From: Julian Roth <mathmerizing@gmail.com>
Date: Fri, 5 Jun 2020 19:31:55 +0000 (+0200)
Subject: Added check whether there actually are any user_constraints
X-Git-Tag: v9.3.0-rc1~1476^2~1
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a1b371d93a004e359c0b942071670cb3f5a9f6b0;p=dealii.git

Added check whether there actually are any user_constraints
---

diff --git a/source/multigrid/mg_transfer_prebuilt.cc b/source/multigrid/mg_transfer_prebuilt.cc
index 80d8390171..c250fefb37 100644
--- a/source/multigrid/mg_transfer_prebuilt.cc
+++ b/source/multigrid/mg_transfer_prebuilt.cc
@@ -86,7 +86,10 @@ MGTransferPrebuilt<VectorType>::prolongate(const unsigned int to_level,
          ExcIndexRange(to_level, 1, prolongation_matrices.size() + 1));
 
   VectorType copy_src(src);
-  if (this->mg_constrained_dofs != nullptr)
+  if (this->mg_constrained_dofs != nullptr &&
+      this->mg_constrained_dofs->get_user_constraint_matrix(to_level - 1)
+          .get_local_lines()
+          .size() > 0)
     this->mg_constrained_dofs->get_user_constraint_matrix(to_level - 1)
       .distribute(copy_src);
   prolongation_matrices[to_level - 1]->vmult(dst, copy_src);