diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
new file mode 100644
index 0000000..20279ec
--- /dev/null
+++ b/tests/span_tests.cpp
@@ -0,0 +1,1464 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// 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 <catch/catch.hpp> // for AssertionHandler, StringRef, CHECK, TEST_...
+
+#include <gsl/gsl_byte> // for byte
+#include <gsl/gsl_util> // for narrow_cast, at
+#include <gsl/span>     // for span, span_iterator, operator==, operator!=
+
+#include <array>       // for array
+#include <iostream>    // for ptrdiff_t
+#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 <string>      // for string
+#include <type_traits> // for integral_constant<>::value, is_default_co...
+#include <vector>      // for vector
+
+namespace gsl {
+struct fail_fast;
+}  // namespace gsl
+
+using namespace std;
+using namespace gsl;
+
+namespace
+{
+struct BaseClass
+{
+};
+struct DerivedClass : BaseClass
+{
+};
+}
+
+TEST_CASE("default_constructor")
+{
+    {
+        span<int> s;
+        CHECK((s.size() == 0 && s.data() == nullptr));
+
+        span<const int> cs;
+        CHECK((cs.size() == 0 && cs.data() == nullptr));
+    }
+
+    {
+        span<int, 0> s;
+        CHECK((s.size() == 0 && s.data() == nullptr));
+
+        span<const int, 0> cs;
+        CHECK((cs.size() == 0 && cs.data() == nullptr));
+    }
+
+    {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        span<int, 1> s;
+        CHECK((s.size() == 1 && s.data() == nullptr)); // explains why it can't compile
+#endif
+    }
+
+    {
+        span<int> s{};
+        CHECK((s.size() == 0 && s.data() == nullptr));
+
+        span<const int> cs{};
+        CHECK((cs.size() == 0 && cs.data() == nullptr));
+    }
+}
+
+TEST_CASE("size_optimization")
+{
+    {
+        span<int> s;
+        CHECK(sizeof(s) == sizeof(int*) + sizeof(ptrdiff_t));
+    }
+
+    {
+        span<int, 0> s;
+        CHECK(sizeof(s) == sizeof(int*));
+    }
+}
+
+TEST_CASE("from_nullptr_size_constructor")
+{
+    {
+        span<int> s{nullptr, static_cast<span<int>::index_type>(0)};
+        CHECK((s.size() == 0 && s.data() == nullptr));
+
+        span<const int> cs{nullptr, static_cast<span<int>::index_type>(0)};
+        CHECK((cs.size() == 0 && cs.data() == nullptr));
+    }
+
+    {
+        span<int, 0> s{nullptr, static_cast<span<int>::index_type>(0)};
+        CHECK((s.size() == 0 && s.data() == nullptr));
+
+        span<const int, 0> cs{nullptr, static_cast<span<int>::index_type>(0)};
+        CHECK((cs.size() == 0 && cs.data() == nullptr));
+    }
+
+    {
+        auto workaround_macro = []() {
+            span<int, 1> s{nullptr, static_cast<span<int>::index_type>(0)};
+        };
+        CHECK_THROWS_AS(workaround_macro(), fail_fast);
+    }
+
+    {
+        auto workaround_macro = []() { span<int> s{nullptr, 1}; };
+        CHECK_THROWS_AS(workaround_macro(), fail_fast);
+
+        auto const_workaround_macro = []() { span<const int> cs{nullptr, 1}; };
+        CHECK_THROWS_AS(const_workaround_macro(), fail_fast);
+    }
+
+    {
+        auto workaround_macro = []() { span<int, 0> s{nullptr, 1}; };
+        CHECK_THROWS_AS(workaround_macro(), fail_fast);
+
+        auto const_workaround_macro = []() { span<const int, 0> s{nullptr, 1}; };
+        CHECK_THROWS_AS(const_workaround_macro(), fail_fast);
+    }
+
+    {
+        span<int*> s{nullptr, static_cast<span<int>::index_type>(0)};
+        CHECK((s.size() == 0 && s.data() == nullptr));
+
+        span<const int*> cs{nullptr, static_cast<span<int>::index_type>(0)};
+        CHECK((cs.size() == 0 && cs.data() == nullptr));
+    }
+}
+
+TEST_CASE("from_pointer_length_constructor")
+{
+    int arr[4] = {1, 2, 3, 4};
+
+    {
+        for(int i = 0; i<4 ; ++i)
+        {
+            {
+                span<int> s = { &arr[0], i };
+                CHECK(s.size() == i);
+                CHECK(s.data() == &arr[0]);
+                CHECK(s.empty() == (i == 0));
+                for (int j = 0; j < i; ++j)
+                {
+                    CHECK(arr[j] == s[j]);
+                    CHECK(arr[j] == s.at(j));
+                    CHECK(arr[j] == s(j));
+                }
+            }
+            {
+                span<int> s = { &arr[i], 4-i };
+                CHECK(s.size() == 4-i);
+                CHECK(s.data() == &arr[i]);
+                CHECK(s.empty() == (4-i == 0));
