, 3>, Derived (&)[3]>::value,
+              "!std::is_constructible<gsl::span<Base, 3>, Derived(&)[3]>");
+static_assert(!std::is_constructible<gsl::span<Base, 3>, std::array<Derived, 3>&>::value,
+              "!std::is_constructible<gsl::span<Base, 3>, std::array<Derived, 3>&>");
+
+static_assert(!std::is_constructible<gsl::span<const Base>, Derived (&)[3]>::value,
+              "!std::is_constructible<gsl::span<const Base>, Derived(&)[3]>");
+static_assert(!std::is_constructible<gsl::span<const Base>, const Derived (&)[3]>::value,
+              "!std::is_constructible<gsl::span<const Base>, const Derived(&)[3]>");
+static_assert(!std::is_constructible<gsl::span<const Base>, std::array<Derived, 3>&>::value,
+              "!std::is_constructible<gsl::span<const Base>, std::array<Derived, 3>&>");
+static_assert(!std::is_constructible<gsl::span<const Base>, const std::array<Derived, 3>&>::value,
+              "!std::is_constructible<gsl::span<const Base>, const std::array<Derived, 3>&>");
+static_assert(!std::is_constructible<gsl::span<const Base>, const gsl::span<Derived>&>::value,
+              "!std::is_constructible<gsl::span<const Base>, const gsl::span<Derived>&>");
+static_assert(!std::is_constructible<gsl::span<const Base>, const gsl::span<Derived, 3>&>::value,
+              "!std::is_constructible<gsl::span<const Base>, const gsl::span<Derived, 3>&>");
+static_assert(!std::is_constructible<gsl::span<const Base>, const gsl::span<const Derived>&>::value,
+              "!std::is_constructible<gsl::span<const Base>, const gsl::span<const Derived>&>");
+static_assert(
+    !std::is_constructible<gsl::span<const Base>, const gsl::span<const Derived, 3>&>::value,
+    "!std::is_constructible<gsl::span<const Base>, const gsl::span<const Derived, 3>&>");
+
+static_assert(!std::is_constructible<gsl::span<const Base, 3>, const gsl::span<Derived, 3>&>::value,
+              "!std::is_constructible<gsl::span<const Base, 3>, const gsl::span<Derived, 3>&>");
+static_assert(
+    !std::is_constructible<gsl::span<const Base, 3>, const gsl::span<const Derived, 3>&>::value,
+    "!std::is_constructible<gsl::span<const Base, 3>, const gsl::span<const Derived, 3>&>");
+
+static_assert(!std::is_constructible<gsl::span<const Derived>, std::array<Base, 3>&>::value,
+              "!std::is_constructible<gsl::span<const Derived>, std::array<Base, 3>&>");
+static_assert(!std::is_constructible<gsl::span<const Derived>, const std::array<Base, 3>&>::value,
+              "!std::is_constructible<gsl::span<const Derived>, const std::array<Base, 3>&>");
+
+// no throw copy constructor
+static_assert(std::is_nothrow_copy_constructible<gsl::span<int>>::value,
+              "std::is_nothrow_copy_constructible<gsl::span<int>>");
+static_assert(std::is_nothrow_copy_constructible<gsl::span<int, 3>>::value,
+              "std::is_nothrow_copy_constructible<gsl::span<int, 3>>");
+static_assert(std::is_nothrow_copy_constructible<gsl::span<const int>>::value,
+              "std::is_nothrow_copy_constructible<gsl::span<const int>>");
+static_assert(std::is_nothrow_copy_constructible<gsl::span<const int, 3>>::value,
+              "std::is_nothrow_copy_constructible<gsl::span<const int, 3>>");
+
+// no throw copy assignment
+static_assert(std::is_nothrow_copy_assignable<gsl::span<int>>::value,
+              "std::is_nothrow_copy_assignable<gsl::span<int>>");
+static_assert(std::is_nothrow_copy_assignable<gsl::span<int, 3>>::value,
+              "std::is_nothrow_copy_assignable<gsl::span<int, 3>>");
+static_assert(std::is_nothrow_copy_assignable<gsl::span<const int>>::value,
+              "std::is_nothrow_copy_assignable<gsl::span<const int>>");
+static_assert(std::is_nothrow_copy_assignable<gsl::span<const int, 3>>::value,
+              "std::is_nothrow_copy_assignable<gsl::span<const int, 3>>");
+
+// no throw destruction
+static_assert(std::is_nothrow_destructible<gsl::span<int>>::value,
+              "std::is_nothrow_destructible<gsl::span<int>>");
+static_assert(std::is_nothrow_destructible<gsl::span<int, 3>>::value,
+              "std::is_nothrow_destructible<gsl::span<int, 3>>");
+static_assert(std::is_nothrow_destructible<gsl::span<const int>>::value,
+              "std::is_nothrow_destructible<gsl::span<const int>>");
+
+// conversions
+static_assert(std::is_convertible<int (&)[3], gsl::span<int>>::value,
+              "std::is_convertible<int(&)[3], gsl::span<int>>");
+static_assert(std::is_convertible<int (&)[3], gsl::span<int, 3>>::value,
+              "std::is_convertible<int(&)[3], gsl::span<int, 3>>");
