not_null cleanups and improvements: (#449)

* constexpr all the things.

* remove operator=(const T&)
  * it leaves *this in an invalid state if ensure_invariant fails
  * implicitly converting the T to not_null and then assigning is in every way superior.

* simplify conversion from not_null<U> with constructor delegation.

* remove the converting assignment operator; again let the conversion constructor and self-assignment operator do the work.

* Cover the remaining pointer arithmetic operations as Wakely suggests in issue #447.

* Cleanup not_null conversions from null pointer constants:

  * replace constructor that accepts T with constructor template that accepts U convertible to T

  * remove deleted constructor that accepts int

  * Attempts to initialize with nullptr, 0, 0L, 0LL, etc. all unambiguously select the deleted nullptr_t constructor.

include/gsl/gsl

@@ -62,81 +62,63 @@ using owner = T;
 // Has zero size overhead over T.
 //
 // If T is a pointer (i.e. T == U*) then
-// - allow construction from U* or U&
+// - allow construction from U*
 // - disallow construction from nullptr_t
 // - disallow default construction
-// - ensure construction from U* fails with nullptr
+// - ensure construction from null U* fails
 // - allow implicit conversion to U*
 //
 template <class T>
 class not_null
 {
+public:
     static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
 
-public:
-    not_null(T t) : ptr_(t) { ensure_invariant(); }
-    not_null& operator=(const T& t)
-    {
-        ptr_ = t;
-        ensure_invariant();
-        return *this;
-    }
+    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
+    constexpr not_null(U&& u) : ptr_(std::forward<U>(u)) { Expects(ptr_ != nullptr); }
+
+    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
+    constexpr not_null(const not_null<U>& other) : not_null(other.get()) {}
 
     not_null(const not_null& other) = default;
     not_null& operator=(const not_null& other) = default;
 
-    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
-    not_null(const not_null<U>& other)
+    constexpr T get() const
     {
-        *this = other;
-    }
-
-    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
-    not_null& operator=(const not_null<U>& other)
-    {
-        ptr_ = other.get();
-        return *this;
-    }
-
-    // prevents compilation when someone attempts to assign a nullptr
-    not_null(std::nullptr_t) = delete;
-    not_null(int) = delete;
-    not_null<T>& operator=(std::nullptr_t) = delete;
-    not_null<T>& operator=(int) = delete;
-
-    T get() const
-    {
-#ifdef _MSC_VER
-        __assume(ptr_ != nullptr);
-#endif
+        Ensures(ptr_ != nullptr);
         return ptr_;
-    } // the assume() should help the optimizer
+    }
 
-    operator T() const { return get(); }
-    T operator->() const { return get(); }
+    constexpr operator T() const { return get(); }
+    constexpr T operator->() const { return get(); }
 
-    bool operator==(const T& rhs) const { return ptr_ == rhs; }
-    bool operator!=(const T& rhs) const { return !(*this == rhs); }
-private:
-    T ptr_;
-
-    // we assume that the compiler can hoist/prove away most of the checks inlined from this
-    // function
-    // if not, we could make them optional via conditional compilation
-    void ensure_invariant() const { Expects(ptr_ != nullptr); }
+    // prevents compilation when someone attempts to assign a null pointer constant
+    not_null(std::nullptr_t) = delete;
+    not_null& operator=(std::nullptr_t) = delete;
 
     // unwanted operators...pointers only point to single objects!
-    // TODO ensure all arithmetic ops on this type are unavailable
-    not_null<T>& operator++() = delete;
-    not_null<T>& operator--() = delete;
-    not_null<T> operator++(int) = delete;
-    not_null<T> operator--(int) = delete;
-    not_null<T>& operator+(size_t) = delete;
-    not_null<T>& operator+=(size_t) = delete;
-    not_null<T>& operator-(size_t) = delete;
-    not_null<T>& operator-=(size_t) = delete;
+    not_null& operator++() = delete;
+    not_null& operator--() = delete;
+    not_null operator++(int) = delete;
+    not_null operator--(int) = delete;
+    not_null& operator+=(std::ptrdiff_t) = delete;
+    not_null& operator-=(std::ptrdiff_t) = delete;
+    void operator[](std::ptrdiff_t) const = delete;
+
+private:
+    T ptr_;
 };
 
+// more unwanted operators
+template <class T, class U>
+std::ptrdiff_t operator-(const not_null<T>&, const not_null<U>&) = delete;
+template <class T>
+not_null<T> operator-(const not_null<T>&, std::ptrdiff_t)  = delete;
+template <class T>
+not_null<T> operator+(const not_null<T>&, std::ptrdiff_t) = delete;
+template <class T>
+not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
+
 } // namespace gsl
 
 namespace std