Mark not_null constructors as noexcept when underlying type can be mo…

…ved with no exception (#1135)

This enables possible optimisations for trivial types. This also avoids a bug
in std::variant::emplace from GNU's libstdc++.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106547

include/gsl/pointers

@@ -100,19 +100,19 @@ public:
     static_assert(details::is_comparable_to_nullptr<T>::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))
+    constexpr not_null(U&& u) noexcept(std::is_nothrow_move_constructible<T>::value) : ptr_(std::forward<U>(u))
     {
         Expects(ptr_ != nullptr);
     }
 
     template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
-    constexpr not_null(T u) : ptr_(std::move(u))
+    constexpr not_null(T u) noexcept(std::is_nothrow_move_constructible<T>::value) : ptr_(std::move(u))
     {
         Expects(ptr_ != nullptr);
     }
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr not_null(const not_null<U>& other) : not_null(other.get())
+    constexpr not_null(const not_null<U>& other) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null(other.get())
     {}
 
     not_null(const not_null& other) = default;

tests/notnull_tests.cpp

@@ -24,6 +24,7 @@
 #include <sstream>   // for operator<<, ostringstream, basic_ostream:...
 #include <string>    // for basic_string, operator==, string, operator<<
 #include <typeinfo>  // for type_info
+#include <variant>   // for variant, monostate, get
 
 #include "deathTestCommon.h"
 using namespace gsl;
@@ -514,6 +515,17 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
     }
 #endif
 }
+
+TEST(notnull_tests, TestVariantEmplace)
+{
+    int i = 0;
+    std::variant<std::monostate, not_null<int*>> v;
+    v.emplace<not_null<int*>>(&i);
+
+    EXPECT_FALSE(v.valueless_by_exception());
+    EXPECT_TRUE(v.index() == 1);
+    EXPECT_TRUE(std::get<not_null<int*>>(v) == &i);
+}
 #endif // #if defined(__cplusplus) && (__cplusplus >= 201703L)
 
 TEST(notnull_tests, TestMakeNotNull)