Extracted enable_if_convertible trait to a separate header.

Keep including the new header in iterator_adaptor.hpp for backward
compatibility, until downstream users are updated to include the new
header.

include/boost/iterator/enable_if_convertible.hpp

@@ -0,0 +1,84 @@
+/*
+ * 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) 2025 Andrey Semashev
+ */
+
+#ifndef BOOST_ITERATOR_ENABLE_IF_CONVERTIBLE_HPP_INCLUDED_
+#define BOOST_ITERATOR_ENABLE_IF_CONVERTIBLE_HPP_INCLUDED_
+
+#include <type_traits>
+
+namespace boost {
+namespace iterators {
+namespace detail {
+
+//
+// Result type used in enable_if_convertible meta function.
+// This can be an incomplete type, as only pointers to
+// enable_if_convertible< ... >::type are used.
+// We could have used void for this, but conversion to
+// void* is just too easy.
+//
+struct enable_type;
+
+} // namespace detail
+
+//
+// enable_if for use in adapted iterators constructors.
+//
+// In order to provide interoperability between adapted constant and
+// mutable iterators, adapted iterators will usually provide templated
+// conversion constructors of the following form
+//
+// template <class BaseIterator>
+// class adapted_iterator :
+//   public iterator_adaptor< adapted_iterator<Iterator>, Iterator >
+// {
+// public:
+//
+//   ...
+//
+//   template <class OtherIterator>
+//   adapted_iterator(
+//       OtherIterator const& it
+//     , typename enable_if_convertible<OtherIterator, Iterator>::type* = 0);
+//
+//   ...
+// };
+//
+// enable_if_convertible is used to remove those overloads from the overload
+// set that cannot be instantiated. For all practical purposes only overloads
+// for constant/mutable interaction will remain. This has the advantage that
+// meta functions like boost::is_convertible do not return false positives,
+// as they can only look at the signature of the conversion constructor
+// and not at the actual instantiation.
+//
+// enable_if_interoperable can be safely used in user code. It falls back to
+// always enabled for compilers that don't support enable_if or is_convertible.
+// There is no need for compiler specific workarounds in user code.
+//
+// The operators implementation relies on boost::is_convertible not returning
+// false positives for user/library defined iterator types. See comments
+// on operator implementation for consequences.
+//
+template< typename From, typename To >
+struct enable_if_convertible :
+    public std::enable_if<
+        std::is_convertible< From, To >::value,
+        boost::iterators::detail::enable_type
+    >
+{};
+
+template< typename From, typename To >
+using enable_if_convertible_t = typename enable_if_convertible< From, To >::type;
+
+} // namespace iterators
+
+using iterators::enable_if_convertible;
+
+} // namespace boost
+
+#endif // BOOST_ITERATOR_ENABLE_IF_CONVERTIBLE_HPP_INCLUDED_

include/boost/iterator/filter_iterator.hpp

@@ -11,6 +11,7 @@
 
 #include <boost/iterator/iterator_adaptor.hpp>
 #include <boost/iterator/iterator_categories.hpp>
+#include <boost/iterator/enable_if_convertible.hpp>
 #include <boost/core/use_default.hpp>
 
 namespace boost {

include/boost/iterator/indirect_iterator.hpp

@@ -11,6 +11,7 @@
 #include <type_traits>
 
 #include <boost/iterator/iterator_adaptor.hpp>
+#include <boost/iterator/enable_if_convertible.hpp>
 
 #include <boost/pointee.hpp>
 #include <boost/indirect_reference.hpp>
@@ -84,7 +85,7 @@ public:
         typename Category2,
         typename Reference2,
         typename Difference2,
-        typename = typename enable_if_convertible< Iterator2, Iterator >::type
+        typename = enable_if_convertible_t< Iterator2, Iterator >
     >
     indirect_iterator(indirect_iterator< Iterator2, Value2, Category2, Reference2, Difference2 > const& y) :
         super_t(y.base())

include/boost/iterator/iterator_adaptor.hpp

@@ -14,6 +14,7 @@
 #include <boost/iterator/iterator_categories.hpp>
 #include <boost/iterator/iterator_facade.hpp>
 #include <boost/iterator/iterator_traits.hpp>
+#include <boost/iterator/enable_if_convertible.hpp> // for backward compatibility; remove once downstream users are updated
 