+static_assert(std::is_convertible<int (&)[3], gsl::span<const int>>::value,
+              "std::is_convertible<int(&)[3], gsl::span<const int>>");
+
+static_assert(std::is_convertible<const int (&)[3], gsl::span<const int>>::value,
+              "std::is_convertible<const int(&)[3], gsl::span<const int>>");
+
+static_assert(std::is_convertible<const gsl::span<int>&, gsl::span<int>>::value,
+              "std::is_convertible<const gsl::span<int>&, gsl::span<int>>");
+static_assert(std::is_convertible<const gsl::span<int>&, gsl::span<const int>>::value,
+              "std::is_convertible<const gsl::span<int>&, gsl::span<const int>>");
+
+static_assert(std::is_convertible<const gsl::span<int, 3>&, gsl::span<int>>::value,
+              "std::is_convertible<const gsl::span<int, 3>&, gsl::span<int>>");
+static_assert(std::is_convertible<const gsl::span<int, 3>&, gsl::span<int, 3>>::value,
+              "std::is_convertible<const gsl::span<int, 3>&, gsl::span<int, 3>>");
+static_assert(std::is_convertible<const gsl::span<int, 3>&, gsl::span<const int>>::value,
+              "std::is_convertible<const gsl::span<int, 3>&, gsl::span<const int>>");
+static_assert(std::is_convertible<const gsl::span<int, 3>&, gsl::span<const int, 3>>::value,
+              "std::is_convertible<const gsl::span<int, 3>&, gsl::span<const int, 3>>");
+static_assert(std::is_convertible<const gsl::span<int, 500>&, gsl::span<int>>::value,
+              "std::is_convertible<const gsl::span<int, 500>&, gsl::span<int>>");
+static_assert(std::is_convertible<const gsl::span<int, 500>&, gsl::span<const int>>::value,
+              "std::is_convertible<const gsl::span<int, 500>&, gsl::span<const int>>");
+
+static_assert(std::is_convertible<const gsl::span<const int>&, gsl::span<const int>>::value,
+              "std::is_convertible<const gsl::span<const int>&, gsl::span<const int>>");
+
+static_assert(std::is_convertible<const gsl::span<const int, 3>&, gsl::span<const int>>::value,
+              "std::is_convertible<const gsl::span<const int, 3>&, gsl::span<const int>>");
+static_assert(std::is_convertible<const gsl::span<const int, 3>&, gsl::span<const int, 3>>::value,
+              "std::is_convertible<const gsl::span<const int, 3>&, gsl::span<const int, 3>>");
+static_assert(std::is_convertible<const gsl::span<const int, 500>&, gsl::span<const int>>::value,
+              "std::is_convertible<const gsl::span<const int, 500>&, gsl::span<const int>>");
+
+static_assert(std::is_convertible<std::array<int, 3>&, gsl::span<int>>::value,
+              "std::is_convertible<std::array<int, 3>&, gsl::span<int>>");
+static_assert(std::is_convertible<std::array<int, 3>&, gsl::span<int, 3>>::value,
+              "std::is_convertible<std::array<int, 3>&, gsl::span<int, 3>>");
+static_assert(std::is_convertible<std::array<int, 3>&, gsl::span<const int>>::value,
+              "std::is_convertible<std::array<int, 3>&, gsl::span<const int>>");
+
+static_assert(std::is_convertible<const std::array<int, 3>&, gsl::span<const int>>::value,
+              "std::is_convertible<const std::array<int, 3>&, gsl::span<const int>>");
+
+
+#if __cplusplus >= 201703l
+template <typename U, typename = void>
+static constexpr bool AsWritableBytesCompilesFor = false;
+
+template <typename U>
+static constexpr bool AsWritableBytesCompilesFor<U, void_t<decltype(as_writable_bytes(declval<U>()))>> =
+    true;
+
+static_assert(AsWritableBytesCompilesFor<gsl::span<int>>,
+                "AsWritableBytesCompilesFor<gsl::span<int>>");
+static_assert(AsWritableBytesCompilesFor<gsl::span<int, 9>>,
+                "AsWritableBytesCompilesFor<gsl::span<int, 9>>");
+static_assert(!AsWritableBytesCompilesFor<gsl::span<const int>>,
+                "!AsWritableBytesCompilesFor<gsl::span<const int>>");
+static_assert(!AsWritableBytesCompilesFor<gsl::span<const int, 9>>,
+                "!AsWritableBytesCompilesFor<gsl::span<const int, 9>>");
+#endif // __cplusplus >= 201703l
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index c41e013..2c4a19c 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -25,15 +25,11 @@
 #include <iterator>    // for reverse_iterator, operator-, operator==
 #include <memory>      // for unique_ptr, shared_ptr, make_unique, allo...
 #include <regex>       // for match_results, sub_match, match_results<>...
-#include <stddef.h>    // for ptrdiff_t
+#include <cstddef>     // for ptrdiff_t
 #include <string>      // for string
 #include <type_traits> // for integral_constant<>::value, is_default_co...
 #include <vector>      // for vector
-
-namespace gsl
-{
-struct fail_fast;
-} // namespace gsl
+#include <utility>
 
