diff --git a/include/gsl/gsl_util b/include/gsl/gsl_util
index d771414..585b3f9 100644
--- a/include/gsl/gsl_util
+++ b/include/gsl/gsl_util
@@ -32,6 +32,12 @@
 
 #endif            // _MSC_VER
 
+#if defined(__cplusplus) && (__cplusplus >= 201703L)
+#define GSL_NODISCARD [[nodiscard]]
+#else
+#define GSL_NODISCARD
+#endif // defined(__cplusplus) && (__cplusplus >= 201703L)
+
 namespace gsl
 {
 //
@@ -67,13 +73,13 @@ private:
 
 // finally() - convenience function to generate a final_action
 template <class F>
-final_action<F> finally(const F& f) noexcept
+GSL_NODISCARD final_action<F> finally(const F& f) noexcept
 {
     return final_action<F>(f);
 }
 
 template <class F>
-final_action<F> finally(F&& f) noexcept
+GSL_NODISCARD final_action<F> finally(F&& f) noexcept
 {
     return final_action<F>(std::forward<F>(f));
 }
diff --git a/include/gsl/pointers b/include/gsl/pointers
index 0ec0fc3..bb80b92 100644
--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -63,7 +63,7 @@ template <class T>
 class not_null
 {
 public:
-    static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
+    static_assert(std::is_convertible<decltype(std::declval<T>() != nullptr), bool>::value, "T cannot be compared to nullptr.");
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr not_null(U&& u) : ptr_(std::forward<U>(u))
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 2bd1bbe..a4bd7e6 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -1,37 +1,42 @@
 cmake_minimum_required(VERSION 3.0.2)
 
 project(GSLTests CXX)
+include(FindPkgConfig)
 
 # will make visual studio generated project group files
 set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 
-configure_file(CMakeLists.txt.in googletest-download/CMakeLists.txt)
-execute_process(
-    COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
-    RESULT_VARIABLE result
-    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/googletest-download
-)
-if(result)
-    message(FATAL_ERROR "CMake step for googletest failed: ${result}")
+pkg_search_module(GTestMain gtest_main)
+if (NOT GTestMain_FOUND)
+    configure_file(CMakeLists.txt.in googletest-download/CMakeLists.txt)
+    execute_process(
+        COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
+        RESULT_VARIABLE result
+        WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/googletest-download
+    )
+    if(result)
+        message(FATAL_ERROR "CMake step for googletest failed: ${result}")
+    endif()
+
+    execute_process(
+        COMMAND ${CMAKE_COMMAND} --build .
+        RESULT_VARIABLE result
+        WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/googletest-download
+    )
+    if(result)
+        message(FATAL_ERROR "CMake step for googletest failed: ${result}")
+    endif()
+
+    set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+    set(GTestMain_LIBRARIES gtest_main)
+
+    add_subdirectory(
+        ${CMAKE_CURRENT_BINARY_DIR}/googletest-src
+        ${CMAKE_CURRENT_BINARY_DIR}/googletest-build
+        EXCLUDE_FROM_ALL
+    )
 endif()
 
-execute_process(
-    COMMAND ${CMAKE_COMMAND} --build .
-    RESULT_VARIABLE result
-    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/googletest-download
-)
-if(result)
-    message(FATAL_ERROR "CMake step for googletest failed: ${result}")
-endif()
-
-set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-
-add_subdirectory(
-    ${CMAKE_CURRENT_BINARY_DIR}/googletest-src
-    ${CMAKE_CURRENT_BINARY_DIR}/googletest-build
-    EXCLUDE_FROM_ALL
-)
-
 if (MSVC AND (GSL_CXX_STANDARD EQUAL 17))
     set(GSL_CPLUSPLUS_OPT -Zc:__cplusplus -permissive-)
 endif()
@@ -149,7 +154,7 @@ function(add_gsl_test name)
     target_link_libraries(${name}
         GSL
         gsl_tests_config
-        gtest_main
+        ${GTestMain_LIBRARIES}
     )
     add_test(
         ${name}
@@ -254,7 +259,7 @@ function(add_gsl_test_noexcept name)
     target_link_libraries(${name}
         GSL
         gsl_tests_config_noexcept
-        gtest_main
+        ${GTestMain_LIBRARIES}
     )
     add_test(
       ${name}