Extracted is_iterator trait to a separate header.

Also, reworked is_iterator to be more robust against various corner cases
and added dedicated tests for the type trait.

include/boost/iterator/is_iterator.hpp

@@ -0,0 +1,148 @@
+/*
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * https://www.boost.org/LICENSE_1_0.txt)
+ *
+ * Copyright (c) 2023 Andrey Semashev
+ */
+/*!
+ * \file iterator/is_iterator.hpp
+ *
+ * This header contains definition of the \c is_iterator type trait.
+ */
+
+#ifndef BOOST_ITERATOR_IS_ITERATOR_HPP_INCLUDED_
+#define BOOST_ITERATOR_IS_ITERATOR_HPP_INCLUDED_
+
+#include <cstddef>
+#include <boost/config.hpp>
+#include <boost/type_traits/integral_constant.hpp>
+#include <boost/type_traits/negation.hpp>
+#include <boost/type_traits/conjunction.hpp>
+#include <boost/type_traits/is_complete.hpp>
+#include <boost/type_traits/is_function.hpp>
+#if !defined(BOOST_NO_CXX17_ITERATOR_TRAITS)
+#include <iterator>
+#endif
+
+#ifdef BOOST_HAS_PRAGMA_ONCE
+#pragma once
+#endif
+
+namespace boost {
+namespace iterators {
+namespace detail {
+
+// The trait attempts to detect if the T type is an iterator class. Class-type iterators are assumed
+// to have the nested type iterator_category. Strictly speaking, this is not required to be the
+// case (e.g. a user can specialize iterator_traits for T without defining T::iterator_category).
+// Still, this is a good heuristic in practice, and we can't do anything better anyway.
+// Since C++17 we can test for iterator_traits<T>::iterator_category presence instead as it is
+// required to be only present for iterators.
+namespace has_iterator_category_detail {
+
+typedef char yes_type;
+struct no_type { char padding[2]; };
+
+template< typename T >
+yes_type check(
+#if !defined(BOOST_NO_CXX17_ITERATOR_TRAITS)
+    typename std::iterator_traits< T >::iterator_category*
+#else
+    typename T::iterator_category*
+#endif
+);
+template< typename >
+no_type check(...);
+
+} // namespace has_iterator_category_detail
+
+template< typename T >
+struct is_iterator_impl :
+    public boost::integral_constant<
+        bool,
+        sizeof(has_iterator_category_detail::check< T >(0)) == sizeof(has_iterator_category_detail::yes_type)
+    >
+{
+};
+
+template< typename T >
+struct is_iterator_impl< T* > :
+    public boost::conjunction<
+        boost::is_complete< T >,
+        boost::negation< boost::is_function< T > >
+    >::type
+{
+};
+
+template< typename T, typename U >
+struct is_iterator_impl< T U::* > :
+    public boost::false_type
+{
+};
+
+template< typename T >
+struct is_iterator_impl< T& > :
+    public boost::false_type
+{
+};
+
+template< typename T, std::size_t N >
+struct is_iterator_impl< T[N] > :
+    public boost::false_type
+{
+};
+
+#if !defined(BOOST_TT_HAS_WORKING_IS_COMPLETE)
+template< typename T >
+struct is_iterator_impl< T[] > :
+    public boost::false_type
+{
+};
+
+template< >
+struct is_iterator_impl< void > :
+    public boost::false_type
+{
+};
+
+template< >
+struct is_iterator_impl< void* > :
+    public boost::false_type
+{
+};
+#endif // !defined(BOOST_TT_HAS_WORKING_IS_COMPLETE)
+
+} // namespace detail
+
+/*!
+ * \brief The type trait detects whether the type \c T is an iterator type.
+ *
+ * The type trait yields \c true if its argument type \c T, after stripping top level
+ * cv qualifiers, is one of the following:
+ *
+ * - A pointer type, other than a pointer to function, a pointer to a class member,
+ *   or a pointer to an incomplete type, including `void`.
+ * - A class type for which an iterator category is obtainable. Prior to C++17,
+ *   the iterator category must be defined as a public `T::iterator_category` type.
+ *   Since C++17, the expression `std::iterator_traits< T >::iterator_category` must
+ *   be valid and produce the iterator category type.
+ *
+ * Otherwise, the type trait yields \c false.
+ */
+template< typename T >
+struct is_iterator : public detail::is_iterator_impl< T >::type {};
+template< typename T >
+struct is_iterator< const T > : public detail::is_iterator_impl< T >::type {};
+template< typename T >
+struct is_iterator< volatile T > : public detail::is_iterator_impl< T >::type {};
+template< typename T >
+struct is_iterator< const volatile T > : public detail::is_iterator_impl< T >::type {};
+
+} // namespace iterators
+
+using iterators::is_iterator;
+
+} // namespace boost
+
+#endif // BOOST_ITERATOR_IS_ITERATOR_HPP_INCLUDED_