Simplify predicate type use

- Renamed first_t to first_of
- Added first_of_t type alias for first_of<...>::type
- changed std::integral_constant<bool, ...> to detail::bool_constant<...>

Author: jagerman

include/pybind11/common.h

@@ -350,18 +350,19 @@ using any_of_t = negation<std::is_same<bools<Predicate<Ts>::value...>,
                                        bools<always_false<Ts>::value...>>>;
 
 // Extracts the first type from the template parameter pack matching the predicate, or void if none match.
-template <template<class> class Predicate, class... Ts> struct first_t;
-template <template<class> class Predicate> struct first_t<Predicate> {
+template <template<class> class Predicate, class... Ts> struct first_of;
+template <template<class> class Predicate> struct first_of<Predicate> {
     using type = void;
 };
 template <template<class> class Predicate, class T, class... Ts>
-struct first_t<Predicate, T, Ts...> {
+struct first_of<Predicate, T, Ts...> {
     using type = typename std::conditional<
         Predicate<T>::value,
         T,
-        typename first_t<Predicate, Ts...>::type
+        typename first_of<Predicate, Ts...>::type
     >::type;
 };
+template <template<class> class Predicate, class... T> using first_of_t = typename first_of<Predicate, T...>::type;
 
 // Counts the number of types in the template parameter pack matching the predicate
 template <template<typename> class Predicate, typename... Ts> struct count_t;

include/pybind11/pybind11.h

@@ -822,20 +822,20 @@ template <typename type_, typename... options>
 class class_ : public detail::generic_type {
     template <typename T> using is_holder = detail::is_holder_type<type_, T>;
     template <typename T> using is_subtype = std::is_base_of<type_, T>;
-    template <typename T> using is_base_class = std::integral_constant<bool, std::is_base_of<T, type_>::value && !std::is_same<T, type_>::value>;
+    template <typename T> using is_base_class = detail::bool_constant<std::is_base_of<T, type_>::value && !std::is_same<T, type_>::value>;
 #if defined(_MSC_VER) // MSVC fails with the below type alias, unable to find the is_holder, is_subtype, etc. predicate types
     template <typename T> struct is_valid_class_option :
-        std::integral_constant<bool, is_holder<T>::value || is_subtype<T>::value || is_base_class<T>::value> {};
+        detail::bool_constant<is_holder<T>::value || is_subtype<T>::value || is_base_class<T>::value> {};
 #else
     template <typename T> using is_valid_class_option =
-        std::integral_constant<bool, is_holder<T>::value || is_subtype<T>::value || is_base_class<T>::value>;
+        detail::bool_constant<is_holder<T>::value || is_subtype<T>::value || is_base_class<T>::value>;
 #endif
 
-    using extracted_holder_t = typename detail::first_t<is_holder, options...>::type;
+    using extracted_holder_t = typename detail::first_of_t<is_holder, options...>;
 
 public:
     using type = type_;
-    using type_alias = typename detail::first_t<is_subtype, options...>::type;
+    using type_alias = detail::first_of_t<is_subtype, options...>;
     constexpr static bool has_alias = !std::is_void<type_alias>::value;
     using holder_type = typename std::conditional<
         std::is_void<extracted_holder_t>::value,