diff --git a/.gitmodules b/.gitmodules
index d9229ae..b198402 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "tests/unittest-cpp"]
 	path = tests/unittest-cpp
-	url = https://github.com/Microsoft/unittest-cpp.git
+	url = https://github.com/unittest-cpp/unittest-cpp.git
diff --git a/include/gsl/gsl b/include/gsl/gsl
index 7ef7742..63c9e7b 100644
--- a/include/gsl/gsl
+++ b/include/gsl/gsl
@@ -118,6 +118,42 @@ std::ostream& operator<<(std::ostream& os, const not_null<T>& val)
     return os;
 }
 
+template <class T, class U>
+auto operator==(const not_null<T>& lhs, const not_null<U>& rhs) -> decltype(lhs.get() == rhs.get())
+{
+    return lhs.get() == rhs.get();
+}
+
+template <class T, class U>
+auto operator!=(const not_null<T>& lhs, const not_null<U>& rhs) -> decltype(lhs.get() != rhs.get())
+{
+    return lhs.get() != rhs.get();
+}
+
+template <class T, class U>
+auto operator<(const not_null<T>& lhs, const not_null<U>& rhs) -> decltype(lhs.get() < rhs.get())
+{
+    return lhs.get() < rhs.get();
+}
+
+template <class T, class U>
+auto operator<=(const not_null<T>& lhs, const not_null<U>& rhs) -> decltype(lhs.get() <= rhs.get())
+{
+    return lhs.get() <= rhs.get();
+}
+
+template <class T, class U>
+auto operator>(const not_null<T>& lhs, const not_null<U>& rhs) -> decltype(lhs.get() > rhs.get())
+{
+    return lhs.get() > rhs.get();
+}
+
+template <class T, class U>
+auto operator>=(const not_null<T>& lhs, const not_null<U>& rhs) -> decltype(lhs.get() >= rhs.get())
+{
+    return lhs.get() >= rhs.get();
+}
+
 // more unwanted operators
 template <class T, class U>
 std::ptrdiff_t operator-(const not_null<T>&, const not_null<U>&) = delete;
diff --git a/include/gsl/span b/include/gsl/span
index 957c9e6..3a8d31d 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -133,7 +133,7 @@ namespace details
         using element_type_ = typename Span::element_type;
     public:
         using iterator_category = std::random_access_iterator_tag;
-        using value_type = std::remove_const_t<element_type_>;
+        using value_type = std::remove_cv_t<element_type_>;
         using difference_type = typename Span::index_type;
 
         using reference =
@@ -337,6 +337,7 @@ 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 pointer = element_type*;
     using reference = element_type&;
@@ -346,6 +347,8 @@ public:
     using reverse_iterator = std::reverse_iterator<iterator>;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
+    using size_type = index_type;
+
     constexpr static const index_type extent = Extent;
 
     // [span.cons], span constructors, copy, assignment, and destructor
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 2ed2170..be9e65c 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -18,15 +18,15 @@ add_definitions(-DGSL_THROW_ON_CONTRACT_VIOLATION)
 if(MSVC) # has the support we need
     # remove unnecessary warnings about unchecked iterators
     add_definitions(-D_SCL_SECURE_NO_WARNINGS)
-    add_compile_options(/EHsc /W4)
+    add_compile_options(/EHsc /W4 /WX)
 else()
     include(CheckCXXCompilerFlag)
     CHECK_CXX_COMPILER_FLAG("-std=c++14" COMPILER_SUPPORTS_CXX14)
     CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
     if(COMPILER_SUPPORTS_CXX14)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread -fno-strict-aliasing -std=c++14 -O3 -Wall -Wextra -Wpedantic -Wno-missing-braces -Wconversion -Wsign-conversion -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Woverloaded-virtual")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread -fno-strict-aliasing -std=c++14 -Wall -Wextra -Wpedantic -Wno-missing-braces -Wconversion -Wsign-conversion -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Woverloaded-virtual")
     elseif(COMPILER_SUPPORTS_CXX11)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread -fno-strict-aliasing -std=c++11 -O3 -Wall -Wextra -Wpedantic -Wno-missing-braces -Wconversion -Wsign-conversion -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Woverloaded-virtual")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread -fno-strict-aliasing -std=c++11 -Wall -Wextra -Wpedantic -Wno-missing-braces -Wconversion -Wsign-conversion -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Woverloaded-virtual")
     else()
       message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
     endif()
diff --git a/tests/at_tests.cpp b/tests/at_tests.cpp
index 79f8ddd..db89c34 100644
--- a/tests/at_tests.cpp
+++ b/tests/at_tests.cpp
@@ -45,8 +45,8 @@ SUITE(at_tests)
         const std::array<int, 4>& c_a = a;
 
         for (int i = 0; i < 4; ++i) {
-            CHECK(&gsl::at(a, i) == &a[i]);
-            CHECK(&gsl::at(c_a, i) == &a[i]);
+            CHECK(&gsl::at(a, i) == &a[static_cast<std::size_t>(i)]);
+            CHECK(&gsl::at(c_a, i) == &a[static_cast<std::size_t>(i)]);
         }
 