 using namespace std;
 using namespace gsl;
@@ -110,17 +106,17 @@ TEST(span_test, from_nullptr_size_constructor)
         std::abort();
     });
     {
-        span<int> s{nullptr, narrow_cast<span<int>::index_type>(0)};
+        span<int> s{nullptr, narrow_cast<span<int>::size_type>(0)};
         EXPECT_TRUE(s.size() == 0);
         EXPECT_TRUE(s.data() == nullptr);
 
-        span<int> cs{nullptr, narrow_cast<span<int>::index_type>(0)};
+        span<int> cs{nullptr, narrow_cast<span<int>::size_type>(0)};
         EXPECT_TRUE(cs.size() == 0);
         EXPECT_TRUE(cs.data() == nullptr);
     }
     {
         auto workaround_macro = []() {
-            const span<int, 1> s{nullptr, narrow_cast<span<int>::index_type>(0)};
+            const span<int, 1> s{nullptr, narrow_cast<span<int>::size_type>(0)};
         };
         EXPECT_DEATH(workaround_macro(), deathstring);
     }
@@ -139,11 +135,11 @@ TEST(span_test, from_nullptr_size_constructor)
         EXPECT_DEATH(const_workaround_macro(), deathstring);
     }
     {
-        span<int*> s{nullptr, narrow_cast<span<int>::index_type>(0)};
+        span<int*> s{nullptr, narrow_cast<span<int>::size_type>(0)};
         EXPECT_TRUE(s.size() == 0);
         EXPECT_TRUE(s.data() == nullptr);
 
-        span<const int*> cs{nullptr, narrow_cast<span<int>::index_type>(0)};
+        span<const int*> cs{nullptr, narrow_cast<span<int>::size_type>(0)};
         EXPECT_TRUE(cs.size() == 0);
         EXPECT_TRUE(cs.data() == nullptr);
     }
@@ -161,29 +157,21 @@ TEST(span_test, from_pointer_length_constructor)
         for (int i = 0; i < 4; ++i)
         {
             {
-                span<int> s = {&arr[0], i};
-                EXPECT_TRUE(s.size() == i);
+                span<int> s = {&arr[0], narrow_cast<std::size_t>(i)};
+                EXPECT_TRUE(s.size() == narrow_cast<std::size_t>(i));
                 EXPECT_TRUE(s.data() == &arr[0]);
                 EXPECT_TRUE(s.empty() == (i == 0));
                 for (int j = 0; j < i; ++j)
-                {
-                    EXPECT_TRUE(arr[j] == s[j]);
-                    EXPECT_TRUE(arr[j] == s.at(j));
-                    EXPECT_TRUE(arr[j] == s(j));
-                }
+                    EXPECT_TRUE(arr[j] == s[narrow_cast<std::size_t>(j)]);
             }
             {
-                span<int> s = {&arr[i], 4 - narrow_cast<ptrdiff_t>(i)};
-                EXPECT_TRUE(s.size() == 4 - i);
+                span<int> s = {&arr[i], 4 - narrow_cast<std::size_t>(i)};
+                EXPECT_TRUE(s.size() == 4 - narrow_cast<std::size_t>(i));
                 EXPECT_TRUE(s.data() == &arr[i]);
                 EXPECT_TRUE(s.empty() == ((4 - i) == 0));
 
