Add detection trait utility.

Author: eieio

include/nop/traits/is_detected.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2019 The Native Object Protocols Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef LIBNOP_INCLUDE_NOP_TRAITS_IS_DETECTED_H_
+#define LIBNOP_INCLUDE_NOP_TRAITS_IS_DETECTED_H_
+
+#include <type_traits>
+
+#include <nop/traits/void.h>
+
+namespace nop {
+
+namespace detail {
+
+template <template <typename...> class Trait, typename Expression,
+          typename... Args>
+struct IsDetectedType : std::false_type {};
+
+template <template <typename...> class Trait, typename... Args>
+struct IsDetectedType<Trait, Void<Trait<Args...>>, Args...> : std::true_type {};
+
+}  // namespace detail
+
+// Trait that determines whether the given Trait template is successfully
+// evaluated with the given Args. This makes it possible to express SFINAE
+// detection expressions as templated aliases instead of full structure
+// definitions with partial specializations.
+//
+// For example, a callable detector may be implemented as follows:
+//
+// template <typename Callable, typename... Args>
+// using CallableTest =
+//     decltype(std::declval<Callable>()(std::declval<Args>()...));
+//
+// template <typename Callable, typename... Args>
+// using IsCallable = IsDetected<CallableTest, Callable, Args...>;
+//
+template <template <typename...> class Trait, typename... Args>
+using IsDetected = detail::IsDetectedType<Trait, void, Args...>;
+
+}  // namespace nop
+
+#endif  // LIBNOP_INCLUDE_NOP_TRAITS_IS_DETECTED_H_