Reformat and simplify code.

* Constrain distance function using is_iterator type trait

* Add enable_if include

* Fix template syntax in distance function

* Fix syntax issue in distance enable_if

* Test which overload of distance is called

Added a test for custom distance function using Foo struct.

* Fix wrong spelling of std::ptrdiff_t

* Move Foo struct to global scope

* Andrey magic

* Fix headers, add copyright

* Constrain advance to iterators

* Test contraint of advance function with int overload

Added overload for advance function to handle integers.

* Fix enable_if condition for advance function

* Fix template syntax in advance constraint

* Fix advance test

* Remove constexpr specifier

* Make advance overload require conversion in parameter

* Explain choice of type long for n parameter

.github/workflows/ci.yml

@@ -297,7 +297,7 @@ jobs:
             cxxstd: "11,14,17,20,2b"
             os: macos-14
           - toolset: clang
-            cxxstd: "11,14,17,20,2b"
+            cxxstd: "11,14,17,20,23"
             os: macos-15
 
           - name: CMake tests

include/boost/iterator/advance.hpp

@@ -7,7 +7,10 @@
 #ifndef BOOST_ITERATOR_ADVANCE_HPP
 #define BOOST_ITERATOR_ADVANCE_HPP
 
+#include <type_traits>
+
 #include <boost/config.hpp>
+#include <boost/iterator/is_iterator.hpp>
 #include <boost/iterator/iterator_categories.hpp>
 
 namespace boost {
@@ -56,7 +59,8 @@ inline BOOST_CXX14_CONSTEXPR void advance_impl(RandomAccessIterator& it, Distanc
 namespace advance_adl_barrier {
 
 template< typename InputIterator, typename Distance >
-inline BOOST_CXX14_CONSTEXPR void advance(InputIterator& it, Distance n)
+inline BOOST_CXX14_CONSTEXPR typename std::enable_if< is_iterator< InputIterator >::value >::type
+advance(InputIterator& it, Distance n)
 {
     detail::advance_impl(it, n, typename iterator_traversal< InputIterator >::type());
 }

include/boost/iterator/distance.hpp

@@ -1,4 +1,5 @@
 // Copyright (C) 2017 Michel Morin.
+// Copyright (C) 2026 Jeremy W. Murphy
 //
 // Distributed under the Boost Software License, Version 1.0.
 // (See accompanying file LICENSE_1_0.txt or copy at
@@ -7,7 +8,10 @@
 #ifndef BOOST_ITERATOR_DISTANCE_HPP
 #define BOOST_ITERATOR_DISTANCE_HPP
 
+#include <type_traits>
+
 #include <boost/config.hpp>
+#include <boost/iterator/is_iterator.hpp>
 #include <boost/iterator/iterator_categories.hpp>
 #include <boost/iterator/iterator_traits.hpp>
 
@@ -40,8 +44,10 @@ distance_impl(RandomAccessIterator first, RandomAccessIterator last, random_acce
 namespace distance_adl_barrier {
 
 template< typename SinglePassIterator >
-inline BOOST_CXX14_CONSTEXPR typename iterator_difference< SinglePassIterator >::type
-distance(SinglePassIterator first, SinglePassIterator last)
+inline BOOST_CXX14_CONSTEXPR typename std::enable_if<
+    is_iterator< SinglePassIterator >::value,
+    iterator_difference< SinglePassIterator >
+>::type::type distance(SinglePassIterator first, SinglePassIterator last)
 {
     return detail::distance_impl(first, last, typename iterator_traversal< SinglePassIterator >::type());
 }

include/boost/iterator/is_iterator.hpp

@@ -139,10 +139,19 @@ 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 {};
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+template< typename T >
+BOOST_INLINE_VARIABLE constexpr bool is_iterator_v = iterators::is_iterator< T >::value;
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
 } // namespace iterators
 
 using iterators::is_iterator;
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+using iterators::is_iterator_v;
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
 } // namespace boost
 
 #endif // BOOST_ITERATOR_IS_ITERATOR_HPP_INCLUDED_

include/boost/iterator/is_lvalue_iterator.hpp

@@ -4,6 +4,7 @@
 #ifndef IS_LVALUE_ITERATOR_DWA2003112_HPP
 #define IS_LVALUE_ITERATOR_DWA2003112_HPP
 
