a std::decay_t lookalike

Summary: Appears to be measurably faster to compile under clang as compared to `std::decay_t`.

Reviewed By: Alfus, luciang

Differential Revision: D32946561

fbshipit-source-id: 028c79633095ae9d6dc75d967282b53b32986736

folly/Utility.h

@@ -60,6 +60,23 @@ T declval() noexcept;
 #define FOLLY_DECLVAL(...) ::folly::detail::declval<__VA_ARGS__>()
 #endif
 
+namespace detail {
+template <typename T>
+T decay_(T) noexcept;
+
+//  decay_t
+//
+//  Like std::decay_t but possibly faster to compile.
+//
+//  Marked as detail since this differs from std::decay_t in some respects:
+//  * incomplete decayed types are forbidden
+//  * non-moveable decayed types are forbidden
+//
+//  mimic: std::decay_t, C++14
+template <typename T>
+using decay_t = decltype(detail::decay_(FOLLY_DECLVAL(T &&)));
+} // namespace detail
+
 /**
  *  copy
  *
@@ -103,9 +120,9 @@ T declval() noexcept;
  */
 
 template <typename T>
-constexpr typename std::decay<T>::type copy(T&& value) noexcept(
-    noexcept(typename std::decay<T>::type(std::forward<T>(value)))) {
-  return std::forward<T>(value);
+constexpr detail::decay_t<T> copy(T&& value) noexcept(
+    noexcept(detail::decay_t<T>(static_cast<T&&>(value)))) {
+  return static_cast<T&&>(value);
 }
 
 /**

folly/test/UtilityTest.cpp

@@ -32,6 +32,31 @@ static_assert(std::is_same_v<decltype(FOLLY_DECLVAL(int&)), int&>);
 static_assert(std::is_same_v<decltype(FOLLY_DECLVAL(int&&)), int&&>);
 static_assert(noexcept(FOLLY_DECLVAL(int)));
 
+// Tests for folly::decay_t:
+
+template <typename T>
+using dec = folly::detail::decay_t<T>;
+static_assert(std::is_same_v<int, dec<int>>);
+static_assert(std::is_same_v<int, dec<int&>>);
+static_assert(std::is_same_v<int, dec<int&&>>);
+static_assert(std::is_same_v<int, dec<int const>>);
+static_assert(std::is_same_v<int, dec<int const&&>>);
+static_assert(std::is_same_v<int, dec<int const&>>);
+static_assert(std::is_same_v<int, dec<int volatile>>);
+static_assert(std::is_same_v<int, dec<int volatile&>>);
+static_assert(std::is_same_v<int, dec<int volatile&&>>);
+static_assert(std::is_same_v<int, dec<int const volatile>>);
+static_assert(std::is_same_v<int, dec<int const volatile&>>);
+static_assert(std::is_same_v<int, dec<int const volatile&&>>);
+static_assert(std::is_same_v<int*, dec<int*>>);
+static_assert(std::is_same_v<int*, dec<int[]>>);
+static_assert(std::is_same_v<int*, dec<int[7]>>);
+static_assert(std::is_same_v<int*, dec<int*&>>);
+static_assert(std::is_same_v<int*, dec<int (&)[]>>);
+static_assert(std::is_same_v<int*, dec<int (&)[7]>>);
+static_assert(std::is_same_v<int (*)(), dec<int (*)()>>);
+static_assert(std::is_same_v<int (*)(), dec<int (&)()>>);
+
 TEST_F(UtilityTest, copy) {
   struct MyData {};
   struct Worker {