                 for (int j = 0; j < 4 - i; ++j)
-                {
-                    EXPECT_TRUE(arr[j + i] == s[j]);
-                    EXPECT_TRUE(arr[j + i] == s.at(j));
-                    EXPECT_TRUE(arr[j + i] == s(j));
-                }
+                    EXPECT_TRUE(arr[j + i] == s[narrow_cast<std::size_t>(j)]);
             }
         }
     }
@@ -198,7 +186,7 @@ TEST(span_test, from_pointer_length_constructor)
 
     {
         int* p = nullptr;
-        span<int> s{p, narrow_cast<span<int>::index_type>(0)};
+        span<int> s{p, narrow_cast<span<int>::size_type>(0)};
         EXPECT_TRUE(s.size() == 0);
         EXPECT_TRUE(s.data() == nullptr);
     }
@@ -219,7 +207,7 @@ TEST(span_test, from_pointer_length_constructor)
 
     {
         int* p = nullptr;
-        auto s = make_span(p, narrow_cast<span<int>::index_type>(0));
+        auto s = make_span(p, narrow_cast<span<int>::size_type>(0));
         EXPECT_TRUE(s.size() == 0);
         EXPECT_TRUE(s.data() == nullptr);
     }
@@ -457,21 +445,21 @@ TEST(span_test, from_array_constructor)
 
      {
          span<int> s{arr};
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() == arr.size());
          EXPECT_TRUE(s.data() == arr.data());
 
          span<const int> cs{arr};
-         EXPECT_TRUE(cs.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(cs.size() == arr.size());
          EXPECT_TRUE(cs.data() == arr.data());
      }
 
      {
          span<int, 4> s{arr};
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() == arr.size());
          EXPECT_TRUE(s.data() == arr.data());
 
          span<const int, 4> cs{arr};
-         EXPECT_TRUE(cs.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(cs.size() == arr.size());
          EXPECT_TRUE(cs.data() == arr.data());
      }
 
@@ -486,7 +474,7 @@ TEST(span_test, from_array_constructor)
 
      {
          span<AddressOverloaded, 4> fs{ao_arr};
-         EXPECT_TRUE(fs.size() == narrow_cast<ptrdiff_t>(ao_arr.size()));
+         EXPECT_TRUE(fs.size() == ao_arr.size());
          EXPECT_TRUE(ao_arr.data() == fs.data());
      }
 
@@ -532,7 +520,7 @@ TEST(span_test, from_array_constructor)
 
      {
          auto s = make_span(arr);
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() == arr.size());
          EXPECT_TRUE(s.data() == arr.data());
      }
 
@@ -561,13 +549,13 @@ TEST(span_test, from_array_constructor)
 
      {
          span<const int> s{arr};
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() == arr.size());
          EXPECT_TRUE(s.data() == arr.data());
      }
 
      {
          span<const int, 4> s{arr};
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() == arr.size());
          EXPECT_TRUE(s.data() == arr.data());
      }
 
@@ -575,7 +563,7 @@ TEST(span_test, from_array_constructor)
 
      {
          span<const AddressOverloaded, 4> s{ao_arr};
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(ao_arr.size()));
+         EXPECT_TRUE(s.size() == ao_arr.size());
          EXPECT_TRUE(s.data() == ao_arr.data());
      }
 
@@ -606,7 +594,7 @@ TEST(span_test, from_array_constructor)
 
      {
          auto s = make_span(arr);
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() == arr.size());
          EXPECT_TRUE(s.data() == arr.data());
      }
  }
@@ -617,13 +605,13 @@ TEST(span_test, from_array_constructor)
 
