added assignment from value, conversion to database_type and appropriate tests

include/matador/utils/identifier.hpp

@@ -16,6 +16,7 @@
 #include <variant>
 
 namespace matador::utils {
+class value;
 class identifier_serializer {
 public:
   virtual ~identifier_serializer() = default;
@@ -94,7 +95,6 @@ public:
   >;
 
   identifier();
-
   template<typename Type, std::enable_if_t<is_identifier_supported_v<Type>, int> = 0>
   explicit identifier(Type &&id)
   : value_(std::forward<Type>(id)) {
@@ -117,17 +117,20 @@ public:
   }
 
   identifier& operator=(const std::string &value);
-
   identifier& operator=(const char *value);
-
   template<typename Type, std::enable_if_t<is_identifier_supported_v<Type>  && std::is_same_v<std::decay_t<Type>, bool>, int> = 0>
-  identifier &operator=(Type &&value) {
+  identifier& operator=(Type &&value) {
     value_ = std::forward<Type>(value);
     return *this;
   }
 
   ~identifier() = default;
 
+  result<void, error> assign(const value &val);
+
+  static result<identifier, error> from_value(const value &val);
+  [[nodiscard]] database_type to_database_type() const;
+
   bool operator==(const identifier &x) const;
   bool operator!=(const identifier &x) const;
   bool operator<(const identifier &x) const;