make capabilities a compile-time test whether backup-restore functionality can be used

src/amdis/ProblemStat.inc.hpp

@@ -479,16 +479,26 @@ writeFiles(AdaptInfo& adaptInfo, bool force)
 }
 
 
+template <class Grid, class GridView,
+  class = decltype(Dune::BackupRestoreFacility<Grid>::backup(std::declval<Grid>(), std::string("")))>
+void backup_impl(Grid const& grid, GridView const& gv, std::string filename, std::true_type)
+{
+  Dune::BackupRestoreFacility<Grid>::backup(grid, filename);
+}
+
+template <class Grid, class GridView, bool B,
+  class = decltype(BackupRestoreByGridFactory<Grid>::backup(std::declval<GridView>(), std::string("")))>
+void backup_impl(Grid const& grid, GridView const& gv, std::string filename, bool_t<B>)
+{
+  warning("Falling back to backup of gridview.");
+  BackupRestoreByGridFactory<Grid>::backup(gv, filename);
+}
+
 template <class Traits>
 void ProblemStat<Traits>::
 backup(std::string const& filename) const
 {
-  if (Dune::Capabilities::hasBackupRestoreFacilities<Grid>::v)
-    Dune::BackupRestoreFacility<Grid>::backup(*grid_, filename);
-  else {
-    warning("Falling back to backup of gridview.");
-    BackupRestoreByGridFactory<Grid>::backup(gridView(), filename);
-  }
+  backup_impl(*grid_, gridView(), filename, bool_t<Dune::Capabilities::hasBackupRestoreFacilities<Grid>::v>{});
 }
 
 
@@ -510,16 +520,26 @@ backup(AdaptInfo& adaptInfo)
 }
 
 
+template <class Grid,
+  class = decltype(Dune::BackupRestoreFacility<Grid>::restore(std::string("")))>
+auto restore_impl(std::string filename, std::true_type)
+{
+  return Dune::BackupRestoreFacility<Grid>::restore(filename);
+}
+
+template <class Grid, bool B,
+  class = decltype(BackupRestoreByGridFactory<Grid>::restore(std::string("")))>
+auto restore_impl(std::string filename, bool_t<B>)
+{
+  return BackupRestoreByGridFactory<Grid>::restore(filename);
+}
+
 template <class Traits>
 auto ProblemStat<Traits>::
 restore(std::string const& filename)
 {
   // restore grid from file
-  std::unique_ptr<Grid> grid;
-  if (Dune::Capabilities::hasBackupRestoreFacilities<Grid>::v)
-    grid.reset(Dune::BackupRestoreFacility<Grid>::restore(filename));
-  else
-    grid.reset(BackupRestoreByGridFactory<Grid>::restore(filename));
+  std::unique_ptr<Grid> grid(restore_impl<Grid>(filename, bool_t<Dune::Capabilities::hasBackupRestoreFacilities<Grid>::v>{}));
   return std::move(grid);
 }