Fix clang compilation issues and warnings

src/scheme/DiscreteFunctionP0.hpp

@@ -19,6 +19,9 @@ class DiscreteFunctionP0 : public IDiscreteFunction
 
   static constexpr HandledItemDataType handled_data_type = HandledItemDataType::value;
 
+  friend class DiscreteFunctionP0<Dimension, std::add_const_t<DataType>>;
+  friend class DiscreteFunctionP0<Dimension, std::remove_const_t<DataType>>;
+
  private:
   std::shared_ptr<const MeshType> m_mesh;
   CellValue<DataType> m_cell_values;

src/scheme/DiscreteFunctionP0Vector.hpp

@@ -22,6 +22,9 @@ class DiscreteFunctionP0Vector : public IDiscreteFunction
 
   static constexpr HandledItemDataType handled_data_type = HandledItemDataType::vector;
 
+  friend class DiscreteFunctionP0Vector<Dimension, std::add_const_t<DataType>>;
+  friend class DiscreteFunctionP0Vector<Dimension, std::remove_const_t<DataType>>;
+
   static_assert(std::is_arithmetic_v<DataType>, "DiscreteFunctionP0Vector are only defined for arithmetic data type");
 
  private:

tests/test_DiscreteFunctionDescriptorP0.cpp

@@ -19,7 +19,8 @@ TEST_CASE("DiscreteFunctionDescriptorP0", "[scheme]")
       REQUIRE(descriptor_copy.type() == DiscreteFunctionType::P0);
     }
 
-    DiscreteFunctionDescriptorP0 descriptor_move{std::move(DiscreteFunctionDescriptorP0{})};
+    DiscreteFunctionDescriptorP0 temp;
+    DiscreteFunctionDescriptorP0 descriptor_move{std::move(temp)};
     REQUIRE(descriptor_move.type() == DiscreteFunctionType::P0);
   }
 }

tests/test_DiscreteFunctionDescriptorP0Vector.cpp

@@ -19,7 +19,8 @@ TEST_CASE("DiscreteFunctionDescriptorP0Vector", "[scheme]")
       REQUIRE(descriptor_copy.type() == DiscreteFunctionType::P0Vector);
     }
 
-    DiscreteFunctionDescriptorP0Vector descriptor_move{std::move(DiscreteFunctionDescriptorP0Vector{})};
+    DiscreteFunctionDescriptorP0Vector temp;
+    DiscreteFunctionDescriptorP0Vector descriptor_move{std::move(temp)};
     REQUIRE(descriptor_move.type() == DiscreteFunctionType::P0Vector);
   }
 }