Added remove_cvref

Summary:
std::remove_cvref is like std::decay, but without the function to
pointer and array to pointer decays

Backport of http://en.cppreference.com/w/cpp/types/remove_cvref

Reviewed By: yfeldblum

Differential Revision: D6639513

fbshipit-source-id: 2a5e252678aacc09acf6ce4565872e7efb9b48f3

folly/Traits.h

@@ -155,6 +155,18 @@ namespace folly {
 template <typename T>
 using _t = typename T::type;
 
+/**
+ * A type trait to remove all const volatile and reference qualifiers on a
+ * type T
+ */
+template <typename T>
+struct remove_cvref {
+  using type =
+      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
+};
+template <typename T>
+using remove_cvref_t = typename remove_cvref<T>::type;
+
 /**
  *  type_t
  *

folly/test/TraitsTest.cpp

@@ -265,3 +265,49 @@ TEST(Traits, type_t) {
       (::std::is_constructible<::container<std::string>, some_tag, float>::
            value));
 }
+
+TEST(Traits, remove_cvref) {
+  using folly::remove_cvref;
+  using folly::remove_cvref_t;
+
+  // test all possible c-ref qualifiers without volatile
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int&&>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int&&>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int&>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int&>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int const>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int const>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int const&>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int const&>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int const&&>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int const&&>::type, int>::value));
+
+  // test all possible c-ref qualifiers with volatile
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int volatile>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int volatile>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int volatile&&>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int volatile&&>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int volatile&>, int>::value));
+  EXPECT_TRUE((std::is_same<remove_cvref<int volatile&>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int volatile const>, int>::value));
+  EXPECT_TRUE(
+      (std::is_same<remove_cvref<int volatile const>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int volatile const&>, int>::value));
+  EXPECT_TRUE(
+      (std::is_same<remove_cvref<int volatile const&>::type, int>::value));
+
+  EXPECT_TRUE((std::is_same<remove_cvref_t<int volatile const&&>, int>::value));
+  EXPECT_TRUE(
+      (std::is_same<remove_cvref<int volatile const&&>::type, int>::value));
+}