[Hashable] hana::integral_constant with an enum value implements Hashable

Author: Dwarfobserver

Diff for: include/boost/hana/hash.hpp

@@ -80,6 +80,16 @@ BOOST_HANA_NAMESPACE_BEGIN
             }
         };
 
+        template <typename T>
+        struct hash_integral_helper<T,
+            typename std::enable_if<std::is_enum<T>::value>::type
+        > {
+            template <typename X>
+            static constexpr auto apply(X const&) {
+                return hana::type_c<X>;
+            }
+        };
+
         template <>
         struct hash_integral_helper<bool> {
             template <typename X>

Diff for: test/integral_constant/hash.cpp

@@ -107,4 +107,26 @@ int main() {
             hana::type_c<hana::false_>
         ));
     }
+
+    // Enumeration constants should hash to themselves
+    {
+        enum enum_1                  { zero };
+        enum class enum_2            { zero };
+        enum class enum_3 : unsigned { zero };
+
+        BOOST_HANA_CONSTANT_ASSERT(hana::equal(
+            hana::hash(hana::integral_c<enum_1, zero>),
+            hana::type_c<hana::integral_constant<enum_1, zero>>
+        ));
+
+        BOOST_HANA_CONSTANT_ASSERT(hana::equal(
+            hana::hash(hana::integral_c<enum_2, enum_2::zero>),
+            hana::type_c<hana::integral_constant<enum_2, enum_2::zero>>
+        ));
+
+        BOOST_HANA_CONSTANT_ASSERT(hana::equal(
+            hana::hash(hana::integral_c<enum_3, enum_3::zero>),
+            hana::type_c<hana::integral_constant<enum_3, enum_3::zero>>
+        ));
+    }
 }