From ccac0e2af831c8902ea3cc3ba23f5115ba44a07c Mon Sep 17 00:00:00 2001
From: Simon Sticko <simon@sticko.se>
Date: Sat, 7 Sep 2024 14:50:42 +0200
Subject: [PATCH] Make it possible to include nonlinear.h when no external libs
 are enabled

The file nonlinear.h can not be included when none of SUNDIALS,
TRILINOS, or PETSC are enabled. Fix this.
---
 include/deal.II/numerics/nonlinear.h          |  8 ++--
 .../numerics/nonlinear_solver_selector_04.cc  | 37 +++++++++++++++++++
 .../nonlinear_solver_selector_04.output       |  2 +
 3 files changed, 43 insertions(+), 4 deletions(-)
 create mode 100644 tests/numerics/nonlinear_solver_selector_04.cc
 create mode 100644 tests/numerics/nonlinear_solver_selector_04.output

diff --git a/include/deal.II/numerics/nonlinear.h b/include/deal.II/numerics/nonlinear.h
index bde8ee41b6..712db8b5c7 100644
--- a/include/deal.II/numerics/nonlinear.h
+++ b/include/deal.II/numerics/nonlinear.h
@@ -689,15 +689,15 @@ NonlinearSolverSelector<VectorType>::solve(
     }
   else
     {
-      const std::string solvers =
+      const std::string solvers = ""
 #ifdef DEAL_II_WITH_SUNDIALS
-        "kinsol\n"
+                                  "kinsol\n"
 #endif
 #ifdef DEAL_II_TRILINOS_WITH_NOX
-        "NOX\n"
+                                  "NOX\n"
 #endif
 #ifdef DEAL_II_WITH_PETSC
-        "SNES\n"
+                                  "SNES\n"
 #endif
         ;
 
diff --git a/tests/numerics/nonlinear_solver_selector_04.cc b/tests/numerics/nonlinear_solver_selector_04.cc
new file mode 100644
index 0000000000..2ce8c47828
--- /dev/null
+++ b/tests/numerics/nonlinear_solver_selector_04.cc
@@ -0,0 +1,37 @@
+// ------------------------------------------------------------------------
+//
+// SPDX-License-Identifier: LGPL-2.1-or-later
+// Copyright (C) 2022 - 2024 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// Part of the source code is dual licensed under Apache-2.0 WITH
+// LLVM-exception OR LGPL-2.1-or-later. Detailed license information
+// governing the source code and code contributions can be found in
+// LICENSE.md and CONTRIBUTING.md at the top level directory of deal.II.
+//
+// ------------------------------------------------------------------------
+
+
+
+// Tests that nonlinear.h can be included and that an object of
+// NonlinearSolverSelector can be created, even when none of SUNDIALS,
+// TRILINOS, or PETSC are enabled. Test added to trigger the bug, that this was
+// not possibles.
+
+#include <deal.II/base/function.h>
+
+#include <deal.II/numerics/nonlinear.h>
+
+#include "../tests.h"
+
+
+int
+main()
+{
+  initlog();
+
+  NonlinearSolverSelector solver;
+
+  deallog << "OK" << std::endl;
+}
diff --git a/tests/numerics/nonlinear_solver_selector_04.output b/tests/numerics/nonlinear_solver_selector_04.output
new file mode 100644
index 0000000000..0fd8fc12f0
--- /dev/null
+++ b/tests/numerics/nonlinear_solver_selector_04.output
@@ -0,0 +1,2 @@
+
+DEAL::OK
-- 
2.39.5