From fa7eb205d5f18b72c1e04a4229d171a15040bdea Mon Sep 17 00:00:00 2001
From: johannst <johannst@users.noreply.github.com>
Date: Fri, 30 Sep 2022 10:58:40 +0000
Subject: deploy: b6f25b16f1f23b0169e8f076e79ee4964b81db99

---
 development/c++/concepts-11.cc | 53 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)
 create mode 100644 development/c++/concepts-11.cc

(limited to 'development/c++/concepts-11.cc')

diff --git a/development/c++/concepts-11.cc b/development/c++/concepts-11.cc
new file mode 100644
index 0000000..888ff4d
--- /dev/null
+++ b/development/c++/concepts-11.cc
@@ -0,0 +1,53 @@
+// Copyright (C) 2022 johannst
+#include <type_traits>
+
+template<typename T, template<typename> class Checker, typename = void>
+struct is_valid : std::false_type {};
+
+template<typename T, template<typename> class Checker>
+struct is_valid<T, Checker, std::void_t<Checker<T>>> : std::true_type {};
+
+template<typename T, template<typename> class Checker>
+static constexpr bool is_valid_v = is_valid<T, Checker>::value;
+
+// -----------------------------------------------------------------------------
+
+template<typename T, typename R, template<typename> class Checker, typename = void>
+struct is_valid_with_ret : std::false_type {};
+
+template<typename T, typename R, template<typename> class Checker>
+struct is_valid_with_ret<T, R, Checker, std::void_t<Checker<T>>> : std::is_same<R, Checker<T>> {};
+
+template<typename T, typename R, template<typename> class Checker>
+static constexpr bool is_valid_with_ret_v = is_valid_with_ret<T, R, Checker>::value;
+
+// -----------------------------------------------------------------------------
+
+template<typename T>
+struct is_entry {
+    template<typename TT>
+    using init = decltype(std::declval<TT>().init());
+    template<typename TT>
+    using tag = decltype(std::declval<TT>().tag());
+    template<typename TT>
+    using val = decltype(std::declval<TT>().val());
+
+    static constexpr bool value = is_valid_v<T, init> &&
+                                  is_valid_with_ret_v<T, int, tag> &&
+                                  is_valid_with_ret_v<T, typename T::Type, val>;
+};
+
+template<typename T>
+static constexpr bool is_entry_v = is_entry<T>::value;
+
+template<typename E>
+struct Entry {
+    using Type = E;
+    void init();
+    int tag() const;
+    E val() const;
+};
+
+int main() {
+    static_assert(is_entry_v<Entry<bool>>, "");
+}
-- 
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