diff --git a/msim/include/opm/msim/msim.hpp b/msim/include/opm/msim/msim.hpp
index 92565f73ee1..a21c576de53 100644
--- a/msim/include/opm/msim/msim.hpp
+++ b/msim/include/opm/msim/msim.hpp
@@ -42,9 +42,23 @@ class msim {
     void post_step(data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, const time_point& sim_time);
 
 private:
+    void run_step(const WellTestState& wtest_state,
+                  UDQState& udq_state,
+                  data::Solution& sol,
+                  data::Wells& well_data,
+                  data::GroupAndNetworkValues& group_nwrk_data,
+                  size_t report_step,
+                  EclipseIO& io);
+
+    void run_step(const WellTestState& wtest_state,
+                  UDQState& udq_state,
+                  data::Solution& sol,
+                  data::Wells& well_data,
+                  data::GroupAndNetworkValues& group_nwrk_data,
+                  size_t report_step,
+                  double dt,
+                  EclipseIO& io);
 
-    void run_step(WellTestState& wtest_state, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, EclipseIO& io);
-    void run_step(WellTestState& wtest_state, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, double dt, EclipseIO& io);
     void output(const WellTestState& wtest_state, const UDQState& udq_state,
                 size_t report_step, bool substep, double seconds_elapsed,
                 const data::Solution& sol, const data::Wells& well_data,
diff --git a/msim/src/msim.cpp b/msim/src/msim.cpp
index 34566294cfe..9b4064bf098 100644
--- a/msim/src/msim.cpp
+++ b/msim/src/msim.cpp
@@ -148,7 +148,7 @@ void msim::post_step(data::Solution& /* sol */,
 }
 
 
-void msim::run_step(WellTestState& wtest_state,
+void msim::run_step(const WellTestState& wtest_state,
                     UDQState& udq_state,
                     data::Solution& sol,
                     data::Wells& well_data,
@@ -160,7 +160,7 @@ void msim::run_step(WellTestState& wtest_state,
                    report_step, schedule.stepLength(report_step - 1), io);
 }
 
-void msim::run_step(WellTestState& wtest_state,
+void msim::run_step(const WellTestState& wtest_state,
                     UDQState& udq_state,
                     data::Solution& sol,
                     data::Wells& well_data,
diff --git a/opm/input/eclipse/Schedule/KeywordHandlers.cpp b/opm/input/eclipse/Schedule/KeywordHandlers.cpp
index 8ef94af0fba..61c17654d41 100644
--- a/opm/input/eclipse/Schedule/KeywordHandlers.cpp
+++ b/opm/input/eclipse/Schedule/KeywordHandlers.cpp
@@ -70,12 +70,12 @@ namespace Opm {
 
 namespace {
 
-void handleAQUCT(HandlerContext& handlerContext)
+void handleAQUCT(const HandlerContext& handlerContext)
 {
     throw OpmInputError("AQUCT is not supported as SCHEDULE keyword", handlerContext.keyword.location());
 }
 
-void handleAQUFETP(HandlerContext& handlerContext)
+void handleAQUFETP(const HandlerContext& handlerContext)
 {
     throw OpmInputError("AQUFETP is not supported as SCHEDULE keyword", handlerContext.keyword.location());
 }
diff --git a/opm/input/eclipse/Schedule/Schedule.cpp b/opm/input/eclipse/Schedule/Schedule.cpp
index 1b6277f04d1..ad243dc1640 100644
--- a/opm/input/eclipse/Schedule/Schedule.cpp
+++ b/opm/input/eclipse/Schedule/Schedule.cpp
@@ -455,7 +455,7 @@ namespace
 /// \brief Check whether each MS well has COMPSEGS entry andissue error if not.
 /// \param welsegs All wells with a WELSEGS entry together with the location.
 /// \param compegs All wells with a COMPSEGS entry
-void check_compsegs_consistency(Opm::WelSegsSet& welsegs,
+void check_compsegs_consistency(const Opm::WelSegsSet& welsegs,
                                 const std::set<std::string>& compsegs,
                                 const std::vector<::Opm::Well>& wells)
 {
diff --git a/opm/input/eclipse/Schedule/UDQ/UDQConfig.cpp b/opm/input/eclipse/Schedule/UDQ/UDQConfig.cpp
index 44ab3842fb6..d12445bded9 100644
--- a/opm/input/eclipse/Schedule/UDQ/UDQConfig.cpp
+++ b/opm/input/eclipse/Schedule/UDQ/UDQConfig.cpp
@@ -608,7 +608,7 @@ namespace Opm {
     }
 