+                for (int j = 0; j < 4-i; ++j)
+                {
+                    CHECK(arr[j+i] == s[j]);
+                    CHECK(arr[j+i] == s.at(j));
+                    CHECK(arr[j+i] == s(j));
+                }
+            }
+        }
+    }
+
+    {
+        span<int, 2> s{&arr[0], 2};
+        CHECK((s.size() == 2 && s.data() == &arr[0]));
+        CHECK((s[0] == 1 && s[1] == 2));
+    }
+
+    {
+        int* p = nullptr;
+        span<int> s{p, static_cast<span<int>::index_type>(0)};
+        CHECK((s.size() == 0 && s.data() == nullptr));
+    }
+
+    {
+        int* p = nullptr;
+        auto workaround_macro = [=]() { span<int> s{p, 2}; };
+        CHECK_THROWS_AS(workaround_macro(), fail_fast);
+    }
+
+    {
+        auto s = make_span(&arr[0], 2);
+        CHECK((s.size() == 2 && s.data() == &arr[0]));
+        CHECK((s[0] == 1 && s[1] == 2));
+    }
+
+    {
+        int* p = nullptr;
+        auto s = make_span(p, static_cast<span<int>::index_type>(0));
+        CHECK((s.size() == 0 && s.data() == nullptr));
+    }
+
+    {
+        int* p = nullptr;
+        auto workaround_macro = [=]() { make_span(p, 2); };
+        CHECK_THROWS_AS(workaround_macro(), fail_fast);
+    }
+}
+
+TEST_CASE("from_pointer_pointer_constructor")
+{
+    int arr[4] = {1, 2, 3, 4};
+
+    {
+        span<int> s{&arr[0], &arr[2]};
+        CHECK((s.size() == 2 && s.data() == &arr[0]));
+        CHECK((s[0] == 1 && s[1] == 2));
+    }
+
+    {
+        span<int, 2> s{&arr[0], &arr[2]};
+        CHECK((s.size() == 2 && s.data() == &arr[0]));
+        CHECK((s[0] == 1 && s[1] == 2));
+    }
+
+    {
+        span<int> s{&arr[0], &arr[0]};
+        CHECK((s.size() == 0 && s.data() == &arr[0]));
+    }
+
+    {
+        span<int, 0> s{&arr[0], &arr[0]};
+        CHECK((s.size() == 0 && s.data() == &arr[0]));
+    }
+
+    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
+    //{
+    //    auto workaround_macro = [&]() { span<int> s{&arr[1], &arr[0]}; };
+    //    CHECK_THROWS_AS(workaround_macro(), fail_fast);
+    //}
+
+    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
+    //{
+    //    int* p = nullptr;
+    //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
+    //    CHECK_THROWS_AS(workaround_macro(), fail_fast);
+    //}
+
+    {
+        int* p = nullptr;
+        span<int> s{p, p};
+        CHECK((s.size() == 0 && s.data() == nullptr));
+    }
+
+    {
+        int* p = nullptr;
+        span<int, 0> s{p, p};
+        CHECK((s.size() == 0 && s.data() == nullptr));
+    }
+
+    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
+    //{
+    //    int* p = nullptr;
+    //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
+    //    CHECK_THROWS_AS(workaround_macro(), fail_fast);
+    //}
+
+    {
+        auto s = make_span(&arr[0], &arr[2]);
+        CHECK((s.size() == 2 && s.data() == &arr[0]));
+        CHECK((s[0] == 1 && s[1] == 2));
+    }
+
+    {
+        auto s = make_span(&arr[0], &arr[0]);
+        CHECK((s.size() == 0 && s.data() == &arr[0]));
+    }
+
+    {
+        int* p = nullptr;
+        auto s = make_span(p, p);
+        CHECK((s.size() == 0 && s.data() == nullptr));
+    }
+}
+
+TEST_CASE("from_array_constructor")
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+
+    {
+        span<int> s{arr};
+        CHECK((s.size() == 5 && s.data() == &arr[0]));
+    }
+
+    {
+        span<int, 5> s{arr};
+        CHECK((s.size() == 5 && s.data() == &arr[0]));
+    }
+
+    int arr2d[2][3] = {1, 2, 3, 4, 5, 6};
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        span<int, 6> s{arr};
+    }
+
+    {
+        span<int, 0> s{arr};
+        CHECK((s.size() == 0 && s.data() == &arr[0]));
+    }
+
+    {
+        span<int> s{arr2d};
+        CHECK((s.size() == 6 && s.data() == &arr2d[0][0]));
+        CHECK((s[0] == 1 && s[5] == 6));
+    }
+
+    {
+        span<int, 0> s{arr2d};
+        CHECK((s.size() == 0 && s.data() == &arr2d[0][0]));
+    }
+
+    {
+        span<int, 6> s{arr2d};
+    }
+#endif
+    {
+        span<int[3]> s{&(arr2d[0]), 1};
+        CHECK((s.size() == 1 && s.data() == &arr2d[0]));
+    }
+
+    int arr3d[2][3][2] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        span<int> s{arr3d};
+        CHECK((s.size() == 12 && s.data() == &arr3d[0][0][0]));
+        CHECK((s[0] == 1 && s[11] == 12));
+    }
+
+    {
+        span<int, 0> s{arr3d};
+        CHECK((s.size() == 0 && s.data() == &arr3d[0][0][0]));