      {
          span<const int> s{arr};
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() == arr.size());
          EXPECT_TRUE(s.data() == arr.data());
      }
 
      {
          span<const int, 4> s{arr};
-         EXPECT_TRUE(s.size() ==  narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() ==  arr.size());
          EXPECT_TRUE(s.data() == arr.data());
      }
 
@@ -651,7 +639,7 @@ TEST(span_test, from_array_constructor)
 
      {
          auto s = make_span(arr);
-         EXPECT_TRUE(s.size() == narrow_cast<ptrdiff_t>(arr.size()));
+         EXPECT_TRUE(s.size() == arr.size());
          EXPECT_TRUE(s.data() == arr.data());
      }
  }
@@ -663,11 +651,11 @@ TEST(span_test, from_array_constructor)
 
      {
          span<int> s{v};
-         EXPECT_TRUE(s.size() == narrow_cast<std::ptrdiff_t>(v.size()));
+         EXPECT_TRUE(s.size() == v.size());
          EXPECT_TRUE(s.data() == v.data());
 
          span<const int> cs{v};
-         EXPECT_TRUE(cs.size() == narrow_cast<std::ptrdiff_t>(v.size()));
+         EXPECT_TRUE(cs.size() == v.size());
          EXPECT_TRUE(cs.data() == v.data());
      }
 
@@ -677,11 +665,11 @@ TEST(span_test, from_array_constructor)
      {
  #ifdef CONFIRM_COMPILATION_ERRORS
          span<char> s{str};
-         EXPECT_TRUE(s.size() == narrow_cast<std::ptrdiff_t>(str.size()));
+         EXPECT_TRUE(s.size() == str.size());
          EXPECT_TRUE(s.data() == str.data()));
  #endif
          span<const char> cs{str};
-         EXPECT_TRUE(cs.size() == narrow_cast<std::ptrdiff_t>(str.size()));
+         EXPECT_TRUE(cs.size() == str.size());
          EXPECT_TRUE(cs.data() == str.data());
      }
 
@@ -690,7 +678,7 @@ TEST(span_test, from_array_constructor)
          span<char> s{cstr};
  #endif
          span<const char> cs{cstr};
-         EXPECT_TRUE(cs.size() == narrow_cast<std::ptrdiff_t>(cstr.size()));
+         EXPECT_TRUE(cs.size() == cstr.size());
          EXPECT_TRUE(cs.data() == cstr.data());
      }
 
@@ -745,11 +733,11 @@ TEST(span_test, from_array_constructor)
 
      {
          auto s = make_span(v);
-         EXPECT_TRUE(s.size() == narrow_cast<std::ptrdiff_t>(v.size()));
+         EXPECT_TRUE(s.size() == v.size());
          EXPECT_TRUE(s.data() == v.data());
 
          auto cs = make_span(cv);
-         EXPECT_TRUE(cs.size() == narrow_cast<std::ptrdiff_t>(cv.size()));
+         EXPECT_TRUE(cs.size() == cv.size());
          EXPECT_TRUE(cs.data() == cv.data());
      }
  }
@@ -966,7 +954,7 @@ TEST(span_test, from_array_constructor)
          EXPECT_TRUE(av.subspan(5).size() == 0);
          EXPECT_DEATH(av.subspan(6).size(), deathstring);
          const auto av2 = av.subspan(1);
-         for (int i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == i + 2);
+         for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == static_cast<int>(i) + 2);
      }
 
      {
@@ -977,53 +965,7 @@ TEST(span_test, from_array_constructor)
          EXPECT_TRUE(av.subspan(5).size() == 0);
          EXPECT_DEATH(av.subspan(6).size(), deathstring);
          const auto av2 = av.subspan(1);
-         for (int i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == i + 2);
-     }
- }
-
- TEST(span_test, at_call)
- {
-    std::set_terminate([] {
-        std::cerr << "Expected Death. at_call";
-        std::abort();
-    });
-     int arr[4] = {1, 2, 3, 4};
-
-     {
-         span<int> s = arr;
-         EXPECT_TRUE(s.at(0) == 1);
-         EXPECT_DEATH(s.at(5), deathstring);
-     }
-
-     {
-         int arr2d[2] = {1, 6};
-         span<int, 2> s = arr2d;
-         EXPECT_TRUE(s.at(0) == 1);
-         EXPECT_TRUE(s.at(1) == 6);
-         EXPECT_DEATH(s.at(2), deathstring);
-     }
- }
-
- TEST(span_test, operator_function_call)
- {
-    std::set_terminate([] {
-        std::cerr << "Expected Death. operator_function_call";
-        std::abort();
-    });
-     int arr[4] = {1, 2, 3, 4};
-
-     {
-         span<int> s = arr;
-         EXPECT_TRUE(s(0) == 1);
-         EXPECT_DEATH(s(5), deathstring);
-     }
-
-     {
-         int arr2d[2] = {1, 6};
-         span<int, 2> s = arr2d;
-         EXPECT_TRUE(s(0) == 1);
-         EXPECT_TRUE(s(1) == 6);
-         EXPECT_DEATH(s(2), deathstring);
+         for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == static_cast<int>(i) + 2);
      }
  }
 
