Merge branch 'cherry-pick-994f9aca' into 'releases/2.7'

Merge branch 'fix-multigridstep-constructor-bug' into 'master'

See merge request !48

dune/solvers/iterationsteps/multigridstep.hh

@@ -54,7 +54,7 @@ namespace Dune {
 
             setSmoother(preSmoother,postSmoother);
 
-            basesolver_   = baseSolver;
+            basesolver_ = std::shared_ptr<Solver>(baseSolver, [](auto*){} );
 
             this->ignoreNodes_ = ignoreNodes;
         }

dune/solvers/test/multigridtest.cc

@@ -85,6 +85,13 @@ struct MultigridTestSuite
         mgStep.setIgnore(p.ignore);
         mgStep.setBaseSolver(basesolver);
 
+        // create an unused second multigrid step just to check that the constructor works
+        MGStep alternativeStep(p.A, p.u, p.rhs,
+                               1, 3, 3,
+                               &smoother, &smoother,
+                               &basesolver,
+                               &p.ignore);
+
         // create loop solver
         Solver solver(mgStep, maxIterations, tol, p.energyNorm, Solver::FULL);