+    }
+
+    {
+        span<int, 11> s{arr3d};
+    }
+
+    {
+        span<int, 12> s{arr3d};
+        CHECK((s.size() == 12 && s.data() == &arr3d[0][0][0]));
+        CHECK((s[0] == 1 && s[5] == 6));
+    }
+#endif
+    {
+        span<int[3][2]> s{&arr3d[0], 1};
+        CHECK((s.size() == 1 && s.data() == &arr3d[0]));
+    }
+
+    {
+        auto s = make_span(arr);
+        CHECK((s.size() == 5 && s.data() == &arr[0]));
+    }
+
+    {
+        auto s = make_span(&(arr2d[0]), 1);
+        CHECK((s.size() == 1 && s.data() == &arr2d[0]));
+    }
+
+    {
+        auto s = make_span(&arr3d[0], 1);
+        CHECK((s.size() == 1 && s.data() == &arr3d[0]));
+    }
+}
+
+TEST_CASE("from_dynamic_array_constructor")
+{
+    double(*arr)[3][4] = new double[100][3][4];
+
+    {
+        span<double> s(&arr[0][0][0], 10);
+        CHECK((s.size() == 10 && s.data() == &arr[0][0][0]));
+    }
+
+    {
+        auto s = make_span(&arr[0][0][0], 10);
+        CHECK((s.size() == 10 && s.data() == &arr[0][0][0]));
+    }
+
+    delete[] arr;
+}
+
+TEST_CASE("from_std_array_constructor")
+{
+    std::array<int, 4> arr = {1, 2, 3, 4};
+
+    {
+        span<int> s{arr};
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+
+        span<const int> cs{arr};
+        CHECK((cs.size() == narrow_cast<ptrdiff_t>(arr.size()) && cs.data() == arr.data()));
+    }
+
+    {
+        span<int, 4> s{arr};
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+
+        span<const int, 4> cs{arr};
+        CHECK((cs.size() == narrow_cast<ptrdiff_t>(arr.size()) && cs.data() == arr.data()));
+    }
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        span<int, 2> s{arr};
+        CHECK((s.size() == 2 && s.data() == arr.data()));
+
+        span<const int, 2> cs{arr};
+        CHECK((cs.size() == 2 && cs.data() == arr.data()));
+    }
+
+    {
+        span<int, 0> s{arr};
+        CHECK((s.size() == 0 && s.data() == arr.data()));
+
+        span<const int, 0> cs{arr};
+        CHECK((cs.size() == 0 && cs.data() == arr.data()));
+    }
+
+    {
+        span<int, 5> s{arr};
+    }
+
+    {
+        auto get_an_array = []() -> std::array<int, 4> { return {1, 2, 3, 4}; };
+        auto take_a_span = [](span<int> s) { static_cast<void>(s); };
+        // try to take a temporary std::array
+        take_a_span(get_an_array());
+    }
+#endif
+
+    {
+        auto get_an_array = []() -> std::array<int, 4> { return {1, 2, 3, 4}; };
+        auto take_a_span = [](span<const int> s) { static_cast<void>(s); };
+        // try to take a temporary std::array
+        take_a_span(get_an_array());
+    }
+
+    {
+        auto s = make_span(arr);
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+    }
+}
+
+TEST_CASE("from_const_std_array_constructor")
+{
+    const std::array<int, 4> arr = {1, 2, 3, 4};
+
+    {
+        span<const int> s{arr};
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+    }
+
+    {
+        span<const int, 4> s{arr};
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+    }
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        span<const int, 2> s{arr};
+        CHECK((s.size() == 2 && s.data() == arr.data()));
+    }
+
+    {
+        span<const int, 0> s{arr};
+        CHECK((s.size() == 0 && s.data() == arr.data()));
+    }
+
+    {
+        span<const int, 5> s{arr};
+    }
+#endif
+
+    {
+        auto get_an_array = []() -> const std::array<int, 4> { return {1, 2, 3, 4}; };
+        auto take_a_span = [](span<const int> s) { static_cast<void>(s); };
+        // try to take a temporary std::array
+        take_a_span(get_an_array());
+    }
+
+    {
+        auto s = make_span(arr);
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+    }
+}
+
+TEST_CASE("from_std_array_const_constructor")
+{
+    std::array<const int, 4> arr = {1, 2, 3, 4};
+
+    {
+        span<const int> s{arr};
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+    }
+
+    {
+        span<const int, 4> s{arr};
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+    }
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        span<const int, 2> s{arr};
+        CHECK((s.size() == 2 && s.data() == arr.data()));
+    }
+
+    {
+        span<const int, 0> s{arr};
+        CHECK((s.size() == 0 && s.data() == arr.data()));
+    }
+
+    {
+        span<const int, 5> s{arr};
+    }
+
+    {
+        span<int, 4> s{arr};
+    }
+#endif
+
+    {
