concepts: add the supports_iterator_traits concept

concepts.hpp

@@ -251,6 +251,23 @@ namespace cruft::concepts {
 #include <tuple>
 
 namespace cruft::concepts {
+    /// Tests if the type has all typedefs required for use with
+    /// std::iterator_traits.
+    template <typename T>
+    concept supports_iterator_traits = requires
+    {
+        typename T::difference_type;
+        typename T::value_type;
+        typename T::reference;
+        typename T::iterator_category;
+
+        // C++20 defines `pointer` as void if it's not present.
+        #if __cplusplus <= 201703L
+        typename T::pointer;
+        #endif
+    };
+
+
     template <typename T>
     concept arithmetic = std::is_arithmetic_v<T>;