types/traits: add is_orderable trait

types/traits.hpp

@@ -470,4 +470,32 @@ namespace cruft {
 
     template <typename T>
     constexpr auto is_tuple_like_v = is_tuple_like<T>::value;
-}
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Tests if a type is orderable; ie, if it supports the less than operator
+    ///
+    /// We use void_t to detect the presence of an appropriate operator<
+    template <
+        typename ValueT,
+        typename = std::void_t<>
+    >
+    struct is_orderable : public std::false_type {};
+
+
+    //-------------------------------------------------------------------------
+    template <typename ValueT>
+    struct is_orderable<
+        ValueT,
+        std::void_t<
+            decltype (
+                std::declval<ValueT> () < std::declval<ValueT> ()
+            )
+        >
+    > : std::true_type {};
+
+
+    //-----------------------------------------------------------------------------------
+    template <typename ValueT>
+    constexpr auto is_orderable_v = is_orderable<ValueT>::value;
+};