     void UDQConfig::eval_define(const std::size_t report_step,
-                                UDQState&         udq_state,
+                                const UDQState&   udq_state,
                                 UDQContext&       context) const
     {
         auto var_type_bit = [](const UDQVarType var_type)
diff --git a/opm/input/eclipse/Schedule/UDQ/UDQConfig.hpp b/opm/input/eclipse/Schedule/UDQ/UDQConfig.hpp
index 87d11eb03cc..28e8fef5959 100644
--- a/opm/input/eclipse/Schedule/UDQ/UDQConfig.hpp
+++ b/opm/input/eclipse/Schedule/UDQ/UDQConfig.hpp
@@ -196,7 +196,7 @@ namespace Opm {
                          UDQContext&     context) const;
 
         void eval_define(std::size_t report_step,
-                         UDQState&   udq_state,
+                         const UDQState& udq_state,
                          UDQContext& context) const;
 
         void add_named_assign(const std::string&              quantity,
diff --git a/opm/io/eclipse/SummaryNode.cpp b/opm/io/eclipse/SummaryNode.cpp
index 13e72915d34..f8372d2dd56 100644
--- a/opm/io/eclipse/SummaryNode.cpp
+++ b/opm/io/eclipse/SummaryNode.cpp
@@ -151,7 +151,7 @@ std::string Opm::EclIO::SummaryNode::unique_key(number_renderer render_number) c
     if (auto opt = display_number(render_number))
         key_parts.emplace_back(opt.value());
 
-    auto compose_key = [](std::string& key, const std::string& key_part) -> std::string {
+    auto compose_key = [](const std::string& key, const std::string& key_part) -> std::string {
         constexpr auto delimiter { ':' } ;
         return key.empty() ? key_part : key + delimiter + key_part;
     };
diff --git a/opm/output/eclipse/AggregateActionxData.hpp b/opm/output/eclipse/AggregateActionxData.hpp
index ba2fca8bbc1..20e5bbcb809 100644
--- a/opm/output/eclipse/AggregateActionxData.hpp
+++ b/opm/output/eclipse/AggregateActionxData.hpp
@@ -25,9 +25,7 @@
 #include <opm/io/eclipse/PaddedOutputString.hpp>
 
 #include <cstddef>
-#include <string>
 #include <vector>
-#include <map>
 
 namespace Opm {
     class Actdims;
diff --git a/opm/output/eclipse/AggregateNetworkData.cpp b/opm/output/eclipse/AggregateNetworkData.cpp
index 00f5b099a2f..0f20d5aa64a 100644
--- a/opm/output/eclipse/AggregateNetworkData.cpp
+++ b/opm/output/eclipse/AggregateNetworkData.cpp
@@ -120,7 +120,7 @@ std::optional<int> findInVector(const std::vector<T>  & vecOfElements, const T
     return (it != vecOfElements.end()) ? std::optional<int>{std::distance(vecOfElements.begin(), it)} : std::nullopt;
 }
 
-int next_branch(int node_no, std::vector<int>& inlets, std::vector<int>& outlets)
+int next_branch(int node_no, const std::vector<int>& inlets, const std::vector<int>& outlets)
 {
     int nxt_br = 0;
     auto res_inlets = findInVector<int>(inlets, node_no);
diff --git a/opm/output/eclipse/EclipseGridInspector.cpp b/opm/output/eclipse/EclipseGridInspector.cpp
index b667cd0a51d..9835abd642f 100644
--- a/opm/output/eclipse/EclipseGridInspector.cpp
+++ b/opm/output/eclipse/EclipseGridInspector.cpp
@@ -225,31 +225,31 @@ double EclipseGridInspector::cellVolumeVerticalPillars(int i, int j, int k) cons
     // Computing the base area as half the 2d cross product of the diagonals.
     int numxpill = logical_gridsize_[0] + 1;
     int pix = i + j*numxpill;
-    double px[4] = { pillc[6*pix],
-		     pillc[6*(pix + 1)],
-		     pillc[6*(pix + numxpill)],
-		     pillc[6*(pix + numxpill + 1)] };
-    double py[4] = { pillc[6*pix + 1],
-		     pillc[6*(pix + 1) + 1],
-		     pillc[6*(pix + numxpill) + 1],
-		     pillc[6*(pix + numxpill + 1) + 1] };
-    double diag1[2] = { px[3] - px[0], py[3] - py[0] };
-    double diag2[2] = { px[2] - px[1], py[2] - py[1] };
-    double area = 0.5*(diag1[0]*diag2[1] - diag1[1]*diag2[0]);
+    const double px[4] = {pillc[6 * pix],
+                          pillc[6 * (pix + 1)],
+                          pillc[6 * (pix + numxpill)],
+                          pillc[6 * (pix + numxpill + 1)]};
+    const double py[4] = {pillc[6 * pix + 1],
+                          pillc[6 * (pix + 1) + 1],
+                          pillc[6 * (pix + numxpill) + 1],
+                          pillc[6 * (pix + numxpill + 1) + 1]};
+    const double diag1[2] = {px[3] - px[0], py[3] - py[0]};
+    const double diag2[2] = {px[2] - px[1], py[2] - py[1]};
+    const double area = 0.5*(diag1[0] * diag2[1] - diag1[1] * diag2[0]);
 