@@ -1067,28 +1009,28 @@ TEST(span_test, from_array_constructor)
  {
      int a[] = {1, 2, 3, 4};
      span<int> s{a};
- 
+
      EXPECT_TRUE((std::is_same<decltype(s.begin()), decltype(begin(s))>::value));
      EXPECT_TRUE((std::is_same<decltype(s.end()), decltype(end(s))>::value));
- 
+
      EXPECT_TRUE((std::is_same<decltype(s.cbegin()), decltype(cbegin(s))>::value));
      EXPECT_TRUE((std::is_same<decltype(s.cend()), decltype(cend(s))>::value));
- 
+
      EXPECT_TRUE((std::is_same<decltype(s.rbegin()), decltype(rbegin(s))>::value));
      EXPECT_TRUE((std::is_same<decltype(s.rend()), decltype(rend(s))>::value));
- 
+
      EXPECT_TRUE((std::is_same<decltype(s.crbegin()), decltype(crbegin(s))>::value));
      EXPECT_TRUE((std::is_same<decltype(s.crend()), decltype(crend(s))>::value));
- 
-     EXPECT_TRUE(s.begin() == begin(s));
-     EXPECT_TRUE(s.end() == end(s));
- 
-     EXPECT_TRUE(s.cbegin() == cbegin(s));
-     EXPECT_TRUE(s.cend() == cend(s));
- 
+
+     EXPECT_TRUE(s.begin() == std::begin(s));
+     EXPECT_TRUE(s.end() == std::end(s));
+
+     EXPECT_TRUE(s.cbegin() == std::cbegin(s));
+     EXPECT_TRUE(s.cend() == std::cend(s));
+
      EXPECT_TRUE(s.rbegin() == rbegin(s));
      EXPECT_TRUE(s.rend() == rend(s));
- 
+
      EXPECT_TRUE(s.crbegin() == crbegin(s));
      EXPECT_TRUE(s.crend() == crend(s));
  }
@@ -1097,9 +1039,9 @@ TEST(span_test, from_array_constructor)
  {
      int a[] = {1, 2, 3, 4};
      span<int> s{a};
- 
-     EXPECT_TRUE((std::is_same<decltype(s.size()), decltype(ssize(s))>::value));
-     EXPECT_TRUE(s.size() == ssize(s));
+
+     EXPECT_FALSE((std::is_same<decltype(s.size()), decltype(ssize(s))>::value));
+     EXPECT_TRUE(s.size() == static_cast<std::size_t>(ssize(s)));
  }
 
  TEST(span_test, iterator_comparisons)
@@ -1149,6 +1091,28 @@ TEST(span_test, from_array_constructor)
      }
  }
 
+ TEST(span_test, incomparable_iterators)
+ {
+     std::set_terminate([] {
+         std::cerr << "Expected Death. incomparable_iterators";
+         std::abort();
+     });
+
+     int a[] = {1, 2, 3, 4};
+     int b[] = {1, 2, 3, 4};
+     {
+         span<int> s = a;
+         span<int> s2 = b;
+#if (__cplusplus > 201402L)
+         EXPECT_DEATH([[maybe_unused]] bool _ = (s.begin() == s2.begin()), deathstring);
+         EXPECT_DEATH([[maybe_unused]] bool _ = (s.begin() <= s2.begin()), deathstring);
+#else
+         EXPECT_DEATH(bool _ = (s.begin() == s2.begin()), deathstring);
+         EXPECT_DEATH(bool _ = (s.begin() <= s2.begin()), deathstring);
+#endif
+     }
+ }
+
  TEST(span_test, begin_end)
  {
     std::set_terminate([] {
@@ -1484,8 +1448,12 @@ TEST(span_test, from_array_constructor)
 
