Remove && from the template parameter given to declval

It seems redundant as declval already returns a rvalue reference

src/optional.hh

@@ -95,7 +95,7 @@ public:
     template<typename U> using DecayOptional = typename DecayOptionalImpl<U>::Type;
 
     template<typename F>
-    auto map(F f) -> Optional<DecayOptional<decltype(f(std::declval<T&&>()))>>
+    auto map(F f) -> Optional<DecayOptional<decltype(f(std::declval<T>()))>>
     {
         if (not m_valid)
             return {};