[T/MPL] Test current scalar exprtk evaluations

Linking to exprtk is neccessary because MaterialLib designates
private linking to that library.

Tests/CMakeLists.txt

@@ -119,6 +119,7 @@ target_link_libraries(
         $<$<TARGET_EXISTS:SwmmInterface>:SwmmInterface>
         $<$<TARGET_EXISTS:InSituLib>:InSituLib>
         $<$<TARGET_EXISTS:petsc>:petsc>
+        exprtk
     PUBLIC $<$<BOOL:${OGS_USE_MFRONT}>:OgsMFrontBehaviourForUnitTests>
 )
 

Tests/MaterialLib/TestMPLFunction.cpp

+/**
+ * \file
+ * \copyright
+ * Copyright (c) 2012-2024, OpenGeoSys Community (http://www.opengeosys.org)
+ *            Distributed under a Modified BSD License.
+ *              See accompanying file LICENSE.txt or
+ *              http://www.opengeosys.org/project/license
+ */
+
+#include <gtest/gtest.h>
+
+#include <limits>
+
+#include "BaseLib/ConfigTree.h"
+#include "MaterialLib/MPL/Properties/Function.h"
+#include "MathLib/KelvinVector.h"
+#include "MathLib/VectorizedTensor.h"
+
+namespace MPL = MaterialPropertyLib;
+
+struct MPLFunction : public ::testing::Test
+{
+    static double constexpr nan = std::numeric_limits<double>::quiet_NaN();
+    MPL::VariableArray vars;
+};
+
+TEST_F(MPLFunction, ScalarScalar)
+{
+    vars.temperature = 2.;
+
+    MPL::Property const& f = MPL::Function{
+        "test_function", {"temperature"}, {{"temperature", {"1"}}}};
+    ASSERT_EQ(2., f.value<double>(vars, {}, nan, nan));
+    ASSERT_EQ(1.,
+              f.dValue<double>(vars, MPL::Variable::temperature, {}, nan, nan));
+}
+
+TEST_F(MPLFunction, ScalarVector)
+{
+    vars.temperature = 2.;
+
+    MPL::Property const& f =
+        MPL::Function{"test_function",
+                      {"temperature", "temperature^2"},
+                      {{"temperature", {"1", "2*temperature"}}}};
+    ASSERT_EQ((Eigen::Vector2d{2., 4.}),
+              (f.value<Eigen::Vector2d>(vars, {}, nan, nan)));
+    ASSERT_EQ((Eigen::Vector2d{1., 4.}),
+              f.dValue<Eigen::Vector2d>(
+                  vars, MPL::Variable::temperature, {}, nan, nan));
+}
+
+TEST_F(MPLFunction, ScalarUninitialized)
+{
+    // The vars.temperature = is not initialized.
+
+    MPL::Property const& f = MPL::Function{
+        "test_function", {"temperature"}, {{"temperature", {"1"}}}};
+    ASSERT_ANY_THROW(f.value<double>(vars, {}, nan, nan));
+    ASSERT_ANY_THROW(
+        f.dValue<double>(vars, MPL::Variable::temperature, {}, nan, nan));
+}