+        auto s = make_span(arr);
+        CHECK((s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data()));
+    }
+}
+
+TEST_CASE("from_container_constructor")
+{
+    std::vector<int> v = {1, 2, 3};
+    const std::vector<int> cv = v;
+
+    {
+        span<int> s{v};
+        CHECK((s.size() == narrow_cast<std::ptrdiff_t>(v.size()) && s.data() == v.data()));
+
+        span<const int> cs{v};
+        CHECK((cs.size() == narrow_cast<std::ptrdiff_t>(v.size()) && cs.data() == v.data()));
+    }
+
+    std::string str = "hello";
+    const std::string cstr = "hello";
+
+    {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        span<char> s{str};
+        CHECK((s.size() == narrow_cast<std::ptrdiff_t>(str.size()) && s.data() == str.data()));
+#endif
+        span<const char> cs{str};
+        CHECK((cs.size() == narrow_cast<std::ptrdiff_t>(str.size()) && cs.data() == str.data()));
+    }
+
+    {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        span<char> s{cstr};
+#endif
+        span<const char> cs{cstr};
+        CHECK((cs.size() == narrow_cast<std::ptrdiff_t>(cstr.size()) &&
+              cs.data() == cstr.data()));
+    }
+
+    {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        auto get_temp_vector = []() -> std::vector<int> { return {}; };
+        auto use_span = [](span<int> s) { static_cast<void>(s); };
+        use_span(get_temp_vector());
+#endif
+    }
+
+    {
+        auto get_temp_vector = []() -> std::vector<int> { return {}; };
+        auto use_span = [](span<const int> s) { static_cast<void>(s); };
+        use_span(get_temp_vector());
+    }
+
+    {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        auto get_temp_string = []() -> std::string { return {}; };
+        auto use_span = [](span<char> s) { static_cast<void>(s); };
+        use_span(get_temp_string());
+#endif
+    }
+
+    {
+        auto get_temp_string = []() -> std::string { return {}; };
+        auto use_span = [](span<const char> s) { static_cast<void>(s); };
+        use_span(get_temp_string());
+    }
+
+    {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        auto get_temp_vector = []() -> const std::vector<int> { return {}; };
+        auto use_span = [](span<const char> s) { static_cast<void>(s); };
+        use_span(get_temp_vector());
+#endif
+    }
+
+    {
+        auto get_temp_string = []() -> const std::string { return {}; };
+        auto use_span = [](span<const char> s) { static_cast<void>(s); };
+        use_span(get_temp_string());
+    }
+
+    {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        std::map<int, int> m;
+        span<int> s{m};
+#endif
+    }
+
+    {
+        auto s = make_span(v);
+        CHECK((s.size() == narrow_cast<std::ptrdiff_t>(v.size()) && s.data() == v.data()));
+
+        auto cs = make_span(cv);
+        CHECK((cs.size() == narrow_cast<std::ptrdiff_t>(cv.size()) && cs.data() == cv.data()));
+    }
+}
+
+TEST_CASE("from_convertible_span_constructor")
+{
+    {
+        span<DerivedClass> avd;
+        span<const DerivedClass> avcd = avd;
+        static_cast<void>(avcd);
+    }
+
+    {
+    #ifdef CONFIRM_COMPILATION_ERRORS
+        span<DerivedClass> avd;
+        span<BaseClass> avb = avd;
+        static_cast<void>(avb);
+    #endif
+    }
+
+    #ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        span<int> s;
+        span<unsigned int> s2 = s;
+        static_cast<void>(s2);
+    }
+
+    {
+        span<int> s;
+        span<const unsigned int> s2 = s;
+        static_cast<void>(s2);
+    }
+
+    {
+        span<int> s;
+        span<short> s2 = s;
+        static_cast<void>(s2);
+    }
+    #endif
+}
+
+TEST_CASE("copy_move_and_assignment")
+{
+    span<int> s1;
+    CHECK(s1.empty());
+
+    int arr[] = {3, 4, 5};
+
+    span<const int> s2 = arr;
+    CHECK((s2.size() == 3 && s2.data() == &arr[0]));
+
+    s2 = s1;
+    CHECK(s2.empty());
+
+    auto get_temp_span = [&]() -> span<int> { return {&arr[1], 2}; };
+    auto use_span = [&](span<const int> s) { CHECK((s.size() == 2 && s.data() == &arr[1])); };
+    use_span(get_temp_span());
+
+    s1 = get_temp_span();
+    CHECK((s1.size() == 2 && s1.data() == &arr[1]));
+}
+
+TEST_CASE("first")
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+
+    {
+        span<int, 5> av = arr;
+        CHECK(av.first<2>().size() == 2);
+        CHECK(av.first(2).size() == 2);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK(av.first<0>().size() == 0);
