is_detected

Summary: [Folly] `is_detected`, for a common pattern around `void_t`, backported from Library Fundamentals TS v2.

Reviewed By: Mizuchi

Differential Revision: D23367990

fbshipit-source-id: 3a0e9a5b7eb668f872ce699757e32bb620bb5404

folly/Traits.h

@@ -356,6 +356,28 @@ using type_t = typename traits_detail::type_t_<T, Ts...>::type;
 template <class... Ts>
 using void_t = type_t<void, Ts...>;
 
+namespace detail {
+template <typename Void, template <typename...> class, typename...>
+FOLLY_INLINE_VARIABLE constexpr bool //
+    is_detected_ = false;
+template <template <typename...> class T, typename... A>
+FOLLY_INLINE_VARIABLE constexpr bool //
+    is_detected_<void_t<T<A...>>, T, A...> = true;
+} // namespace detail
+
+//  is_detected_v
+//  is_detected
+//
+//  A trait variable and type to test whether some metafunction from types to
+//  types would succeed or fail in substitution over a given set of arguments.
+//
+//  mimic: std::is_detected, std::is_detected_v, Library Fundamentals TS v2
+template <template <typename...> class T, typename... A>
+FOLLY_INLINE_VARIABLE constexpr bool is_detected_v =
+    detail::is_detected_<void, T, A...>;
+template <template <typename...> class T, typename... A>
+struct is_detected : bool_constant<is_detected_v<T, A...>> {};
+
 template <typename T>
 using aligned_storage_for_t =
     typename std::aligned_storage<sizeof(T), alignof(T)>::type;

folly/test/TraitsTest.cpp

@@ -337,6 +337,21 @@ TEST(Traits, type_t) {
            value));
 }
 
+namespace {
+template <typename T, typename V>
+using detector_find = decltype(std::declval<T>().find(std::declval<V>()));
+}
+
+TEST(Traits, is_detected) {
+  EXPECT_TRUE((folly::is_detected<detector_find, std::string, char>::value));
+  EXPECT_FALSE((folly::is_detected<detector_find, double, char>::value));
+}
+
+TEST(Traits, is_detected_v) {
+  EXPECT_TRUE((folly::is_detected_v<detector_find, std::string, char>));
+  EXPECT_FALSE((folly::is_detected_v<detector_find, double, char>));
+}
+
 TEST(Traits, aligned_storage_for_t) {
   struct alignas(2) Foo {
     char data[4];