+#include <boost/config.hpp>
 #include <boost/iterator/detail/type_traits/conjunction.hpp>
 
 #include <iterator>
@@ -71,11 +72,23 @@ struct is_non_const_lvalue_iterator :
 {
 };
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+template< typename T >
+BOOST_INLINE_VARIABLE constexpr bool is_lvalue_iterator_v = iterators::is_lvalue_iterator< T >::value;
+template< typename T >
+BOOST_INLINE_VARIABLE constexpr bool is_non_const_lvalue_iterator_v = iterators::is_non_const_lvalue_iterator< T >::value;
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
 } // namespace iterators
 
 using iterators::is_lvalue_iterator;
 using iterators::is_non_const_lvalue_iterator;
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+using iterators::is_lvalue_iterator_v;
+using iterators::is_non_const_lvalue_iterator_v;
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
 } // namespace boost
 
 #endif // IS_LVALUE_ITERATOR_DWA2003112_HPP

include/boost/iterator/is_readable_iterator.hpp

@@ -6,6 +6,7 @@
 
 #include <iterator>
 #include <type_traits>
+#include <boost/config.hpp>
 
 namespace boost {
 namespace iterators {
@@ -60,10 +61,19 @@ struct is_readable_iterator :
 {
 };
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+template< typename T >
+BOOST_INLINE_VARIABLE constexpr bool is_readable_iterator_v = iterators::is_readable_iterator< T >::value;
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
 } // namespace iterators
 
 using iterators::is_readable_iterator;
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+using iterators::is_readable_iterator_v;
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
 } // namespace boost
 
 #endif // IS_READABLE_ITERATOR_DWA2003112_HPP

test/advance_test.cpp

@@ -20,6 +20,20 @@ void test_advance(Iterator it_from, Iterator it_to, int n)
     BOOST_TEST(it_from == it_to);
 }
 
+// Definitely not an iterator
+struct Foo
+{
+    int x = 0;
+
+    // Don't use type "int" for "n", otherwise it matches literal int exactly
+    // and doesn't demonstrate the effect of enable_if.
+    friend
+    void advance(Foo &value, long n)
+    {
+        value.x += 10 * n;
+    }
+};
+
 int main()
 {
     int array[3] = {1, 2, 3};
@@ -87,5 +101,12 @@ int main()
         );
     }
 
+    {
+        using boost::advance;
+        Foo bar;
+        advance(bar, 3);
+        BOOST_TEST(bar.x == 30);
+    }
+    
     return boost::report_errors();
 }

test/distance_test.cpp

@@ -4,6 +4,7 @@
 // (See accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
+#include <cstddef>
 #include <vector>
 #include <list>
 #include <boost/container/slist.hpp>
@@ -19,6 +20,13 @@ void test_distance(Iterator it_from, Iterator it_to, int n)
     BOOST_TEST(boost::distance(it_from, it_to) == n);
 }
 
+// Definitely not an iterator.
+struct Foo
+{
+    constexpr friend
+    std::ptrdiff_t distance(Foo const &, Foo const &) { return -1; }
+};
+
 int main()
 {
     int array[3] = {1, 2, 3};
@@ -80,5 +88,11 @@ int main()
         );
     }
 
+    {
+        // Make boost::distance visible since we're not actually in the boost namespace here.
+        using boost::distance;
+        auto result = distance(Foo{}, Foo{});
+        BOOST_TEST(result == -1);
+    }
     return boost::report_errors();
 }

test/is_iterator.cpp

@@ -17,6 +17,7 @@
 #include <vector>
 #include <string>
 #include <iterator>
+#include <boost/config.hpp>
 #include <boost/iterator/iterator_adaptor.hpp>
 #include <boost/core/lightweight_test.hpp>
 
