some function trait aliases and variables

Reviewed By: Orvid

Differential Revision: D74496242

fbshipit-source-id: 88ac4378cfd672acca36a12433ff7cbbcd15f2f0

Author: yfeldblum

folly/ExceptionWrapper-inl.h

@@ -25,15 +25,12 @@ struct exception_wrapper::with_exception_from_fn_ {
   };
   struct impl_arg_ {
     template <typename F>
-    using apply = typename function_traits<F>::template argument<0>;
+    using apply = function_arguments_element_t<0, F>;
   };
   struct impl_bye_;
-  template <
-      typename Sig,
-      typename Traits = function_traits<Sig>,
-      std::size_t NArgs = Traits::template arguments<type_pack_size_t>::value>
+  template <typename Sig, std::size_t NArgs = function_arguments_size_v<Sig>>
   using impl_ = conditional_t<
-      Traits::is_variadic,
+      function_is_variadic_v<Sig>,
       impl_var_,
       conditional_t<NArgs == 1, impl_arg_, impl_bye_>>;
 

folly/Traits.h

@@ -200,6 +200,18 @@ struct member_pointer_traits<M O::*> {
   using object_type = O;
 };
 
+/// member_pointer_member_t
+///
+/// The member-type of a pointer-to-member type.
+template <typename P>
+using member_pointer_member_t = typename member_pointer_traits<P>::member_type;
+
+/// member_pointer_object_t
+///
+/// The object-type of a pointer-to-member type.
+template <typename P>
+using member_pointer_object_t = typename member_pointer_traits<P>::object_type;
+
 namespace detail {
 
 struct is_constexpr_default_constructible_ {

folly/channels/detail/MultiplexerTraits.h

@@ -25,32 +25,30 @@ namespace detail {
 
 template <typename MultiplexerType>
 struct MultiplexerTraits {
-  using OnNewSubscriptionPtr = decltype(&MultiplexerType::onNewSubscription);
-  using OnNewSubscriptionTraits = function_traits<
-      typename member_pointer_traits<OnNewSubscriptionPtr>::member_type>;
+  using OnNewSubscriptionSig =
+      member_pointer_member_t<decltype(&MultiplexerType::onNewSubscription)>;
 
-  using OnInputValuePtr = decltype(&MultiplexerType::onInputValue);
-  using OnInputValueTraits = function_traits<
-      typename member_pointer_traits<OnInputValuePtr>::member_type>;
+  using OnInputValueSig =
+      member_pointer_member_t<decltype(&MultiplexerType::onInputValue)>;
 
   // First parameter type of MultiplexerType::onNewSubscription
-  using KeyType = typename OnNewSubscriptionTraits::template argument<0>;
+  using KeyType = function_arguments_element_t<0, OnNewSubscriptionSig>;
 
   // Second parameter type for MultiplexerType::onNewSubscription
   using KeyContextType =
-      std::decay_t<typename OnNewSubscriptionTraits::template argument<1>>;
+      std::decay_t<function_arguments_element_t<1, OnNewSubscriptionSig>>;
 
   // Third parameter type for MultiplexerType::onNewSubscription
   using SubscriptionArgType =
-      typename OnNewSubscriptionTraits::template argument<2>;
+      function_arguments_element_t<2, OnNewSubscriptionSig>;
 
   // First parameter value type of MultiplexerType::onInputValue
   using InputValueType =
-      typename OnInputValueTraits::template argument<0>::element_type;
+      typename function_arguments_element_t<0, OnInputValueSig>::element_type;
 
   // Element type of the returned vector from MultiplexerType::onNewSubscription
   using OutputValueType =
-      typename OnNewSubscriptionTraits::result::StorageType::value_type;
+      typename function_result_t<OnNewSubscriptionSig>::StorageType::value_type;
 };
 } // namespace detail
 } // namespace channels

folly/functional/traits.h

@@ -449,6 +449,51 @@ struct function_traits<R(A..., ...) const volatile && noexcept>
 
 //  ----
 
+//  function_result_t
+//
+//  The result type of the given function type.
+template <typename F>
+using function_result_t = typename function_traits<F>::result;
+
+//  function_arguments_size_t
+//
+//  The size of the arguments list of the given function type, as an
+//  instantiation of integral_constant.
+template <typename F>
+using function_arguments_size_t =
+    typename function_traits<F>::template arguments<type_pack_size_t>;
+
+//  function_arguments_size_t
+//
+//  The size of the arguments list of the given function type.
+template <typename F>
+constexpr std::size_t function_arguments_size_v =
+    function_arguments_size_t<F>::value;
+
+//  function_arguments_element_t
+//
+//  The type of the argument at the given index of the given function type.
+template <std::size_t Idx, typename F>
+using function_arguments_element_t =
+    typename function_traits<F>::template argument<Idx>;
+
+//  function_is_nothrow_v
+//
+//  True precisely when the given function type is marked noexcept.
+template <typename F>
+constexpr bool function_is_nothrow_v = function_traits<F>::is_nothrow;
+
+//  function_is_variadic_v
+//
+//  True precisely when the given function type is variadic.
+//
+//  Note: C-style variadic, like in printf. Not C++-style variadic-template,
+//  since concrete function types cannot also be function type templates.
+template <typename F>
+constexpr bool function_is_variadic_v = function_traits<F>::is_variadic;
+
+//  ----
+
 namespace detail {
 
 template <bool Nx, bool Var, typename R>