Deprecate ToUniquePtr.

It was just introduced to allow to  show a
deprecation warning in case someone consumes a
GridFactory::createGrid() into a raw-pointer.

CHANGELOG.md

@@ -37,6 +37,9 @@
 - `dune_list_filter` is deprecated and will be removed after Dune 2.8. Use
   `list(FILTER ...)` introduced by CMake 3.6 instead.
 
+- `ToUniquePtr` is deprecated and will be removed after Dune 2.8. Use
+  `std::unique_ptr` or `std::shared_ptr` instead.
+
 - Remove the CMake options `DUNE_BUILD_BOTH_LIBS` and
   `DUNE_USE_ONLY_STATIC_LIBS`. Use the default CMake way instead by
   setting `BUILD_SHARED_LIBS` accordingly. Building both static

dune/common/test/to_unique_ptrtest.cc

@@ -22,6 +22,8 @@ int main()
   using namespace Dune;
   TestSuite t;
 
+  DUNE_NO_DEPRECATED_BEGIN
+
   {
     int* ptr = new int(1);
 
@@ -32,10 +34,8 @@ int main()
     ToUniquePtr<int> w2 = makeToUnique<int>(2);
 
     // test conversion to pointer
-    DUNE_NO_DEPRECATED_BEGIN
     int* p1 = f1();
     A* p2 = g();
-    DUNE_NO_DEPRECATED_END
 
     delete p1;
     delete p2;
@@ -89,5 +89,8 @@ int main()
   std::unique_ptr<int> x0{ f_old() };
   std::unique_ptr<int> x1{ f1() };
   std::unique_ptr<int> x2 = f1();
+
+  DUNE_NO_DEPRECATED_END
+
   return t.exit();
 }

dune/common/to_unique_ptr.hh

@@ -4,6 +4,8 @@
 #ifndef DUNE_TO_UNIQUE_PTR_HH
 #define DUNE_TO_UNIQUE_PTR_HH
 
+#warning to_unique_ptr.hh and ToUniquePtr is deprecated. Use std::unique_ptr or std::shared_ptr instead.
+
 #include <memory>
 
 namespace Dune
@@ -12,6 +14,9 @@ namespace Dune
   /// Transfers ownership by cast to any (smart) pointer type. Releases the stored pointer on transfer.
   /// NOTE: This is an intermediate solution to switch to std::unique_ptr in later releases smoothly.
   /**
+   * \deprecated ToUniquePtr is deprecated and will be removed after Dune 2.8.
+   *             Use std::unique_ptr or std::shared_ptr instead.
+   *
    * Example of usage:
    * ```
    * ToUniquePtr<int> f() { return new int(1); }
@@ -30,7 +35,9 @@ namespace Dune
    * ```
    **/
   template <class T>
-  class ToUniquePtr
+  class
+  [[deprecated("Will be removed after Dune 2.8. Use std::unique_ptr or std::shared_ptr instead.")]]
+  ToUniquePtr
       : public std::unique_ptr<T>
   {
     using Super = std::unique_ptr<T>;