Fix build on VS 2022 17.3.1

Author: yuyoyuppe

include/flags/flags.hpp

@@ -85,8 +85,7 @@ template <class E> class flags {
 
 
   template <class FwIter>
-  flags(FwIter b, FwIter e,
-        typename convertible<decltype(*b)>::type = nullptr)
+  flags(FwIter b, FwIter e, typename convertible<decltype(*std::declval<FwIter>())>::type = nullptr)
   noexcept(noexcept(std::declval<flags>().insert(std::declval<FwIter>(),
                                                  std::declval<FwIter>())))
   : val_(0)