+        CHECK(av.first(0).size() == 0);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK(av.first<5>().size() == 5);
+        CHECK(av.first(5).size() == 5);
+    }
+
+    {
+        span<int, 5> av = arr;
+#ifdef CONFIRM_COMPILATION_ERRORS
+        CHECK(av.first<6>().size() == 6);
+        CHECK(av.first<-1>().size() == -1);
+#endif
+        CHECK_THROWS_AS(av.first(6).size(), fail_fast);
+    }
+
+    {
+        span<int> av;
+        CHECK(av.first<0>().size() == 0);
+        CHECK(av.first(0).size() == 0);
+    }
+}
+
+TEST_CASE("last")
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+
+    {
+        span<int, 5> av = arr;
+        CHECK(av.last<2>().size() == 2);
+        CHECK(av.last(2).size() == 2);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK(av.last<0>().size() == 0);
+        CHECK(av.last(0).size() == 0);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK(av.last<5>().size() == 5);
+        CHECK(av.last(5).size() == 5);
+    }
+
+    {
+        span<int, 5> av = arr;
+#ifdef CONFIRM_COMPILATION_ERRORS
+        CHECK(av.last<6>().size() == 6);
+#endif
+        CHECK_THROWS_AS(av.last(6).size(), fail_fast);
+    }
+
+    {
+        span<int> av;
+        CHECK(av.last<0>().size() == 0);
+        CHECK(av.last(0).size() == 0);
+    }
+}
+
+TEST_CASE("subspan")
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+
+    {
+        span<int, 5> av = arr;
+        CHECK((av.subspan<2, 2>().size() == 2));
+        CHECK(decltype(av.subspan<2, 2>())::extent == 2);
+        CHECK(av.subspan(2, 2).size() == 2);
+        CHECK(av.subspan(2, 3).size() == 3);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK((av.subspan<0, 0>().size() == 0));
+        CHECK(decltype(av.subspan<0,0>())::extent == 0);
+        CHECK(av.subspan(0, 0).size() == 0);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK((av.subspan<0, 5>().size() == 5));
+        CHECK(decltype(av.subspan<0, 5>())::extent == 5);
+        CHECK(av.subspan(0, 5).size() == 5);
+
+        CHECK_THROWS_AS(av.subspan(0, 6).size(), fail_fast);
+        CHECK_THROWS_AS(av.subspan(1, 5).size(), fail_fast);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK((av.subspan<4, 0>().size() == 0));
+        CHECK(decltype(av.subspan<4, 0>())::extent == 0);
+        CHECK(av.subspan(4, 0).size() == 0);
+        CHECK(av.subspan(5, 0).size() == 0);
+        CHECK_THROWS_AS(av.subspan(6, 0).size(), fail_fast);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK((av.subspan<1>().size() == 4));
+        CHECK(decltype(av.subspan<1>())::extent == 4);    
+    }
+
+    {
+        span<int> av;
+        CHECK((av.subspan<0, 0>().size() == 0));
+        CHECK((decltype(av.subspan<0, 0>())::extent == 0));
+        CHECK(av.subspan(0, 0).size() == 0);
+        CHECK_THROWS_AS((av.subspan<1, 0>().size()), fail_fast);
+    }
+
+    {
+        span<int> av;
+        CHECK(av.subspan(0).size() == 0);
+        CHECK_THROWS_AS(av.subspan(1).size(), fail_fast);
+    }
+
+    {
+        span<int> av = arr;
+        CHECK(av.subspan(0).size() == 5);
+        CHECK(av.subspan(1).size() == 4);
+        CHECK(av.subspan(4).size() == 1);
+        CHECK(av.subspan(5).size() == 0);
+        CHECK_THROWS_AS(av.subspan(6).size(), fail_fast);
+        const auto av2 = av.subspan(1);
+        for (int i = 0; i < 4; ++i) CHECK(av2[i] == i + 2);
+    }
+
+    {
+        span<int, 5> av = arr;
+        CHECK(av.subspan(0).size() == 5);
+        CHECK(av.subspan(1).size() == 4);
+        CHECK(av.subspan(4).size() == 1);
+        CHECK(av.subspan(5).size() == 0);
+        CHECK_THROWS_AS(av.subspan(6).size(), fail_fast);
+        const auto av2 = av.subspan(1);
+        for (int i = 0; i < 4; ++i) CHECK(av2[i] == i + 2);
+    }
+}
+
+TEST_CASE("at_call")
+{
+    int arr[4] = {1, 2, 3, 4};
+
+    {
+        span<int> s = arr;
+        CHECK(s.at(0) == 1);
+        CHECK_THROWS_AS(s.at(5), fail_fast);
+    }
+
+    {
+        int arr2d[2] = {1, 6};
+        span<int, 2> s = arr2d;
+        CHECK(s.at(0) == 1);
+        CHECK(s.at(1) == 6);
+        CHECK_THROWS_AS(s.at(2), fail_fast);
+    }
+}
+
+TEST_CASE("operator_function_call")
+{
+    int arr[4] = {1, 2, 3, 4};
+
+    {
+        span<int> s = arr;
+        CHECK(s(0) == 1);
+        CHECK_THROWS_AS(s(5), fail_fast);
+    }
+
+    {
+        int arr2d[2] = {1, 6};
+        span<int, 2> s = arr2d;
+        CHECK(s(0) == 1);
