From e974b9d43d4d6bbaeb5b648c8ba2cb0d899b79b8 Mon Sep 17 00:00:00 2001
From: Christos Houtouridis <houtouridis.ch@gmail.com>
Date: Sun, 13 Oct 2019 19:52:54 +0300
Subject: [PATCH] meta: Detection idiom (part 1) from Walter Brown added

---
 include/utl/meta/detection.h | 112 ++++++++++++++++++++++++++++++++++-
 include/utl/meta/invoke.h    |   2 +-
 include/utl/meta/meta.h      |   3 +-
 include/utl/meta/typelist.h  |   2 +-
 test/tests/Tmeta.cpp         |   4 +-
 5 files changed, 116 insertions(+), 7 deletions(-)

diff --git a/include/utl/meta/detection.h b/include/utl/meta/detection.h
index 0ee292c..4135b5f 100644
--- a/include/utl/meta/detection.h
+++ b/include/utl/meta/detection.h
@@ -55,7 +55,117 @@ namespace meta {
       template <typename...> using void_t = void;
    #endif
    //! @}
+
+   /*!
+    * Not a type to use in detected idiom
+    */
+   struct nat_ {
+      nat_()  = delete;
+      ~nat_() = delete;
+      nat_(nat_ const&)         = delete;
+      void operator = (nat_ const&) = delete;
+   };
+
+   //! Detector for detection idiom
+   //! @{
+   namespace detail {
+      template <typename Default,
+                typename AlwaysVoid,
+                template<typename...> class Op, typename... Args>
+      struct detector {
+         using detected = false_;
+         using type = Default;
+      };
+
+      template <typename Default,
+                template<typename...> class Op, typename... Args>
+      struct detector <Default, void_t<Op<Args...>>, Op, Args...> {
+         using detected = true_;
+         using type = Op<Args...>;
+      };
+
+      //! helper for detected_or_t
+      template <typename Default,
+                template<typename...> class Op, typename... Args>
+      using detected_or = detector<Default, void, Op, Args...>;
+   } // namespace detail
+   //! @}
+
+   /*!
+    * detection interface
+    */
+   //! @{
+
+   /*!
+    *  Checks if Op<Args...> is a valid expression without evaluating it.
+    *
+    *  \param  Op       a meta-callback function to pass Args...
+    *  \param  Args...  types to pass to Op for checking
+    *  \return status of the operation [bool_]
+    *    \arg  true_    if Op<Args...> is valid expression
+    *    \arg  false_   if Op<Args...> is not valid
+    *
+    * \example
+    * \code
+    *    // archetypal alias for a copy assignment operation
+    *    template< class T >
+    *    using copy_assign_t = decltype( declval<T&>() = declval<T const &>() );
+    *
+    *    template< class T >
+    *    using is_copy_assignable = is_detected< T, copy_assign_t >;
+    * \endcode
+    */
+   template <template<typename...> class Op, typename... Args>
+   using is_detected = typename detail::detector<nat_, void, Op, Args...>::detected;
+
+   //! Detection predicate
+   template< template<typename...> class Op, typename... Args>
+   constexpr bool is_detected_v = is_detected<Op, Args...>::value;
+   //! @}
+
+   /*!
+    * Detection idiom toolkit
+    */
+   //! @{
+
+   //! evaluates to evaluation of Op<Args...> if it's valid and to nat_ if not
+   template <template<typename...> class Op, typename... Args>
+   using detected_t = eval <
+      detail::detector<nat_, void, Op, Args...>
+   >;
+
+   //! evaluates to evaluation of Op<Args...> if it's valid and to \p Default if not
+   template <typename Default,
+             template<typename...> class Op, typename... Args>
+   using detected_or_t = eval <
+      detail::detected_or<Default, Op, Args...>
+   >;
+
+   //! evaluates to true_ if evaluation of Op<Args...> is \p Expected and to false_ if not
+   template <typename Expected,
+             template<typename...> class Op, typename... Args >
+   using is_detected_exact = same_<Expected, detected_t<Op, Args...> >;
+
+   //! evaluates to true if evaluation of Op<Args...> is \p Expected and to false if not
+   template <typename Expected,
+             template<typename...> class Op, typename... Args >
+   constexpr bool is_detected_exact_v = is_detected_exact< Expected, Op, Args...>::value;
+
+   //! evaluates to true_ if evaluation of Op<Args...> is convertible to \p To
+   //! and to false_ if not
+   template <typename To,
+             template<typename...> class Op, typename... Args >
+   using is_detected_convertible = std::is_convertible< detected_t<Op, Args...>, To >;
+
+   //! evaluates to true if evaluation of Op<Args...> is convertible to \p To
+   //! and to false if not
+   template <typename To,
+             template<typename...> class Op, typename... Args >
+   constexpr bool is_detected_convertible_v =
+      is_detected_convertible<To, Op, Args...>::value;
+   //! @}
+
 }}
 //!@}
 
-#endif /* __utl_meta_void_h__ */
+#endif /* __utl_meta_detection_h__ */
diff --git a/include/utl/meta/invoke.h b/include/utl/meta/invoke.h
index 020c4f1..586d18d 100644
--- a/include/utl/meta/invoke.h
+++ b/include/utl/meta/invoke.h
@@ -22,7 +22,7 @@
 
 #include <utl/core/impl.h>
 #include <utl/meta/integral.h>
-#include <utl/meta/void.h>
+#include <utl/meta/detection.h>
 #include <utl/meta/selection.h>
 #include <utl/meta/logical.h>
 
diff --git a/include/utl/meta/meta.h b/include/utl/meta/meta.h
index 27218a9..aedd778 100644
--- a/include/utl/meta/meta.h
+++ b/include/utl/meta/meta.h
@@ -21,14 +21,13 @@
 #ifndef __utl_meta_meta_h__
 #define __utl_meta_meta_h__
 
-#include <utl/meta/void.h>
 #include <utl/meta/integral.h>
 #include <utl/meta/typelist.h>
 #include <utl/meta/selection.h>
 #include <utl/meta/logical.h>
 #include <utl/meta/operations.h>
 #include <utl/meta/useif.h>
-
+#include <utl/meta/detection.h>
 #include <utl/meta/invoke.h>
 
 #endif /* __utl_meta_meta_h__ */
diff --git a/include/utl/meta/typelist.h b/include/utl/meta/typelist.h
index cb66d46..23d39b4 100644
--- a/include/utl/meta/typelist.h
+++ b/include/utl/meta/typelist.h
@@ -23,7 +23,7 @@
 #include <utl/core/impl.h>
 #include <utl/meta/integral.h>
 #include <utl/meta/idx_sequence.h>
-#include <utl/meta/void.h>
+#include <utl/meta/detection.h>
 #include <utl/meta/invoke.h>
 #include <utl/meta/sfinae.h>
 /*!
diff --git a/test/tests/Tmeta.cpp b/test/tests/Tmeta.cpp
index a05a1cb..7b252a7 100644
--- a/test/tests/Tmeta.cpp
+++ b/test/tests/Tmeta.cpp
@@ -160,9 +160,9 @@ namespace test_meta {
    }
 
    /*
-    * Test void
+    * Test void_t
     */
-   TEST(Tmeta_void, VoidType) {
+   TEST(Tmeta, VoidType) {
       struct Foo {};
       struct Bar {};
       EXPECT_EQ(true, (std::is_same<void, void_t<int, long, Foo, Bar>>()));