not_null and maybe_null variants should only work on pointer types.

include/gsl.h

@@ -102,6 +102,7 @@ typename Cont::value_type& at(Cont& cont, size_t index) { fail_fast_assert(index
 template<class T>
 class not_null
 {
+    static_assert(std::is_pointer<T>::value, "T must be a pointer type");
 public:
     not_null(T t) : ptr_(t) { ensure_invariant(); }
 
@@ -162,6 +163,7 @@ private:
 template<class T>
 class maybe_null_dbg
 {
+    static_assert(std::is_pointer<T>::value, "T must be a pointer type");
 public:
     maybe_null_dbg() : ptr_(nullptr), tested_(false) {}
 
@@ -237,6 +239,7 @@ private:
 template<class T>
 class maybe_null_ret
 {
+    static_assert(std::is_pointer<T>::value, "T must be a pointer type");
 public:
     maybe_null_ret() : ptr_(nullptr) {}
     maybe_null_ret(std::nullptr_t) : ptr_(nullptr) {}

tests/maybenull_tests.cpp

@@ -16,6 +16,7 @@
 
 #include <UnitTest++/UnitTest++.h> 
 #include <gsl.h>
+#include <vector>
 
 using namespace Guide;
 
@@ -27,6 +28,10 @@ SUITE(MaybeNullTests)
 {
     TEST(TestMaybeNull1)
     {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        maybe_null_dbg<std::vector<int>> f(std::vector<int>{1}); // Not a pointer type. Must be tested with a non-integral type.
+        maybe_null_ret<std::vector<int>> g(std::vector<int>{1}); // Not a pointer type. Must be tested with a non-integral type.
+#endif
         int n = 5;
         maybe_null_dbg<int *> opt_n(&n);
         int result = 0;

tests/notnull_tests.cpp

@@ -16,6 +16,7 @@
 
 #include <UnitTest++/UnitTest++.h> 
 #include <gsl.h>
+#include <vector>
 
 using namespace Guide;
 
@@ -48,6 +49,7 @@ SUITE(NotNullTests)
         not_null<int*> p; // yay...does not compile!
         std::unique_ptr<int> up = std::make_unique<int>(120);
         not_null<int*> p = up;
+        not_null<std::vector<int>> f(std::vector<int>{1}); // Not a pointer type. Must be tested with a non-integral type.
 #endif
       int i = 12; 
       auto rp = RefCounted<int>(&i);