From d5970f8ed49a15d19265c71c8618e32a9534eeee Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@gmail.com>
Date: Fri, 3 Jan 2020 13:10:02 +0100
Subject: new_type: implement support for derivation of Hash

---
 include/newtype/derivable.hpp                   |  7 +++++
 include/newtype/impl/type_traits_extensions.hpp | 38 +++++++++++++++++++++++++
 include/newtype/new_type.hpp                    | 16 +++++++++++
 3 files changed, 61 insertions(+)

(limited to 'include')

diff --git a/include/newtype/derivable.hpp b/include/newtype/derivable.hpp
index 95353eb..38a2375 100644
--- a/include/newtype/derivable.hpp
+++ b/include/newtype/derivable.hpp
@@ -35,6 +35,13 @@ namespace nt
      */
     auto constexpr EqBase = derivable<struct eq_base_tag>{};
 
+    /**
+     * @brief A tag to enable derivation of a specialization of std::hash
+     *
+     * @since 1.0.0
+     */
+    auto constexpr Hash = derivable<struct hash_tag>{};
+
     /**
      * @brief A tag to enable derivation of the implicit "conversion to base type" operator
      *
diff --git a/include/newtype/impl/type_traits_extensions.hpp b/include/newtype/impl/type_traits_extensions.hpp
index 10dbc07..d773d40 100644
--- a/include/newtype/impl/type_traits_extensions.hpp
+++ b/include/newtype/impl/type_traits_extensions.hpp
@@ -1,6 +1,8 @@
 #ifndef NEWTYPE_IMPL_TYPE_TRAITS_EXTENSIONS_HPP
 #define NEWTYPE_IMPL_TYPE_TRAITS_EXTENSIONS_HPP
 
+#include <cstddef>
+#include <functional>
 #include <iosfwd>
 #include <type_traits>
 
@@ -1011,6 +1013,42 @@ namespace nt::impl
 
   }  // namespace compound_arithmetic
 
+  inline namespace std_support
+  {
+
+    /**
+     * @brief A trait to test if a given type is hashable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the base case for non-hashable T
+     */
+    template<typename T, typename = void>
+    struct is_hashable : std::false_type
+    {
+    };
+
+    /**
+     * @brief A trait to test if a given type is hashable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the case for hashable T
+     */
+    template<typename T>
+    struct is_hashable<T, std::void_t<decltype(std::declval<std::hash<T> const &>()(std::declval<T const &>()))>>
+        : std::is_same<std::size_t, decltype(std::declval<std::hash<T> const &>()(std::declval<T const &>()))>
+    {
+    };
+
+    /**
+     * @brief A variable template to test if a given type is hashable
+     *
+     * @tparam T The type to test
+     */
+    template<typename T>
+    auto constexpr is_hashable_v = is_hashable<T>::value;
+
+  }  // namespace std_support
+
 }  // namespace nt::impl
 
 #endif
\ No newline at end of file
diff --git a/include/newtype/new_type.hpp b/include/newtype/new_type.hpp
index a59a074..1477c95 100644
--- a/include/newtype/new_type.hpp
+++ b/include/newtype/new_type.hpp
@@ -6,6 +6,7 @@
 #include "newtype/impl/new_type_storage.hpp"
 #include "newtype/impl/type_traits_extensions.hpp"
 
+#include <functional>
 #include <istream>
 #include <ostream>
 #include <type_traits>
@@ -569,4 +570,19 @@ namespace nt
 
 }  // namespace nt
 
+namespace std
+{
+  template<typename BaseType, typename TagType, auto DerivationClause>
+  struct hash<nt::new_type<BaseType, TagType, DerivationClause>>
+  {
+    template<typename BaseTypeT = BaseType, auto DerivationClauseV = DerivationClause>
+    auto constexpr operator()(nt::new_type<BaseType, TagType, DerivationClause> const & object,
+                              std::enable_if_t<DerivationClauseV(nt::Hash) && nt::impl::is_hashable_v<BaseTypeT>> * = nullptr) const
+        -> std::size_t
+    {
+      return std::hash<BaseType>{}(object.decay());
+    }
+  };
+}  // namespace std
+
 #endif
-- 
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