not_null and maybe_null variants should only work on nullptr-assignable types.

This is in accordance with the GSL.View guidance on not_null and maybe_null
types in the CppCoreGuidelines document.

include/gsl.h

@@ -108,6 +108,7 @@ typename Cont::value_type& at(Cont& cont, size_t index) { fail_fast_assert(index
 template<class T>
 class not_null
 {
+    static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
 public:
     not_null(T t) : ptr_(t) { ensure_invariant(); }
 
@@ -168,6 +169,7 @@ private:
 template<class T>
 class maybe_null_dbg
 {
+    static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
 public:
     maybe_null_dbg() : ptr_(nullptr), tested_(false) {}
 
@@ -243,6 +245,7 @@ private:
 template<class T>
 class maybe_null_ret
 {
+    static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
 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;
 
@@ -27,12 +28,24 @@ SUITE(MaybeNullTests)
 {
     TEST(TestMaybeNull1)
     {
+#ifdef CONFIRM_COMPILATION_ERRORS
+        // Forbid non-nullptr assignable types
+        maybe_null_ret<std::vector<int>> f_ret(std::vector<int>{1});
+        maybe_null_ret<std::vector<int>> f_ret(std::vector<int>{1});
+        maybe_null_ret<int> z_ret(10);
+        maybe_null_dbg<std::vector<int>> y_dbg({1,2});
+        maybe_null_dbg<int> z_dbg(10);
+        maybe_null_dbg<std::vector<int>> y_dbg({1,2});
+#endif
         int n = 5;
         maybe_null_dbg<int *> opt_n(&n);
         int result = 0;
         bool threw = false;
 
         CHECK_THROW(result = *opt_n, fail_fast);
+
+        maybe_null_ret<std::shared_ptr<int>> x_ret(std::make_shared<int>(10)); // shared_ptr<int> is nullptr assignable
+        maybe_null_dbg<std::shared_ptr<int>> x_dbg(std::make_shared<int>(10)); // shared_ptr<int> is nullptr assignable
     }
 
     TEST(TestMaybeNull2)

tests/notnull_tests.cpp

@@ -16,6 +16,7 @@
 
 #include <UnitTest++/UnitTest++.h> 
 #include <gsl.h>
+#include <vector>
 
 using namespace Guide;
 
@@ -48,11 +49,18 @@ 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;
+
+        // Forbid non-nullptr assignable types
+        not_null<std::vector<int>> f(std::vector<int>{1});
+        not_null<int> z(10);
+        not_null<std::vector<int>> y({1,2});
 #endif
       int i = 12; 
       auto rp = RefCounted<int>(&i);
       not_null<int*> p(rp);
       CHECK(p.get() == &i);
+
+      not_null<std::shared_ptr<int>> x(std::make_shared<int>(10)); // shared_ptr<int> is nullptr assignable
     }
 
     TEST(TestNotNullCasting)