  TEST(span_test, as_bytes)
  {
-     int a[] = {1, 2, 3, 4};
+     std::set_terminate([] {
+         std::cerr << "Expected Death. as_bytes";
+         std::abort();
+     });
 
+     int a[] = {1, 2, 3, 4};
      {
          const span<const int> s = a;
          EXPECT_TRUE(s.size() == 4);
@@ -1510,9 +1478,15 @@ TEST(span_test, from_array_constructor)
          EXPECT_TRUE(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
          EXPECT_TRUE(bs.size() == s.size_bytes());
      }
+
+     int b[5] = {1, 2, 3, 4, 5};
+     {
+         span<int> sp(begin(b), static_cast<size_t>(-2));
+         EXPECT_DEATH((void) sp.size_bytes(), deathstring);
+     }
  }
 
- TEST(span_test, as_writeable_bytes)
+ TEST(span_test, as_writable_bytes)
  {
      int a[] = {1, 2, 3, 4};
 
@@ -1521,7 +1495,7 @@ TEST(span_test, from_array_constructor)
          // you should not be able to get writeable bytes for const objects
          span<const int> s = a;
          EXPECT_TRUE(s.size() == 4);
-         span<const byte> bs = as_writeable_bytes(s);
+         span<const byte> bs = as_writable_bytes(s);
          EXPECT_TRUE(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
          EXPECT_TRUE(bs.size() == s.size_bytes());
  #endif
@@ -1529,7 +1503,7 @@ TEST(span_test, from_array_constructor)
 
      {
          span<int> s;
-         const auto bs = as_writeable_bytes(s);
+         const auto bs = as_writable_bytes(s);
          EXPECT_TRUE(bs.size() == s.size());
          EXPECT_TRUE(bs.size() == 0);
          EXPECT_TRUE(bs.size_bytes() == 0);
@@ -1539,7 +1513,7 @@ TEST(span_test, from_array_constructor)
 
      {
          span<int> s = a;
-         const auto bs = as_writeable_bytes(s);
+         const auto bs = as_writable_bytes(s);
          EXPECT_TRUE(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
          EXPECT_TRUE(bs.size() == s.size_bytes());
      }
@@ -1577,12 +1551,16 @@ TEST(span_test, from_array_constructor)
 
      // even when done dynamically
      {
+         /*
+         // this now results in a compile-time error, rather than runtime.
+         // There is no suitable conversion from dynamic span to fixed span.
          span<int> s = arr;
          auto f = [&]() {
              const span<int, 2> s2 = s;
              static_cast<void>(s2);
          };
          EXPECT_DEATH(f(), deathstring);
+         */
      }
 
      // but doing so explicitly is ok
@@ -1597,12 +1575,19 @@ TEST(span_test, from_array_constructor)
          static_cast<void>(s1);
      }
 
-     // ...or dynamically
+    /*
+     // this is not a legal operation in std::span, so we are no longer supporting it
+     // conversion from span<int, 4> to span<int, dynamic_extent> via call to `first`
+     // then convert from span<int, dynamic_extent> to span<int, 1>
+     // The dynamic to fixed extents are not supported in the standard
+     // to make this work, span<int, 1> would need to be span<int>.
      {
+
          // NB: implicit conversion to span<int,1> from span<int>
          span<int, 1> s1 = s4.first(1);
          static_cast<void>(s1);
      }
+     */
 
      // initialization or assignment to static span that requires size INCREASE is not ok.
      int arr2[2] = {1, 2};
@@ -1624,12 +1609,15 @@ TEST(span_test, from_array_constructor)
          EXPECT_DEATH(f(), deathstring);
      }
 
+    /*
+     // This no longer compiles. There is no suitable conversion from dynamic span to a fixed size span.
      // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
      span<int> av = arr2; auto f = [&]() {
          const span<int, 4> _s4 = av;
          static_cast<void>(_s4);
      };
      EXPECT_DEATH(f(), deathstring);
+     */
  }
 
