ENH: reset tmp via assignment from literal nullptr (#1775)

- previously this was marked as '= delete' for consistency with
  assignment from an empty pointer being a runtime error.
  However, these can be considered semantically different and it makes
  sense to permit this as equivalent to reset(nullptr).

  This change does not break existing code since the operator was
  previously unavailable (deleted).

STYLE: refactor tmp operator=(T*)

- delegate to reset() after initial checks

src/OpenFOAM/memory/tmp/tmp.H

@@ -255,9 +255,8 @@ public:
         //  Fatal for a null pointer, or when the pointer is non-unique.
         inline void operator=(T* p);
 
-        //- No assignment from literal nullptr.
-        //  Consistent with run-time check for nullptr on assignment.
-        void operator=(std::nullptr_t) = delete;
+        //- Reset via assignment from literal nullptr
+        inline void operator=(std::nullptr_t) noexcept;
 };
 
 

src/OpenFOAM/memory/tmp/tmpI.H

@@ -464,8 +464,6 @@ inline void Foam::tmp<T>::operator=(tmp<T>&& other) noexcept
 template<class T>
 inline void Foam::tmp<T>::operator=(T* p)
 {
-    clear();
-
     if (!p)
     {
         FatalErrorInFunction
@@ -480,8 +478,14 @@ inline void Foam::tmp<T>::operator=(T* p)
             << abort(FatalError);
     }
 
-    ptr_ = p;
-    type_ = PTR;
+    reset(p);
+}
+
+
+template<class T>
+inline void Foam::tmp<T>::operator=(std::nullptr_t) noexcept
+{
+    reset(nullptr);
 }
 
 

src/OpenFOAM/memory/tmp/tmpNrc.H

@@ -234,9 +234,8 @@ public:
         //  Fatal for a null pointer
         inline void operator=(T* p);
 
-        //- No assignment from literal nullptr.
-        //  Consistent with run-time check for nullptr on assignment.
-        void operator=(std::nullptr_t) = delete;
+        //- Reset via assignment from literal nullptr
+        inline void operator=(std::nullptr_t) noexcept;
 
         //- Conversion to tmp - releases pointer or copies reference
         inline operator tmp<T>();

src/OpenFOAM/memory/tmp/tmpNrcI.H

@@ -414,8 +414,6 @@ inline void Foam::tmpNrc<T>::operator=(tmpNrc<T>&& other) noexcept
 template<class T>
 inline void Foam::tmpNrc<T>::operator=(T* p)
 {
-    clear();
-
     if (!p)
     {
         FatalErrorInFunction
@@ -423,8 +421,14 @@ inline void Foam::tmpNrc<T>::operator=(T* p)
             << abort(FatalError);
     }
 
-    ptr_ = p;
-    type_ = PTR;
+    reset(p);
+}
+
+
+template<class T>
+inline void Foam::tmpNrc<T>::operator=(std::nullptr_t) noexcept
+{
+    reset(nullptr);
 }