Add default constructor for LazyInitalizer

This prevents initialization bug from old versions of gcc (pre 7.2)

Signed-off-by: Nikita Alekseev <n.alekseev2104@gmail.com>

# Conflicts:
#	test/benchmark/CMakeLists.txt
#	test/benchmark/bm_proto_creation.cpp

shared_model/backend/protobuf/common_objects/noncopyable_proto.hpp

@@ -18,7 +18,7 @@ class NonCopyableProto : public Iface {
   using TransportType = Proto;
 
   template <typename Transport>
-  NonCopyableProto(Transport &&ref) : proto_(std::forward<Transport>(ref)){};
+  NonCopyableProto(Transport &&ref) : proto_(std::forward<Transport>(ref)){}
 
   const Proto &getTransport() const {
     return proto_;

shared_model/utils/lazy_initializer.hpp

@@ -35,6 +35,23 @@ namespace shared_model {
       using GeneratorType = std::function<Target()>;
 
      public:
+      /**
+       * Default constructor prevents compilation error in pre 7.2 gcc.
+       *
+       * Short explanation: gcc needs default constructor
+       * when the constructor is inherited (via using Base::Base)
+       * if the inherited class has default initalizers for any of its members.
+       * This can be found in shared_model/backend/protobuf/protobuf/block.hpp
+       * where Lazy fields are initialized via default initializers.
+       *
+       * Note that this does not result in default constructor being called, so
+       * the resulting code is still correct. This is an issue of gcc compiler
+       * which is resolved in 7.2
+       *
+       * For more details: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67054
+       */
+      LazyInitializer();
+
       template <typename T>
       explicit LazyInitializer(T &&generator)
           : generator_(std::forward<T>(generator)) {}