         CHECK_THROW(gsl::at(a, -1), fail_fast);
@@ -61,8 +61,8 @@ SUITE(at_tests)
         const std::vector<int>& c_a = a;
 
         for (int i = 0; i < 4; ++i) {
-            CHECK(&gsl::at(a, i) == &a[i]);
-            CHECK(&gsl::at(c_a, i) == &a[i]);
+            CHECK(&gsl::at(a, i) == &a[static_cast<std::size_t>(i)]);
+            CHECK(&gsl::at(c_a, i) == &a[static_cast<std::size_t>(i)]);
         }
 
         CHECK_THROW(gsl::at(a, -1), fail_fast);
@@ -87,7 +87,7 @@ SUITE(at_tests)
     }
 }
 
-#if !defined(_MSC_VER) || (defined(__clang__) || _MSC_VER >= 1910)
+#if !defined(_MSC_VER) || defined(__clang__) || _MSC_VER >= 1910
 static constexpr bool test_constexpr()
 {
     int a1[4] = { 1, 2, 3, 4 };
@@ -98,7 +98,9 @@ static constexpr bool test_constexpr()
     for (int i = 0; i < 4; ++i) {
         if (&gsl::at(a1, i) != &a1[i]) return false;
         if (&gsl::at(c_a1, i) != &a1[i]) return false;
-        if (&gsl::at(c_a2, i) != &c_a2[i]) return false;
+        // requires C++17:
+        // if (&gsl::at(a2, i) != &a2[static_cast<std::size_t>(i)]) return false;
+        if (&gsl::at(c_a2, i) != &c_a2[static_cast<std::size_t>(i)]) return false;
         if (gsl::at({1,2,3,4}, i) != i+1) return false;
     }
 
diff --git a/tests/notnull_tests.cpp b/tests/notnull_tests.cpp
index 201b797..1c2b50e 100644
--- a/tests/notnull_tests.cpp
+++ b/tests/notnull_tests.cpp
@@ -17,6 +17,8 @@
 #include <UnitTest++/UnitTest++.h> 
 #include <gsl/gsl>
 #include <vector>
+#include <memory>
+#include <string>
 
 using namespace gsl;
 
@@ -33,6 +35,54 @@ struct RefCounted
     T* p_;
 };
 
+// user defined smart pointer with comparison operators returning non bool value
+template <typename T>
+struct CustomPtr
+{
+	CustomPtr(T* p) : p_(p) {}
+	operator T*() { return p_; }
+	bool operator !=(std::nullptr_t)const { return p_ != nullptr; }
+	T* p_ = nullptr;
+};
+
+template <typename T, typename U>
+std::string operator==(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
+{
+    return reinterpret_cast<const void*>(lhs.p_) == reinterpret_cast<const void*>(rhs.p_) ? "true" : "false";
+}
+
+template <typename T, typename U>
+std::string operator!=(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
+{
+	return reinterpret_cast<const void*>(lhs.p_) != reinterpret_cast<const void*>(rhs.p_) ? "true" : "false";
+}
+
+template <typename T, typename U>
+std::string operator<(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
+{
+	return reinterpret_cast<const void*>(lhs.p_) < reinterpret_cast<const void*>(rhs.p_) ? "true" : "false";
+}
+
+template <typename T, typename U>
+std::string operator>(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
+{
+	return reinterpret_cast<const void*>(lhs.p_) > reinterpret_cast<const void*>(rhs.p_) ? "true" : "false";
+}
+
+template <typename T, typename U>
+std::string operator<=(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
+{
+	return reinterpret_cast<const void*>(lhs.p_) <= reinterpret_cast<const void*>(rhs.p_) ? "true" : "false";
+}
+
+template <typename T, typename U>
+std::string operator>=(CustomPtr<T> const& lhs, CustomPtr<U> const& rhs)
+{
+	return reinterpret_cast<const void*>(lhs.p_) >= reinterpret_cast<const void*>(rhs.p_) ? "true" : "false";
+}
+
+
+
 SUITE(NotNullTests)
 {
 
