Trait: extend trait is_variant

The trait is populated with getter by index and emplace method.
We will need an ability to get an alternative by index to iterate over
all alternatives and detect all accepted families in decoder. Emplace is
needed to write a value to variant - now the trait does not offer such
an option.

Author: drewdzzz

src/Utils/Traits.hpp

@@ -543,7 +543,9 @@ struct is_variant_h<T, std::void_t<
 	decltype(std::declval<T>().index()),
 	decltype(std::variant_size<T>::value),
 	typename std::variant_alternative<0, T>::type,
-	decltype(tnt::get<std::variant_alternative_t<0, T>>(std::declval<T>()))>>
+	decltype(tnt::get<0>(std::declval<T>())),
+	decltype(tnt::get<std::variant_alternative_t<0, T>>(std::declval<T>())),
+	decltype(std::declval<T>().template emplace<0>(tnt::get<0>(std::declval<T>())))>>
 : std::is_same<size_t, std::decay_t<decltype(std::declval<T>().index())>> {};
 } //namespace details {
 

test/TraitsUnitTest.cpp

@@ -643,6 +643,21 @@ struct CustomVariant {
 	size_t index() const { return m_index; }
 	template <class T> T& get() { return std::get<T&>(std::tie(i, d)); }
 	template <class T> const T& get() const { return std::get<const T&>(std::tie(i, d)); }
+	template <size_t I> auto& get() { return std::get<I>(std::tie(i, d)); }
+	template <size_t I> const auto& get() const { return std::get<I>(std::tie(i, d)); }
+	template <size_t I, class ...Args> void emplace(Args&&... args)
+	{
+		m_index = I;
+		auto &v = std::get<I>(std::tie(i, d));
+		using value_t = std::remove_cv_t<std::remove_reference_t<decltype(v)>>;
+		static_assert(std::is_constructible_v<value_t, Args...>);
+		emplace_impl(v, std::forward<Args>(args)...);
+	}
+private:
+	template <class T, class ...Args> void emplace_impl(T &v, Args&&... args)
+	{
+		v = T(std::forward<Args>(args)...);
+	}
 };
 
 namespace std