Try to avoid UB in thrust::reference

We were dereferencing a nullptr because we wanted to avoid default constructing a system.

This is obvious UB and prevents us from running the tests with sanitizers, so just default construct the systems.

Addresses #1645

thrust/testing/memory.cu

@@ -22,6 +22,8 @@
 class my_memory_system : public thrust::device_execution_policy<my_memory_system>
 {
 public:
+  my_memory_system() = default;
+
   my_memory_system(int)
       : correctly_dispatched(false)
       , num_copies(0)
@@ -43,14 +45,11 @@ public:
   }
 
 private:
-  bool correctly_dispatched;
+  bool correctly_dispatched = true;
 
   // count the number of copies so that we can validate
   // that dispatch does not introduce any
-  unsigned int num_copies;
-
-  // disallow default construction
-  my_memory_system();
+  unsigned int num_copies = 0;
 };
 
 namespace my_old_namespace

thrust/thrust/detail/reference.h

@@ -163,10 +163,7 @@ class reference
    */
   _CCCL_HOST_DEVICE void swap(derived_type& other)
   {
-    // Avoid default-constructing a system; instead, just use a null pointer
-    // for dispatch. This assumes that `get_value` will not access any system
-    // state.
-    typename thrust::iterator_system<pointer>::type* system = nullptr;
+    typename thrust::iterator_system<pointer>::type system;
     swap(system, other);
   }
 
@@ -179,10 +176,7 @@ class reference
   // about what system the object is on.
   _CCCL_HOST_DEVICE operator value_type() const
   {
-    // Avoid default-constructing a system; instead, just use a null pointer
-    // for dispatch. This assumes that `get_value` will not access any system
-    // state.
-    typename thrust::iterator_system<pointer>::type* system = nullptr;
+    typename thrust::iterator_system<pointer>::type system;
     return convert_to_value_type(system);
   }
 
@@ -324,10 +318,10 @@ class reference
   }
 
   template <typename System>
-  _CCCL_HOST_DEVICE value_type convert_to_value_type(System* system) const
+  _CCCL_HOST_DEVICE value_type convert_to_value_type(System& system) const
   {
     using thrust::system::detail::generic::select_system;
-    return strip_const_get_value(select_system(*system));
+    return strip_const_get_value(select_system(system));
   }
 
   template <typename System>
@@ -340,20 +334,17 @@ class reference
   }
 
   template <typename System0, typename System1, typename OtherPointer>
-  _CCCL_HOST_DEVICE void assign_from(System0* system0, System1* system1, OtherPointer src)
+  _CCCL_HOST_DEVICE void assign_from(System0& system0, System1& system1, OtherPointer src)
   {
     using thrust::system::detail::generic::select_system;
-    strip_const_assign_value(select_system(*system0, *system1), src);
+    strip_const_assign_value(select_system(system0, system1), src);
   }
 
   template <typename OtherPointer>
   _CCCL_HOST_DEVICE void assign_from(OtherPointer src)
   {
-    // Avoid default-constructing systems; instead, just use a null pointer
-    // for dispatch. This assumes that `get_value` will not access any system
-    // state.
-    typename thrust::iterator_system<pointer>::type* system0      = nullptr;
-    typename thrust::iterator_system<OtherPointer>::type* system1 = nullptr;
+    typename thrust::iterator_system<pointer>::type system0;
+    typename thrust::iterator_system<OtherPointer>::type system1;
     assign_from(system0, system1, src);
   }
 
@@ -367,12 +358,12 @@ class reference
   }
 
   template <typename System>
-  _CCCL_HOST_DEVICE void swap(System* system, derived_type& other)
+  _CCCL_HOST_DEVICE void swap(System& system, derived_type& other)
   {
     using thrust::system::detail::generic::iter_swap;
     using thrust::system::detail::generic::select_system;
 
-    iter_swap(select_system(*system, *system), ptr, other.ptr);
+    iter_swap(select_system(system, system), ptr, other.ptr);
   }
 };