     // Computing the average of the z-differences along each pillar.
-    int delta[3] = { 1,
-		     2*logical_gridsize_[0],
-		     4*logical_gridsize_[0]*logical_gridsize_[1] };
+    const int delta[3] = {1,
+                          2 * logical_gridsize_[0],
+                          4 * logical_gridsize_[0] * logical_gridsize_[1]};
     int ix = 2*(i*delta[0] + j*delta[1] + k*delta[2]);
-    double cellz[8] = { z[ix], z[ix + delta[0]],
-			z[ix + delta[1]], z[ix + delta[1] + delta[0]],
-			z[ix + delta[2]], z[ix + delta[2] + delta[0]],
-			z[ix + delta[2] + delta[1]], z[ix + delta[2] + delta[1] + delta[0]] };
-    double diffz[4] = { cellz[4] - cellz[0],
-			cellz[5] - cellz[1],
-			cellz[6] - cellz[2],
-			cellz[7] - cellz[3] };
+    const double cellz[8] = {z[ix], z[ix + delta[0]],
+                             z[ix + delta[1]], z[ix + delta[1] + delta[0]],
+                             z[ix + delta[2]], z[ix + delta[2] + delta[0]],
+                             z[ix + delta[2] + delta[1]], z[ix + delta[2] + delta[1] + delta[0]]};
+    const double diffz[4] = {cellz[4] - cellz[0],
+                             cellz[5] - cellz[1],
+                             cellz[6] - cellz[2],
+                             cellz[7] - cellz[3]};
     double averzdiff = 0.25*std::accumulate(diffz, diffz + 4, 0.0);
     return averzdiff*area;
 }
diff --git a/opm/output/eclipse/report/WELSPECS.cpp b/opm/output/eclipse/report/WELSPECS.cpp
index 6ad5b6534f0..91449c6a072 100644
--- a/opm/output/eclipse/report/WELSPECS.cpp
+++ b/opm/output/eclipse/report/WELSPECS.cpp
@@ -435,7 +435,7 @@ namespace {
         std::string grid_block(const context&, std::size_t, std::size_t) const {
             const std::array<int,3> ijk { connection.getI() + 1, connection.getJ() + 1, connection.getK() + 1 } ;
 
-            auto compose_coordinates { [](std::string& out, int in) -> std::string {
+            auto compose_coordinates { [](const std::string& out, const int in) -> std::string {
                 constexpr auto delimiter { ',' } ;
                 std::string coordinate_part { std::to_string(in) } ;
                 right_align(coordinate_part, 3);
diff --git a/test_util/EclFilesComparator.cpp b/test_util/EclFilesComparator.cpp
index 04d6e4127bb..ba57582b0d2 100644
--- a/test_util/EclFilesComparator.cpp
+++ b/test_util/EclFilesComparator.cpp
@@ -26,9 +26,7 @@
 #include <algorithm>
 #include <cmath>
 #include <iostream>
-#include <iomanip>
 #include <numeric>
-#include <set>
 #include <type_traits>
 #include <vector>
 
@@ -46,7 +44,14 @@
 using Opm::EclIO::EGrid;
 
 template <typename T>
-void ECLFilesComparator::printValuesForCell(const std::string& keyword, const std::string& reference, size_t kw_size, size_t cell, EGrid *grid, const T& value1, const T& value2) const {
+void ECLFilesComparator::
+printValuesForCell(const std::string& keyword,
+                   const std::string& reference,
+                   size_t kw_size, size_t cell,
+                   const EGrid *grid,
+                   const T& value1,
+                   const T& value2) const
+{
     if (grid) {
         int nActive = grid->activeCells();
         int nTot = grid->totalNumberOfCells();
@@ -85,15 +90,24 @@ void ECLFilesComparator::printValuesForCell(const std::string& keyword, const st
               << "(first value, second value) = (" << value1 << ", " << value2 << ")\n\n";
 }
 