+        CHECK(s(1) == 6);
+        CHECK_THROWS_AS(s(2), fail_fast);
+    }
+}
+
+TEST_CASE("iterator_default_init")
+{
+    span<int>::iterator it1;
+    span<int>::iterator it2;
+    CHECK(it1 == it2);
+}
+
+TEST_CASE("const_iterator_default_init")
+{
+    span<int>::const_iterator it1;
+    span<int>::const_iterator it2;
+    CHECK(it1 == it2);
+}
+
+TEST_CASE("iterator_conversions")
+{
+    span<int>::iterator badIt;
+    span<int>::const_iterator badConstIt;
+    CHECK(badIt == badConstIt);
+
+    int a[] = {1, 2, 3, 4};
+    span<int> s = a;
+
+    auto it = s.begin();
+    auto cit = s.cbegin();
+
+    CHECK(it == cit);
+    CHECK(cit == it);
+
+    span<int>::const_iterator cit2 = it;
+    CHECK(cit2 == cit);
+
+    span<int>::const_iterator cit3 = it + 4;
+    CHECK(cit3 == s.cend());
+}
+
+TEST_CASE("iterator_comparisons")
+{
+    int a[] = {1, 2, 3, 4};
+    {
+        span<int> s = a;
+        span<int>::iterator it = s.begin();
+        auto it2 = it + 1;
+        span<int>::const_iterator cit = s.cbegin();
+
+        CHECK(it == cit);
+        CHECK(cit == it);
+        CHECK(it == it);
+        CHECK(cit == cit);
+        CHECK(cit == s.begin());
+        CHECK(s.begin() == cit);
+        CHECK(s.cbegin() == cit);
+        CHECK(it == s.begin());
+        CHECK(s.begin() == it);
+
+        CHECK(it != it2);
+        CHECK(it2 != it);
+        CHECK(it != s.end());
+        CHECK(it2 != s.end());
+        CHECK(s.end() != it);
+        CHECK(it2 != cit);
+        CHECK(cit != it2);
+
+        CHECK(it < it2);
+        CHECK(it <= it2);
+        CHECK(it2 <= s.end());
+        CHECK(it < s.end());
+        CHECK(it <= cit);
+        CHECK(cit <= it);
+        CHECK(cit < it2);
+        CHECK(cit <= it2);
+        CHECK(cit < s.end());
+        CHECK(cit <= s.end());
+
+        CHECK(it2 > it);
+        CHECK(it2 >= it);
+        CHECK(s.end() > it2);
+        CHECK(s.end() >= it2);
+        CHECK(it2 > cit);
+        CHECK(it2 >= cit);
+    }
+}
+
+TEST_CASE("begin_end")
+{
+    {
+        int a[] = {1, 2, 3, 4};
+        span<int> s = a;
+
+        span<int>::iterator it = s.begin();
+        span<int>::iterator it2 = std::begin(s);
+        CHECK(it == it2);
+
+        it = s.end();
+        it2 = std::end(s);
+        CHECK(it == it2);
+    }
+
+    {
+        int a[] = {1, 2, 3, 4};
+        span<int> s = a;
+
+        auto it = s.begin();
+        auto first = it;
+        CHECK(it == first);
+        CHECK(*it == 1);
+
+        auto beyond = s.end();
+        CHECK(it != beyond);
+        CHECK_THROWS_AS(*beyond, fail_fast);
+
+        CHECK(beyond - first == 4);
+        CHECK(first - first == 0);
+        CHECK(beyond - beyond == 0);
+
+        ++it;
+        CHECK(it - first == 1);
+        CHECK(*it == 2);
+        *it = 22;
+        CHECK(*it == 22);
+        CHECK(beyond - it == 3);
+
+        it = first;
+        CHECK(it == first);
+        while (it != s.end()) {
+            *it = 5;
+            ++it;
+        }
+
+        CHECK(it == beyond);
+        CHECK(it - beyond == 0);
+
+        for (const auto& n : s) {
+            CHECK(n == 5);
+        }
+    }
+}
+
+TEST_CASE("cbegin_cend")
+{
+    {
+        int a[] = {1, 2, 3, 4};
+        span<int> s = a;
+
+        span<int>::const_iterator cit = s.cbegin();
+        span<int>::const_iterator cit2 = std::cbegin(s);
+        CHECK(cit == cit2);
+
+        cit = s.cend();
+        cit2 = std::cend(s);
+        CHECK(cit == cit2);
+    }
+
+    {
+        int a[] = {1, 2, 3, 4};
+        span<int> s = a;
+
+        auto it = s.cbegin();
+        auto first = it;
+        CHECK(it == first);
+        CHECK(*it == 1);
+
+        auto beyond = s.cend();
+        CHECK(it != beyond);
+        CHECK_THROWS_AS(*beyond, fail_fast);
+
+        CHECK(beyond - first == 4);
+        CHECK(first - first == 0);
+        CHECK(beyond - beyond == 0);
+
+        ++it;
+        CHECK(it - first == 1);
+        CHECK(*it == 2);
+        CHECK(beyond - it == 3);
+
+        int last = 0;
+        it = first;
+        CHECK(it == first);
+        while (it != s.cend()) {
+            CHECK(*it == last + 1);
+
+            last = *it;
+            ++it;
+        }
+
+        CHECK(it == beyond);
+        CHECK(it - beyond == 0);
+    }
+}
+
+TEST_CASE("rbegin_rend")
+{
+    {
+        int a[] = {1, 2, 3, 4};
+        span<int> s = a;
+
+        auto it = s.rbegin();
+        auto first = it;