 #include <boost/iterator/detail/config_def.hpp>
 
@@ -25,65 +26,6 @@ namespace iterators {
 // explicitly in order to specify that the default should be used.
 using boost::use_default;
 
-namespace detail {
-
-//
-// Result type used in enable_if_convertible meta function.
-// This can be an incomplete type, as only pointers to
-// enable_if_convertible< ... >::type are used.
-// We could have used void for this, but conversion to
-// void* is just too easy.
-//
-struct enable_type;
-
-} // namespace detail
-
-//
-// enable_if for use in adapted iterators constructors.
-//
-// In order to provide interoperability between adapted constant and
-// mutable iterators, adapted iterators will usually provide templated
-// conversion constructors of the following form
-//
-// template <class BaseIterator>
-// class adapted_iterator :
-//   public iterator_adaptor< adapted_iterator<Iterator>, Iterator >
-// {
-// public:
-//
-//   ...
-//
-//   template <class OtherIterator>
-//   adapted_iterator(
-//       OtherIterator const& it
-//     , typename enable_if_convertible<OtherIterator, Iterator>::type* = 0);
-//
-//   ...
-// };
-//
-// enable_if_convertible is used to remove those overloads from the overload
-// set that cannot be instantiated. For all practical purposes only overloads
-// for constant/mutable interaction will remain. This has the advantage that
-// meta functions like boost::is_convertible do not return false positives,
-// as they can only look at the signature of the conversion constructor
-// and not at the actual instantiation.
-//
-// enable_if_interoperable can be safely used in user code. It falls back to
-// always enabled for compilers that don't support enable_if or is_convertible.
-// There is no need for compiler specific workarounds in user code.
-//
-// The operators implementation relies on boost::is_convertible not returning
-// false positives for user/library defined iterator types. See comments
-// on operator implementation for consequences.
-//
-template< typename From, typename To >
-struct enable_if_convertible :
-    public std::enable_if<
-        std::is_convertible< From, To >::value,
-        boost::iterators::detail::enable_type
-    >
-{};
-
 //
 // Default template argument handling for iterator_adaptor
 //
@@ -297,7 +239,6 @@ private: // data members
 } // namespace iterators
 
 using iterators::iterator_adaptor;
-using iterators::enable_if_convertible;
 
 } // namespace boost
 

include/boost/iterator/permutation_iterator.hpp

@@ -12,7 +12,7 @@
 
 #include <boost/core/use_default.hpp>
 #include <boost/iterator/iterator_adaptor.hpp>
-
+#include <boost/iterator/enable_if_convertible.hpp>
 
 namespace boost {
 namespace iterators {

include/boost/iterator/reverse_iterator.hpp

@@ -8,6 +8,7 @@
 #define BOOST_REVERSE_ITERATOR_23022003THW_HPP
 
 #include <boost/iterator/iterator_adaptor.hpp>
+#include <boost/iterator/enable_if_convertible.hpp>
 
 namespace boost {
 namespace iterators {

include/boost/iterator/transform_iterator.hpp

@@ -12,6 +12,8 @@
 #include <boost/core/use_default.hpp>
 #include <boost/iterator/iterator_adaptor.hpp>
 #include <boost/iterator/iterator_categories.hpp>
+#include <boost/iterator/enable_if_convertible.hpp>
+
 #include <boost/utility/result_of.hpp>
 
 #include <type_traits>

include/boost/iterator/zip_iterator.hpp

@@ -10,7 +10,7 @@
 
 #include <boost/iterator/iterator_traits.hpp>
 #include <boost/iterator/iterator_facade.hpp>
-#include <boost/iterator/iterator_adaptor.hpp> // for enable_if_convertible
+#include <boost/iterator/enable_if_convertible.hpp>
 #include <boost/iterator/iterator_categories.hpp>
 #include <boost/iterator/minimum_category.hpp>
 

test/iterator_adaptor_test.cpp

@@ -15,6 +15,7 @@
 #include <boost/iterator/iterator_adaptor.hpp>
 #include <boost/iterator/is_readable_iterator.hpp>
 #include <boost/iterator/is_lvalue_iterator.hpp>
+#include <boost/iterator/enable_if_convertible.hpp>
 #include <boost/pending/iterator_tests.hpp>
 
 # include <boost/core/lightweight_test.hpp>