-template void ECLFilesComparator::printValuesForCell<bool>       (const std::string& keyword, const std::string& reference, size_t kw_size, size_t cell, EGrid *grid, const bool&        value1, const bool&        value2) const;
-template void ECLFilesComparator::printValuesForCell<int>        (const std::string& keyword, const std::string& reference, size_t kw_size, size_t cell, EGrid *grid, const int&         value1, const int&         value2) const;
-template void ECLFilesComparator::printValuesForCell<double>     (const std::string& keyword, const std::string& reference, size_t kw_size, size_t cell, EGrid *grid, const double&      value1, const double&      value2) const;
-template void ECLFilesComparator::printValuesForCell<std::string>(const std::string& keyword, const std::string& reference, size_t kw_size, size_t cell, EGrid *grid, const std::string& value1, const std::string& value2) const;
+#define INSTANTIATE_PRINTCELL(T) \
+    template void ECLFilesComparator::printValuesForCell(const std::string&, \
+                                                         const std::string&, \
+                                                         size_t, size_t,     \
+                                                         const EGrid *grid,  \
+                                                         const T&,           \
+                                                         const T&) const;
+
 // Hack to work around case where std::vector<bool>::const_reference is not a bool. If it is we will initialize printValuesForCell<char> otherwise printValuesForCell<std::vector<bool>::const_reference>
 using boolConstReference = typename std::vector<bool>::const_reference;
 using boolTypeHelper = typename std::remove_const<typename std::remove_reference<boolConstReference>::type>::type;
 using boolType = typename std::conditional<std::is_same<boolTypeHelper, bool>::value, char, boolTypeHelper>::type;
-template void ECLFilesComparator::printValuesForCell<boolType>       (const std::string& keyword, const std::string& reference, size_t kw_size, size_t cell, EGrid *grid, const boolType& value1, const boolType& value2) const;
+
+INSTANTIATE_PRINTCELL(bool)
+INSTANTIATE_PRINTCELL(int)
+INSTANTIATE_PRINTCELL(double)
+INSTANTIATE_PRINTCELL(std::string)
+INSTANTIATE_PRINTCELL(boolType)
 
 ECLFilesComparator::ECLFilesComparator(const std::string& basename1,
                                        const std::string& basename2,
diff --git a/test_util/EclFilesComparator.hpp b/test_util/EclFilesComparator.hpp
index 193fc98dada..6fd54aafd99 100644
--- a/test_util/EclFilesComparator.hpp
+++ b/test_util/EclFilesComparator.hpp
@@ -81,7 +81,7 @@ class ECLFilesComparator {
                             const std::string& reference,
                             size_t kw_size,
                             size_t cell,
-                            Opm::EclIO::EGrid *grid,
+                            const Opm::EclIO::EGrid *grid,
                             const T& value1,
                             const T& value2) const;
 
diff --git a/test_util/convertECL.cpp b/test_util/convertECL.cpp
index 6b4b719ec02..da90cf1279b 100644
--- a/test_util/convertECL.cpp
+++ b/test_util/convertECL.cpp
@@ -84,14 +84,17 @@ void writeArray(std::string name, eclArrType arrType, T& file1, int index, int r
 }
 
 
-void writeC0nnArray(std::string name, int elementSize, EclFile& file1, int index, EclOutput& outFile)
+void writeC0nnArray(const std::string& name, int elementSize, EclFile& file1, int index, EclOutput& outFile)
 {
     auto vect = file1.get<std::string>(index);
     outFile.write(name, vect, elementSize);
 }
 
 
-void writeArrayList(std::vector<EclEntry>& arrayList, std::vector<int>& elementSizeList, EclFile file1, EclOutput& outFile) {
+void writeArrayList(std::vector<EclEntry>& arrayList,
+                    const std::vector<int>& elementSizeList,
+                    EclFile& file1, EclOutput& outFile)
+{
 
     for (size_t index = 0; index < arrayList.size(); index++) {
         std::string name = std::get<0>(arrayList[index]);
diff --git a/tests/parser/ScheduleTests.cpp b/tests/parser/ScheduleTests.cpp
index 87581b897de..0a97d052a62 100644
--- a/tests/parser/ScheduleTests.cpp
+++ b/tests/parser/ScheduleTests.cpp
@@ -5290,7 +5290,8 @@ bool compare_dates(const time_point& t, int year, int month, int day) {
     return t == TimeService::from_time_t( asTimeT( TimeStampUTC(year, month, day)));
 }
 
-bool compare_dates(const time_point& t, std::array<int, 3>& ymd) {
+bool compare_dates(const time_point& t, const std::array<int, 3>& ymd)
+{
     return compare_dates(t, ymd[0], ymd[1], ymd[2]);
 }
 
diff --git a/tests/test_EclIO.cpp b/tests/test_EclIO.cpp
index 17a1481ba9c..2db89b89ffd 100644
--- a/tests/test_EclIO.cpp
+++ b/tests/test_EclIO.cpp
@@ -75,7 +75,9 @@ bool operator==(const std::vector<T> & t1, const std::vector<T> & t2)
     return std::equal(t1.begin(), t1.end(), t2.begin(), t2.end());
 }
 
-void write_header(std::ofstream& ofileH, std::string& arrName, int size, std::string arrtype){
+void write_header(std::ofstream& ofileH, const std::string& arrName,
+                  int size, const std::string& arrtype)
+{
 
     int bhead = flipEndianInt(16);
     int fsize = flipEndianInt(size);