From ab825037d0b45f23ede61f600652da38defbc99d Mon Sep 17 00:00:00 2001
From: Martin Moene <martin.moene@gmail.com>
Date: Mon, 28 Sep 2015 07:48:39 +0200
Subject: [PATCH] Add assignment from related types

---
 include/gsl.h           |  6 ++++++
 tests/notnull_tests.cpp | 20 ++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/gsl.h b/include/gsl.h
index fed3b6c..dc4d425 100644
--- a/include/gsl.h
+++ b/include/gsl.h
@@ -129,6 +129,12 @@ public:
     not_null<T>& operator=(std::nullptr_t) = delete;
 	not_null<T>& operator=(int) = delete;
     
+    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
+    not_null<T> & operator=( not_null<U> const & other ) 
+    { 
+        ptr_ = other.get(); ensure_invariant(); return *this; 
+    }
+
     T get() const {
 #ifdef _MSC_VER
         __assume(ptr_ != nullptr);
diff --git a/tests/notnull_tests.cpp b/tests/notnull_tests.cpp
index 7232840..69aebaa 100644
--- a/tests/notnull_tests.cpp
+++ b/tests/notnull_tests.cpp
@@ -86,6 +86,26 @@ SUITE(NotNullTests)
 
         int* q = nullptr;
         CHECK_THROW(p = q, fail_fast);
+
+        // Allows assignment from a not_null related pointer type.
+        {            
+            MyDerived derived;
+            not_null<MyDerived*> p = &derived;
+            not_null<MyBase*> q = p;
+            
+            q = p;
+
+            CHECK(q == p);
+        }
+
+        // Terminates assignment from related pointer types for null pointer value.
+        {
+            MyDerived* z = nullptr;
+            MyDerived derived;
+            not_null<MyBase*> p = &derived;
+
+            CHECK_THROW(p = z, fail_fast);            
+        }
     }
 }