move conjunction/disjunction to the top of detail

src/json.hpp.re2c

@@ -168,6 +168,25 @@ using is_unscoped_enum =
 
 namespace detail
 {
+// Implementation of 2 C++17 constructs: conjunction, negation.
+// This is needed to avoid evaluating all the traits in a condition
+//
+// For example: not std::is_same<void, T>::value and has_value_type<T>::value
+// will not compile when T = void (on MSVC at least)
+// Whereas conjunction<negation<std::is_same<void, T>>, has_value_type<T>>::value
+// will stop evaluating if negation<...>::value == false
+//
+// Please note that those constructs must be used with caution, since symbols can
+// become very long quickly (which can slow down compilation and cause MSVC internal compiler errors)
+// Only use it when you have too (see example ahead)
+template <class...> struct conjunction : std::true_type {};
+template <class B1> struct conjunction<B1> : B1 {};
+template <class B1, class... Bn>
+struct conjunction<B1, Bn...>
+: std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
+
+template <class B> struct negation : std::integral_constant < bool, !B::value > {};
+
 template <typename Json> std::string type_name(const  Json& j)
 {
     switch (j.m_type)
@@ -317,25 +336,6 @@ struct external_constructor<value_t::object>
     }
 };
 
-// Implementation of 2 C++17 constructs: conjunction, negation.
-// This is needed to avoid evaluating all the traits in a condition
-//
-// For example: not std::is_same<void, T>::value and has_value_type<T>::value
-// will not compile when T = void (on MSVC at least)
-// Whereas conjunction<negation<std::is_same<void, T>>, has_value_type<T>>::value
-// will stop evaluating if negation<...>::value == false
-//
-// Please note that those constructs must be used with caution, since symbols can
-// become very long quickly (which can slow down compilation and cause MSVC internal compiler errors)
-// Only use it when you have too (see example ahead)
-template <class...> struct conjunction : std::true_type {};
-template <class B1> struct conjunction<B1> : B1 {};
-template <class B1, class... Bn>
-struct conjunction<B1, Bn...>
-: std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
-
-template <class B> struct negation : std::integral_constant < bool, !B::value > {};
-
 /*!
 @brief Helper to determine whether there's a key_type for T.