+        CHECK(it == first);
+        CHECK(*it == 4);
+
+        auto beyond = s.rend();
+        CHECK(it != beyond);
+        CHECK_THROWS_AS(*beyond, fail_fast);
+
+        CHECK(beyond - first == 4);
+        CHECK(first - first == 0);
+        CHECK(beyond - beyond == 0);
+
+        ++it;
+        CHECK(it - first == 1);
+        CHECK(*it == 3);
+        *it = 22;
+        CHECK(*it == 22);
+        CHECK(beyond - it == 3);
+
+        it = first;
+        CHECK(it == first);
+        while (it != s.rend()) {
+            *it = 5;
+            ++it;
+        }
+
+        CHECK(it == beyond);
+        CHECK(it - beyond == 0);
+
+        for (const auto& n : s) {
+            CHECK(n == 5);
+        }
+    }
+}
+
+TEST_CASE("crbegin_crend")
+{
+    {
+        int a[] = {1, 2, 3, 4};
+        span<int> s = a;
+
+        auto it = s.crbegin();
+        auto first = it;
+        CHECK(it == first);
+        CHECK(*it == 4);
+
+        auto beyond = s.crend();
+        CHECK(it != beyond);
+        CHECK_THROWS_AS(*beyond, fail_fast);
+
+        CHECK(beyond - first == 4);
+        CHECK(first - first == 0);
+        CHECK(beyond - beyond == 0);
+
+        ++it;
+        CHECK(it - first == 1);
+        CHECK(*it == 3);
+        CHECK(beyond - it == 3);
+
+        it = first;
+        CHECK(it == first);
+        int last = 5;
+        while (it != s.crend()) {
+            CHECK(*it == last - 1);
+            last = *it;
+
+            ++it;
+        }
+
+        CHECK(it == beyond);
+        CHECK(it - beyond == 0);
+    }
+}
+
+TEST_CASE("comparison_operators")
+{
+    {
+        span<int> s1;
+        span<int> s2;
+        CHECK(s1 == s2);
+        CHECK(!(s1 != s2));
+        CHECK(!(s1 < s2));
+        CHECK(s1 <= s2);
+        CHECK(!(s1 > s2));
+        CHECK(s1 >= s2);
+        CHECK(s2 == s1);
+        CHECK(!(s2 != s1));
+        CHECK(!(s2 < s1));
+        CHECK(s2 <= s1);
+        CHECK(!(s2 > s1));
+        CHECK(s2 >= s1);
+    }
+
+    {
+        int arr[] = {2, 1};
+        span<int> s1 = arr;
+        span<int> s2 = arr;
+
+        CHECK(s1 == s2);
+        CHECK(!(s1 != s2));
+        CHECK(!(s1 < s2));
+        CHECK(s1 <= s2);
+        CHECK(!(s1 > s2));
+        CHECK(s1 >= s2);
+        CHECK(s2 == s1);
+        CHECK(!(s2 != s1));
+        CHECK(!(s2 < s1));
+        CHECK(s2 <= s1);
+        CHECK(!(s2 > s1));
+        CHECK(s2 >= s1);
+    }
+
+    {
+        int arr[] = {2, 1}; // bigger
+
+        span<int> s1;
+        span<int> s2 = arr;
+
+        CHECK(s1 != s2);
+        CHECK(s2 != s1);
+        CHECK(!(s1 == s2));
+        CHECK(!(s2 == s1));
+        CHECK(s1 < s2);
+        CHECK(!(s2 < s1));
+        CHECK(s1 <= s2);
+        CHECK(!(s2 <= s1));
+        CHECK(s2 > s1);
+        CHECK(!(s1 > s2));
+        CHECK(s2 >= s1);
+        CHECK(!(s1 >= s2));
+    }
+
+    {
+        int arr1[] = {1, 2};
+        int arr2[] = {1, 2};
+        span<int> s1 = arr1;
+        span<int> s2 = arr2;
+
+        CHECK(s1 == s2);
+        CHECK(!(s1 != s2));
+        CHECK(!(s1 < s2));
+        CHECK(s1 <= s2);
+        CHECK(!(s1 > s2));
+        CHECK(s1 >= s2);
+        CHECK(s2 == s1);
+        CHECK(!(s2 != s1));
+        CHECK(!(s2 < s1));
+        CHECK(s2 <= s1);
+        CHECK(!(s2 > s1));
+        CHECK(s2 >= s1);
+    }
+
+    {
+        int arr[] = {1, 2, 3};
+
+        span<int> s1 = {&arr[0], 2}; // shorter
+        span<int> s2 = arr;          // longer
+
+        CHECK(s1 != s2);
+        CHECK(s2 != s1);
+        CHECK(!(s1 == s2));
+        CHECK(!(s2 == s1));
+        CHECK(s1 < s2);
+        CHECK(!(s2 < s1));
+        CHECK(s1 <= s2);
+        CHECK(!(s2 <= s1));
+        CHECK(s2 > s1);
+        CHECK(!(s1 > s2));
+        CHECK(s2 >= s1);
+        CHECK(!(s1 >= s2));
+    }
+
+    {
+        int arr1[] = {1, 2}; // smaller
+        int arr2[] = {2, 1}; // bigger
+
+        span<int> s1 = arr1;
+        span<int> s2 = arr2;
+
+        CHECK(s1 != s2);
+        CHECK(s2 != s1);
+        CHECK(!(s1 == s2));
+        CHECK(!(s2 == s1));
+        CHECK(s1 < s2);
+        CHECK(!(s2 < s1));
+        CHECK(s1 <= s2);
+        CHECK(!(s2 <= s1));
+        CHECK(s2 > s1);
+        CHECK(!(s1 > s2));
+        CHECK(s2 >= s1);
+        CHECK(!(s1 >= s2));
+    }
+}
+
+TEST_CASE("as_bytes")
+{
+    int a[] = {1, 2, 3, 4};
+
+    {
+        const span<const int> s = a;
+        CHECK(s.size() == 4);
+        const span<const byte> bs = as_bytes(s);
