diff --git a/include/gsl_util.h b/include/gsl_util.h
index 316f2a1..508672c 100644
--- a/include/gsl_util.h
+++ b/include/gsl_util.h
@@ -106,6 +106,7 @@ inline T narrow(U u)
     T t = narrow_cast<T>(u);
     if (static_cast<U>(t) != u)
         throw narrowing_error();
+#pragma warning(suppress:4127) // suppress warning from MSVC compiler about constant in if-test
     if (!details::is_same_signedness<T, U>::value && ((t < T{}) != (u < U{})))
         throw narrowing_error();
     return t;
diff --git a/include/span.h b/include/span.h
index 7df4964..b8c1196 100644
--- a/include/span.h
+++ b/include/span.h
@@ -21,16 +21,9 @@
 
 #include "gsl_assert.h"
 #include "gsl_util.h"
-#include <algorithm>
 #include <array>
-#include <cassert>
-#include <cstddef>
-#include <cstdint>
-#include <functional>
-#include <iterator>
 #include <limits>
-#include <new>
-#include <numeric>
+#include <iterator>
 #include <stdexcept>
 #include <type_traits>
 #include <utility>
@@ -79,6 +72,129 @@
 namespace gsl
 {
 
+// [views.constants], constants  
+constexpr const std::ptrdiff_t dynamic_extent = -1;
+
+
+// [span], class template span 
+template <class ElementType, std::ptrdiff_t Extent = dynamic_extent>
+class span {
+public:
+    // constants and types 
+    using element_type = ElementType;
+    using index_type = std::ptrdiff_t;
+    using pointer = element_type*;
+    using reference = element_type&;
+#if 0  // TODO
+    using iterator = /*implementation-defined */;
+    using const_iterator = /* implementation-defined */;
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+#endif 
+    constexpr static const index_type extent = Extent;
+
+    // [span.cons], span constructors, copy, assignment, and destructor 
+    constexpr span() noexcept : data_(nullptr), size_(0)
+    { static_assert(extent == dynamic_extent || extent == 0, "Cannot default initialize a fixed-length span."); }
+    constexpr span(nullptr_t) noexcept : span() {}
+    constexpr span(pointer ptr, index_type count) : data_(ptr), size_(count)
+    { Expects((!ptr && count == 0) || (ptr && count >= 0)); }
+#if 0 // TODO
+    constexpr span(pointer firstElem, pointer lastElem);
+    template <size_t N>
+    constexpr span(element_type(&arr)[N]);
+    template <size_t N>
+    constexpr span(array<remove_const_t<element_type>, N>& arr);
+    template <size_t N>
+    constexpr span(const array<remove_const_t<element_type>, N>& arr);
+    template <class Container>
+    constexpr span(Container& cont);
+    template <class Container>
+    span(const Container&&) = delete;
+    constexpr span(const span& other) noexcept = default;
+    constexpr span(span&& other) noexcept = default;
+    template <class OtherElementType, ptrdiff_t OtherExtent>
+    constexpr span(const span<OtherElementType, OtherExtent>& other);
+    template <class OtherElementType, ptrdiff_t OtherExtent>
+    constexpr span(span<OtherElementType, OtherExtent>&& other);
+    ~span() noexcept = default;
+    constexpr span& operator=(const span& other) noexcept = default;
+    constexpr span& operator=(span&& other) noexcept = default;
+
+    // [span.sub], span subviews  
+    template <ptrdiff_t Count>
+    constexpr span<element_type, Count> first() const;
+    template <ptrdiff_t Count>
+    constexpr span<element_type, Count> last() const;
+    template <ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent>
+    constexpr span<element_type, Count> subspan() const;
+    constexpr span<element_type, dynamic_extent> first(index_type count) const;
+    constexpr span<element_type, dynamic_extent> last(index_type count) const;
+    constexpr span<element_type, dynamic_extent> subspan(index_type offset, index_type count = dynamic_extent) const;
+#endif
+    // [span.obs], span observers 
+    constexpr index_type length() const noexcept { return size(); }
+    constexpr index_type size() const noexcept { return size_;  }
+    constexpr index_type length_bytes() const noexcept { return size_bytes(); }
+    constexpr index_type size_bytes() const noexcept { return size() * sizeof(element_type); }
+    constexpr bool empty() const noexcept { return size() == 0; }
+
+#if 0 // TODO
+    // [span.elem], span element access 
+    constexpr reference operator[](index_type idx) const;
+    constexpr reference operator()(index_type idx) const;
+#endif 
+    constexpr pointer data() const noexcept { return data_; }
+#if 0 // TODO
+    // [span.iter], span iterator support 
+    iterator begin() const noexcept;
+    iterator end() const noexcept;
+ 
+    const_iterator cbegin() const noexcept;
+    const_iterator cend() const noexcept;
+
+    reverse_iterator rbegin() const noexcept;
+    reverse_iterator rend() const noexcept;
+
+    const_reverse_iterator crbegin() const noexcept;
+    const_reverse_iterator crend() const noexcept;
+#endif
+private:
+    pointer data_;
+    index_type size_;
+};
+
+
+#if 0 // TODO
+// [span.comparison], span comparison operators 
+template <class ElementType, ptrdiff_t Extent>
+constexpr bool operator==(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r) const noexcept;
+
+template <class ElementType, ptrdiff_t Extent>
+constexpr bool operator!=(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r) const noexcept;
+
+template <class ElementType, ptrdiff_t Extent>
+constexpr bool operator<(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r) const noexcept;
+
+template <class ElementType, ptrdiff_t Extent>
+constexpr bool operator<=(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r) const noexcept;
+
+template <class ElementType, ptrdiff_t Extent>
+constexpr bool operator>(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r) const noexcept;
+
+template <class ElementType, ptrdiff_t Extent>
+constexpr bool operator>=(const span<ElementType, Extent>& l, const span<ElementType, Extent>& r) const noexcept;
+#endif
+
+
+#if 0 // TODO
+// [span.objectrep], views of object representation 
+template <class ElementType, ptrdiff_t Extent>
+constexpr span<const char, ((Extent == dynamic_extent) ? dynamic_extent : (sizeof(ElementType) * Extent))> as_bytes(span<ElementType, Extent> s) noexcept;
+
+template <class ElementType, ptrdiff_t Extent>
+constexpr span<char, ((Extent == dynamic_extent) ? dynamic_extent : (sizeof(ElementType) * Extent))> as_writeable_bytes(span<ElementType, Extent>) noexcept;
+#endif
 
 } // namespace gsl
 
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index 83eec0e..233ab00 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -39,7 +39,6 @@ struct DerivedClass : BaseClass
 
 SUITE(span_tests)
 {
-#if 0
     TEST(default_constructor)
     {
         {
@@ -116,6 +115,7 @@ SUITE(span_tests)
         }
     }
 
+#if 0
     TEST(from_nullptr_length_constructor)
     {
         {