Constrain distance function using is_iterator type trait

include/boost/iterator/distance.hpp

@@ -8,6 +8,8 @@
 #define BOOST_ITERATOR_DISTANCE_HPP
 
 #include <boost/config.hpp>
+#include <boost/core/enable_if.hpp>
+#include <boost/iterator/is_iterator.hpp>
 #include <boost/iterator/iterator_categories.hpp>
 #include <boost/iterator/iterator_traits.hpp>
 
@@ -40,7 +42,11 @@ distance_impl(RandomAccessIterator first, RandomAccessIterator last, random_acce
 namespace distance_adl_barrier {
 
 template< typename SinglePassIterator >
-inline BOOST_CXX14_CONSTEXPR typename iterator_difference< SinglePassIterator >::type
+inline BOOST_CXX14_CONSTEXPR
+typename std::enable_if<
+    boost::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());

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();
 }