+        CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
+        CHECK(bs.size() == s.size_bytes());
+    }
+
+    {
+        span<int> s;
+        const auto bs = as_bytes(s);
+        CHECK(bs.size() == s.size());
+        CHECK(bs.size() == 0);
+        CHECK(bs.size_bytes() == 0);
+        CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
+        CHECK(bs.data() == nullptr);
+    }
+
+    {
+        span<int> s = a;
+        const auto bs = as_bytes(s);
+        CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
+        CHECK(bs.size() == s.size_bytes());
+    }
+}
+
+TEST_CASE("as_writeable_bytes")
+{
+    int a[] = {1, 2, 3, 4};
+
+    {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        // you should not be able to get writeable bytes for const objects
+        span<const int> s = a;
+        CHECK(s.size() == 4);
+        span<const byte> bs = as_writeable_bytes(s);
+        CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
+        CHECK(bs.size() == s.size_bytes());
+#endif
+    }
+
+    {
+        span<int> s;
+        const auto bs = as_writeable_bytes(s);
+        CHECK(bs.size() == s.size());
+        CHECK(bs.size() == 0);
+        CHECK(bs.size_bytes() == 0);
+        CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
+        CHECK(bs.data() == nullptr);
+    }
+
+    {
+        span<int> s = a;
+        const auto bs = as_writeable_bytes(s);
+        CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
+        CHECK(bs.size() == s.size_bytes());
+    }
+}
+
+TEST_CASE("fixed_size_conversions")
+{
+    int arr[] = {1, 2, 3, 4};
+
+    // converting to an span from an equal size array is ok
+    span<int, 4> s4 = arr;
+    CHECK(s4.size() == 4);
+
+    // converting to dynamic_range is always ok
+    {
+        span<int> s = s4;
+        CHECK(s.size() == s4.size());
+        static_cast<void>(s);
+    }
+
+// initialization or assignment to static span that REDUCES size is NOT ok
+#ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        span<int, 2> s = arr;
+    }
+    {
+        span<int, 2> s2 = s4;
+        static_cast<void>(s2);
+    }
+#endif
+
+    // even when done dynamically
+    {
+        span<int> s = arr;
+        auto f = [&]() {
+            span<int, 2> s2 = s;
+            static_cast<void>(s2);
+        };
+        CHECK_THROWS_AS(f(), fail_fast);
+    }
+
+    // but doing so explicitly is ok
+
+    // you can convert statically
+    {
+        const span<int, 2> s2 = {arr, 2};
+        static_cast<void>(s2);
+    }
+    {
+        const span<int, 1> s1 = s4.first<1>();
+        static_cast<void>(s1);
+    }
+
+    // ...or dynamically
+    {
+        // 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};
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        span<int, 4> s3 = arr2;
+    }
+    {
+        span<int, 2> s2 = arr2;
+        span<int, 4> s4a = s2;
+    }
+#endif
+    {
+        auto f = [&]() {
+            span<int, 4> _s4 = {arr2, 2};
+            static_cast<void>(_s4);
+        };
+        CHECK_THROWS_AS(f(), fail_fast);
+    }
+
+    // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
+    span<int> av = arr2;
+    auto f = [&]() {
+        span<int, 4> _s4 = av;
+        static_cast<void>(_s4);
+    };
+    CHECK_THROWS_AS(f(), fail_fast);
+}
+
+TEST_CASE("interop_with_std_regex")
+{
+    char lat[] = {'1', '2', '3', '4', '5', '6', 'E', 'F', 'G'};
+    span<char> s = lat;
+    const auto f_it = s.begin() + 7;
+
+    std::match_results<span<char>::iterator> match;
+
+    std::regex_match(s.begin(), s.end(), match, std::regex(".*"));
+    CHECK(match.ready());
+    CHECK(!match.empty());
+    CHECK(match[0].matched);
+    CHECK(match[0].first == s.begin());
+    CHECK(match[0].second == s.end());
+
+    std::regex_search(s.begin(), s.end(), match, std::regex("F"));
+    CHECK(match.ready());
+    CHECK(!match.empty());
+    CHECK(match[0].matched);
+    CHECK(match[0].first == f_it);
+    CHECK(match[0].second == (f_it + 1));
+}
+
+TEST_CASE("interop_with_gsl_at")
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+    span<int> s{arr};
+    CHECK((at(s, 0) == 1 && at(s, 1) == 2));
+}
+
+TEST_CASE("default_constructible")
+{
+    CHECK((std::is_default_constructible<span<int>>::value));
+    CHECK((std::is_default_constructible<span<int, 0>>::value));
+    CHECK((!std::is_default_constructible<span<int, 42>>::value));
+}