Keep detail::negation out of json_view.hpp's enable_ifs

include-what-you-use asks json_view.hpp to include
detail/meta/type_traits.hpp for it, which the single header must not do
(the amalgamation would duplicate json's internals). The two conditions of
value() go through a trait in value.hpp, like the others.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/value.hpp

@@ -12,7 +12,7 @@
 #include <cstdint> // int64_t
 #include <map> // map
 #include <string> // basic_string
-#include <type_traits> // enable_if, is_constructible
+#include <type_traits> // decay, enable_if, is_arithmetic, is_constructible, is_integral, is_same
 #include <unordered_map> // unordered_map
 #include <vector> // vector
 
@@ -109,6 +109,11 @@ struct is_string_key : std::is_constructible<Key, const char*, std::size_t> {};
 template<typename T>
 struct is_non_bool_arithmetic : conjunction<std::is_arithmetic<T>, negation<std::is_same<T, bool>>> {};
 
+/// the default values of value() other than C strings, which have their own
+/// overload (returning string_t)
+template<typename T>
+struct is_non_c_string_default : negation<std::is_same<typename std::decay<T>::type, const char*>> {};
+
 /// whether View::get<T>() converts to T without materialize(); get_to()
 /// assigns these, and passes the others to from_json
 template<typename T, typename BasicJsonType, typename View>