@@ -160,5 +161,25 @@ int main()
     BOOST_TEST(!boost::iterators::is_iterator< int (&)(int) >::value);
     BOOST_TEST(!boost::iterators::is_iterator< int (&)[10] >::value);
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+    BOOST_TEST(boost::iterators::is_iterator_v< int* >);
+    BOOST_TEST(boost::iterators::is_iterator_v< const int* >);
+    BOOST_TEST(boost::iterators::is_iterator_v< complete* >);
+    BOOST_TEST(boost::iterators::is_iterator_v< std::reverse_iterator< int* > >);
+    BOOST_TEST(boost::iterators::is_iterator_v< std::reverse_iterator< complete* > >);
+    BOOST_TEST(boost::iterators::is_iterator_v< adapted_iterator< int* > >);
+
+    BOOST_TEST(!boost::iterators::is_iterator_v< int >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< void* >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< int (int) >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< int (*)(int) >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< int complete::* >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< int (complete::*)(int) >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< int*[] >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< int*[10] >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< int& >);
+    BOOST_TEST(!boost::iterators::is_iterator_v< int*& >);
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
     return boost::report_errors();
 }

test/is_lvalue_iterator.cpp

@@ -2,11 +2,12 @@
 // subject to the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
+#include <boost/iterator/is_lvalue_iterator.hpp>
 #include <deque>
 #include <iterator>
 #include <cstddef> // std::ptrdiff_t
+#include <boost/config.hpp>
 #include <boost/noncopyable.hpp>
-#include <boost/iterator/is_lvalue_iterator.hpp>
 
 struct v
 {
@@ -168,5 +169,25 @@ int main()
     static_assert(!boost::is_non_const_lvalue_iterator<constant_lvalue_iterator<float>>::value,
                   "boost::is_non_const_lvalue_iterator<constant_lvalue_iterator<float>>::value is expected to be false.");
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+    static_assert(boost::is_lvalue_iterator_v<v*>,
+                  "boost::is_lvalue_iterator_v<v*> is expected to be true.");
+    static_assert(boost::is_lvalue_iterator_v<v const*>,
+                  "boost::is_lvalue_iterator_v<v const*> is expected to be true.");
+    static_assert(!boost::is_lvalue_iterator_v<proxy_iterator<int>>,
+                  "boost::is_lvalue_iterator_v<proxy_iterator<int>> is expected to be false.");
+    static_assert(!boost::is_lvalue_iterator_v<value_iterator>,
+                  "boost::is_lvalue_iterator_v<value_iterator> is expected to be false.");
+
+    static_assert(boost::is_non_const_lvalue_iterator_v<v*>,
+                  "boost::is_non_const_lvalue_iterator_v<v*> is expected to be true.");
+    static_assert(!boost::is_non_const_lvalue_iterator_v<v const*>,
+                  "boost::is_non_const_lvalue_iterator_v<v const*> is expected to be false.");
+    static_assert(!boost::is_non_const_lvalue_iterator_v<proxy_iterator<int>>,
+                  "boost::is_non_const_lvalue_iterator_v<proxy_iterator<int>> is expected to be false.");
+    static_assert(!boost::is_non_const_lvalue_iterator_v<value_iterator>,
+                  "boost::is_non_const_lvalue_iterator_v<value_iterator> is expected to be false.");
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
     return 0;
 }

test/is_readable_iterator.cpp

@@ -2,11 +2,12 @@
 // subject to the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
+#include <boost/iterator/is_readable_iterator.hpp>
 #include <deque>
 #include <iterator>
 #include <cstddef> // std::ptrdiff_t
+#include <boost/config.hpp>
 #include <boost/noncopyable.hpp>
-#include <boost/iterator/is_readable_iterator.hpp>
 
 struct v
 {
@@ -97,5 +98,16 @@ int main()
     static_assert(boost::is_readable_iterator<noncopyable_iterator>::value,
                   "boost::is_readable_iterator<noncopyable_iterator>::value is expected to be true.");
 
+#if !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+    static_assert(boost::is_readable_iterator_v<v*>,
+                  "boost::is_readable_iterator_v<v*> is expected to be true.");
+    static_assert(boost::is_readable_iterator_v<v const*>,
+                  "boost::is_readable_iterator_v<v const*> is expected to be true.");
+    static_assert(boost::is_readable_iterator_v<proxy_iterator>,
+                  "boost::is_readable_iterator_v<proxy_iterator> is expected to be true.");
+    static_assert(!boost::is_readable_iterator_v<proxy_iterator2>,
+                  "boost::is_readable_iterator_v<proxy_iterator2> is expected to be false.");
+#endif // !defined(BOOST_NO_CXX14_VARIABLE_TEMPLATES)
+
     return 0;
 }