  TEST(span_test, interop_with_std_regex)
diff --git a/tests/string_span_tests.cpp b/tests/string_span_tests.cpp
index e501bba..7a9f7fb 100644
--- a/tests/string_span_tests.cpp
+++ b/tests/string_span_tests.cpp
@@ -84,7 +84,7 @@ czstring_span<> CreateTempName(string_span<> span)
 {
     Expects(span.size() > 1);
 
-    int last = 0;
+    std::size_t last = 0;
     if (span.size() > 4) {
         span[0] = 't';
         span[1] = 'm';
@@ -101,7 +101,7 @@ cwzstring_span<> CreateTempNameW(wstring_span<> span)
 {
     Expects(span.size() > 1);
 
-    int last = 0;
+    std::size_t last = 0;
     if (span.size() > 4) {
         span[0] = L't';
         span[1] = L'm';
@@ -118,7 +118,7 @@ cu16zstring_span<> CreateTempNameU16(u16string_span<> span)
 {
     Expects(span.size() > 1);
 
-    int last = 0;
+    std::size_t last = 0;
     if (span.size() > 4) {
         span[0] = u't';
         span[1] = u'm';
@@ -135,7 +135,7 @@ cu32zstring_span<> CreateTempNameU32(u32string_span<> span)
 {
     Expects(span.size() > 1);
 
-    int last = 0;
+    std::size_t last = 0;
     if (span.size() > 4) {
         span[0] = U't';
         span[1] = U'm';
@@ -163,14 +163,14 @@ TEST(string_span_tests, TestConstructFromStdString)
 {
     std::string s = "Hello there world";
     cstring_span<> v = s;
-    EXPECT_TRUE(v.length() == static_cast<cstring_span<>::index_type>(s.length()));
+    EXPECT_TRUE(v.length() == static_cast<cstring_span<>::size_type>(s.length()));
 }
 
 TEST(string_span_tests, TestConstructFromStdVector)
 {
     std::vector<char> vec(5, 'h');
     string_span<> v{vec};
-    EXPECT_TRUE(v.length() == static_cast<string_span<>::index_type>(vec.size()));
+    EXPECT_TRUE(v.length() == static_cast<string_span<>::size_type>(vec.size()));
 }
 
 TEST(string_span_tests, TestStackArrayConstruction)
@@ -232,7 +232,7 @@ TEST(string_span_tests, TestToString)
     char stack_string[] = "Hello";
     cstring_span<> v = ensure_z(stack_string);
     auto s2 = gsl::to_string(v);
-    EXPECT_TRUE(static_cast<cstring_span<>::index_type>(s2.length()) == v.length());
+    EXPECT_TRUE(static_cast<cstring_span<>::size_type>(s2.length()) == v.length());
     EXPECT_TRUE(s2.length() == static_cast<size_t>(5));
 }
 
@@ -245,7 +245,7 @@ TEST(string_span_tests, TestToBasicString)
     char stack_string[] = "Hello";
     cstring_span<> v = ensure_z(stack_string);
     auto s2 = gsl::to_basic_string<char, std::char_traits<char>, ::std::allocator<char>>(v);
-    EXPECT_TRUE(static_cast<cstring_span<>::index_type>(s2.length()) == v.length());
+    EXPECT_TRUE(static_cast<cstring_span<>::size_type>(s2.length()) == v.length());
     EXPECT_TRUE(s2.length() == static_cast<size_t>(5));
 }
 
@@ -1206,12 +1206,12 @@ TEST(string_span_tests, as_bytes)
     EXPECT_TRUE(bs.size() == s.size_bytes());
 }
 
-TEST(string_span_tests, as_writeable_bytes)
+TEST(string_span_tests, as_writable_bytes)
 {
     wchar_t buf[]{L"qwerty"};
     wzstring_span<> v(buf);
     const auto s = v.as_string_span();
-    const auto bs = as_writeable_bytes(s);
+    const auto bs = as_writable_bytes(s);
     EXPECT_TRUE(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
     EXPECT_TRUE(bs.size() == s.size_bytes());
 }
diff --git a/tests/utils_tests.cpp b/tests/utils_tests.cpp
index b6b5fc9..f7f7ce3 100644
--- a/tests/utils_tests.cpp
+++ b/tests/utils_tests.cpp
@@ -23,6 +23,7 @@
 #include <limits>      // for numeric_limits
 #include <stdint.h>    // for uint32_t, int32_t
 #include <type_traits> // for is_same
+#include <cstddef>     // for std::ptrdiff_t
 
 using namespace gsl;