From 3e90982d29b54523c3a617cfff7ec6bcc42ba1e8 Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@gmail.com>
Date: Thu, 8 Jun 2023 13:49:36 +0200
Subject: concepts: replace is_hashable_v

---
 source/lib/include/newtype/concepts.hpp               | 19 +++++++++++++++++++
 .../include/newtype/impl/type_traits_extensions.hpp   | 19 -------------------
 source/lib/include/newtype/newtype.hpp                |  7 +++----
 source/tests/src/hash.cpp                             | 19 +++++++++++++------
 4 files changed, 35 insertions(+), 29 deletions(-)
 create mode 100644 source/lib/include/newtype/concepts.hpp

diff --git a/source/lib/include/newtype/concepts.hpp b/source/lib/include/newtype/concepts.hpp
new file mode 100644
index 0000000..a29ba37
--- /dev/null
+++ b/source/lib/include/newtype/concepts.hpp
@@ -0,0 +1,19 @@
+#ifndef NEWTYPE_CONCEPTS_HPP
+#define NEWTYPE_CONCEPTS_HPP
+
+#include <concepts>
+#include <cstddef>
+#include <functional>
+
+namespace nt::concepts
+{
+  template<typename SubjectType>
+  concept hashable = requires(SubjectType subject) {
+    {
+      std::hash<SubjectType>{}(subject)
+    } -> std::convertible_to<std::size_t>;
+  };
+
+}  // namespace nt::concepts
+
+#endif
\ No newline at end of file
diff --git a/source/lib/include/newtype/impl/type_traits_extensions.hpp b/source/lib/include/newtype/impl/type_traits_extensions.hpp
index dc41649..bdd1cba 100644
--- a/source/lib/include/newtype/impl/type_traits_extensions.hpp
+++ b/source/lib/include/newtype/impl/type_traits_extensions.hpp
@@ -469,25 +469,6 @@ namespace nt::impl
 
   }  // namespace compound_arithmetic
 
-  inline namespace std_support
-  {
-
-    template<typename T, typename = void>
-    struct is_hashable : std::false_type
-    {
-    };
-
-    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 &>()))>
-    {
-    };
-
-    template<typename T>
-    auto constexpr is_hashable_v = is_hashable<T>::value;
-
-  }  // namespace std_support
-
   inline namespace iterable_begin
   {
     template<typename T, typename = void>
diff --git a/source/lib/include/newtype/newtype.hpp b/source/lib/include/newtype/newtype.hpp
index 2e71553..b833c10 100644
--- a/source/lib/include/newtype/newtype.hpp
+++ b/source/lib/include/newtype/newtype.hpp
@@ -1,6 +1,7 @@
 #ifndef NEWTYPE_NEWTYPE_HPP
 #define NEWTYPE_NEWTYPE_HPP
 
+#include "newtype/concepts.hpp"
 #include "newtype/derivable.hpp"
 #include "newtype/deriving.hpp"
 #include "newtype/impl/new_type_iterator_types.hpp"
@@ -535,12 +536,10 @@ namespace nt
 namespace std
 {
   template<typename BaseType, typename TagType, auto DerivationClause>
+    requires(nt::concepts::hashable<BaseType> && DerivationClause(nt::Hash))
   struct hash<nt::new_type<BaseType, TagType, DerivationClause>>
   {
-    template<typename BaseTypeT = BaseType>
-    auto constexpr operator()(nt::new_type<BaseType, TagType, DerivationClause> const & object,
-                              std::enable_if_t<DerivationClause(nt::Hash) && nt::impl::is_hashable_v<BaseTypeT>> * = nullptr) const
-        -> std::size_t
+    auto constexpr operator()(nt::new_type<BaseType, TagType, DerivationClause> const & object) const
     {
       return std::hash<BaseType>{}(object.decay());
     }
diff --git a/source/tests/src/hash.cpp b/source/tests/src/hash.cpp
index e3f624f..bcf793c 100644
--- a/source/tests/src/hash.cpp
+++ b/source/tests/src/hash.cpp
@@ -1,31 +1,36 @@
+#include "newtype/concepts.hpp"
 #include "newtype/derivable.hpp"
 #include "newtype/deriving.hpp"
 #include "newtype/impl/type_traits_extensions.hpp"
 #include "newtype/newtype.hpp"
 
+#include <catch2/catch_template_test_macros.hpp>
 #include <catch2/catch_test_macros.hpp>
 
+#include <string>
 #include <unordered_map>
 
-SCENARIO("Hash", "[hash]")
+TEMPLATE_TEST_CASE("Hash", "[hash]", std::string, int)
 {
   GIVEN("A new_type not deriving nt::Hash")
   {
-    using type_alias = nt::new_type<int, struct tag>;
+    using type_alias = nt::new_type<TestType, struct tag>;
+    static_assert(nt::concepts::hashable<typename type_alias::base_type>);
 
     THEN("it is not hashable")
     {
-      STATIC_REQUIRE_FALSE(nt::impl::is_hashable_v<type_alias>);
+      STATIC_REQUIRE_FALSE(nt::concepts::hashable<type_alias>);
     }
   }
 
   GIVEN("A new_type over a hashable type deriving nt::Hash")
   {
-    using type_alias = nt::new_type<int, struct tag, deriving(nt::Hash)>;
+    using type_alias = nt::new_type<TestType, struct tag, deriving(nt::Hash)>;
+    static_assert(nt::concepts::hashable<typename type_alias::base_type>);
 
     THEN("it is hashable")
     {
-      STATIC_REQUIRE(nt::impl::is_hashable_v<type_alias>);
+      STATIC_REQUIRE(nt::concepts::hashable<type_alias>);
     }
   }
 
@@ -35,16 +40,18 @@ SCENARIO("Hash", "[hash]")
     {
     };
     using type_alias = nt::new_type<non_hashable, struct tag, deriving(nt::Hash)>;
+    static_assert(!nt::concepts::hashable<typename type_alias::base_type>);
 
     THEN("it is not hashable")
     {
-      STATIC_REQUIRE_FALSE(nt::impl::is_hashable_v<type_alias>);
+      STATIC_REQUIRE_FALSE(nt::concepts::hashable<type_alias>);
     }
   }
 
   GIVEN("A hashable new_type")
   {
     using type_alias = nt::new_type<int, struct tag, deriving(nt::Hash)>;
+    static_assert(nt::concepts::hashable<typename type_alias::base_type>);
 
     THEN("it can be used a the key in an unordered_map")
     {
-- 
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