@@ -95,6 +145,101 @@ SUITE(NotNullTests)
         int* q = nullptr;
         CHECK_THROW(p = q, fail_fast);
     }
+
+    TEST(TestNotNullRawPointerComparison)
+    {
+        int ints[2] = {42, 43};
+        int* p1 = &ints[0];
+        const int* p2 = &ints[1];
+
+        using NotNull1 = not_null<decltype(p1)>;
+        using NotNull2 = not_null<decltype(p2)>;
+
+        CHECK((NotNull1(p1) == NotNull1(p1)) == true);
+        CHECK((NotNull1(p1) == NotNull2(p2)) == false);
+
+        CHECK((NotNull1(p1) != NotNull1(p1)) == false);
+        CHECK((NotNull1(p1) != NotNull2(p2)) == true);
+
+        CHECK((NotNull1(p1) < NotNull1(p1)) == false);
+        CHECK((NotNull1(p1) < NotNull2(p2)) == (p1 < p2));
+        CHECK((NotNull2(p2) < NotNull1(p1)) == (p2 < p1));
+
+        CHECK((NotNull1(p1) > NotNull1(p1)) == false);
+        CHECK((NotNull1(p1) > NotNull2(p2)) == (p1 > p2));
+        CHECK((NotNull2(p2) > NotNull1(p1)) == (p2 > p1));
+
+        CHECK((NotNull1(p1) <= NotNull1(p1)) == true);
+        CHECK((NotNull1(p1) <= NotNull2(p2)) == (p1 <= p2));
+        CHECK((NotNull2(p2) <= NotNull1(p1)) == (p2 <= p1));
+
+        CHECK((NotNull1(p1) >= NotNull1(p1)) == true);
+        CHECK((NotNull1(p1) >= NotNull2(p2)) == (p1 >= p2));
+        CHECK((NotNull2(p2) >= NotNull1(p1)) == (p2 >= p1));
+    }
+
+    TEST(TestNotNullSharedPtrComparison)
+    {
+        auto sp1 = std::make_shared<int>(42);
+        auto sp2 = std::make_shared<const int>(43);
+
+        using NotNullSp1 = not_null<decltype(sp1)>;
+        using NotNullSp2 = not_null<decltype(sp2)>;
+
+        CHECK((NotNullSp1(sp1) == NotNullSp1(sp1)) == true);
+        CHECK((NotNullSp1(sp1) == NotNullSp2(sp2)) == false);
+
+        CHECK((NotNullSp1(sp1) != NotNullSp1(sp1)) == false);
+        CHECK((NotNullSp1(sp1) != NotNullSp2(sp2)) == true);
+
+        CHECK((NotNullSp1(sp1) < NotNullSp1(sp1)) == false);
+        CHECK((NotNullSp1(sp1) < NotNullSp2(sp2)) == (sp1 < sp2));
+        CHECK((NotNullSp2(sp2) < NotNullSp1(sp1)) == (sp2 < sp1));
+
+        CHECK((NotNullSp1(sp1) > NotNullSp1(sp1)) == false);
+        CHECK((NotNullSp1(sp1) > NotNullSp2(sp2)) == (sp1 > sp2));
+        CHECK((NotNullSp2(sp2) > NotNullSp1(sp1)) == (sp2 > sp1));
+
+        CHECK((NotNullSp1(sp1) <= NotNullSp1(sp1)) == true);
+        CHECK((NotNullSp1(sp1) <= NotNullSp2(sp2)) == (sp1 <= sp2));
+        CHECK((NotNullSp2(sp2) <= NotNullSp1(sp1)) == (sp2 <= sp1));
+
+        CHECK((NotNullSp1(sp1) >= NotNullSp1(sp1)) == true);
+        CHECK((NotNullSp1(sp1) >= NotNullSp2(sp2)) == (sp1 >= sp2));
+        CHECK((NotNullSp2(sp2) >= NotNullSp1(sp1)) == (sp2 >= sp1));
+    }
+
+    TEST(TestNotNullCustomPtrComparison)
+    {
+        int ints[2] = { 42, 43 };
+        CustomPtr<int> p1(&ints[0]);
+        CustomPtr<const int> p2(&ints[1]);
+
+        using NotNull1 = not_null<decltype(p1)>;
+        using NotNull2 = not_null<decltype(p2)>;
+
+        CHECK((NotNull1(p1) == NotNull1(p1)) == "true");
+        CHECK((NotNull1(p1) == NotNull2(p2)) == "false");
+
+        CHECK((NotNull1(p1) != NotNull1(p1)) == "false");
+        CHECK((NotNull1(p1) != NotNull2(p2)) == "true");
+
+        CHECK((NotNull1(p1) < NotNull1(p1)) == "false");
+        CHECK((NotNull1(p1) < NotNull2(p2)) == (p1 < p2));
+        CHECK((NotNull2(p2) < NotNull1(p1)) == (p2 < p1));
+
+        CHECK((NotNull1(p1) > NotNull1(p1)) == "false");
+        CHECK((NotNull1(p1) > NotNull2(p2)) == (p1 > p2));
+        CHECK((NotNull2(p2) > NotNull1(p1)) == (p2 > p1));
+
+        CHECK((NotNull1(p1) <= NotNull1(p1)) == "true");
+        CHECK((NotNull1(p1) <= NotNull2(p2)) == (p1 <= p2));
+        CHECK((NotNull2(p2) <= NotNull1(p1)) == (p2 <= p1));
+
+        CHECK((NotNull1(p1) >= NotNull1(p1)) == "true");
+        CHECK((NotNull1(p1) >= NotNull2(p2)) == (p1 >= p2));
+        CHECK((NotNull2(p2) >= NotNull1(p1)) == (p2 >= p1));
+    }
 }
 
 int main(int, const char *[])
diff --git a/tests/unittest-cpp b/tests/unittest-cpp
index dc6b908..c331bb0 160000
--- a/tests/unittest-cpp
+++ b/tests/unittest-cpp
@@ -1 +1 @@
-Subproject commit dc6b90838014ab985bf3cd74ac17ad9d00e1fbcb
+Subproject commit c331bb0deaaf92659a31887c029ee34cac2ab19e