Some refactoring and const correctness fixes.

src/libslic3r/Config.hpp

@@ -289,13 +289,13 @@ public:
             throw std::runtime_error("ConfigOptionVector::set_at(): Assigning an incompatible type");
     }
 
-    T& get_at(size_t i)
+    const T& get_at(size_t i) const
     {
         assert(! this->values.empty());
         return (i < this->values.size()) ? this->values[i] : this->values.front();
     }
 
-    const T& get_at(size_t i) const { return const_cast<ConfigOptionVector<T>*>(this)->get_at(i); }
+    T& get_at(size_t i) { return const_cast<T&>(std::as_const(*this).get_at(i)); }
 
     // Resize this vector by duplicating the /*last*/first value.
     // If the current vector is empty, the default value is used instead.

src/libslic3r/GCode/ToolOrdering.cpp

@@ -394,7 +394,21 @@ void ToolOrdering::collect_extruder_statistics(bool prime_multi_material)
     }
 }
 
-
+const LayerTools& ToolOrdering::tools_for_layer(coordf_t print_z) const
+{
+    auto it_layer_tools = std::lower_bound(m_layer_tools.begin(), m_layer_tools.end(), LayerTools(print_z - EPSILON));
+    assert(it_layer_tools != m_layer_tools.end());
+    coordf_t dist_min = std::abs(it_layer_tools->print_z - print_z);
+    for (++ it_layer_tools; it_layer_tools != m_layer_tools.end(); ++ it_layer_tools) {
+        coordf_t d = std::abs(it_layer_tools->print_z - print_z);
+        if (d >= dist_min)
+            break;
+        dist_min = d;
+    }
+    -- it_layer_tools;
+    assert(dist_min < EPSILON);
+    return *it_layer_tools;
+}
 
 // This function is called from Print::mark_wiping_extrusions and sets extruder this entity should be printed with (-1 .. as usual)
 void WipingExtrusions::set_extruder_override(const ExtrusionEntity* entity, unsigned int copy_id, int extruder, unsigned int num_of_copies)

src/libslic3r/GCode/ToolOrdering.hpp

@@ -5,6 +5,8 @@
 
 #include "../libslic3r.h"
 
+#include <utility>
+
 namespace Slic3r {
 
 class Print;
@@ -123,25 +125,9 @@ public:
     // For a multi-material print, the printing extruders are ordered in the order they shall be primed.
     const std::vector<unsigned int>& all_extruders() const { return m_all_printing_extruders; }
 
-    template<class Self> static auto tools_for_layer(Self& self, coordf_t print_z) -> decltype (*self.m_layer_tools.begin())
-    {
-        auto it_layer_tools = std::lower_bound(self.m_layer_tools.begin(), self.m_layer_tools.end(), LayerTools(print_z - EPSILON));
-        assert(it_layer_tools != self.m_layer_tools.end());
-        coordf_t dist_min = std::abs(it_layer_tools->print_z - print_z);
-        for (++ it_layer_tools; it_layer_tools != self.m_layer_tools.end(); ++it_layer_tools) {
-            coordf_t d = std::abs(it_layer_tools->print_z - print_z);
-            if (d >= dist_min)
-                break;
-            dist_min = d;
-        }
-        -- it_layer_tools;
-        assert(dist_min < EPSILON);
-        return *it_layer_tools;
-    }
-
     // Find LayerTools with the closest print_z.
-    LayerTools&			tools_for_layer(coordf_t print_z) { return tools_for_layer(*this, print_z); }
+    const LayerTools&	tools_for_layer(coordf_t print_z) const;
-    const LayerTools&	tools_for_layer(coordf_t print_z) const { return tools_for_layer(*this, print_z); }
+    LayerTools&			tools_for_layer(coordf_t print_z) { return const_cast<LayerTools&>(std::as_const(*this).tools_for_layer(print_z)); }
 
     const LayerTools&   front()       const { return m_layer_tools.front(); }
     const LayerTools&   back()        const { return m_layer_tools.back(); }