Extracted and unified default template parameter handling helpers.

The new eval_if_default helper replaces both ia_dflt_help and
ia_eval_if_default helpers. Additionally, added a new if_default
helper, which expands to the type or the default parameter directly.

The new helpers are placed in separate headers which can be used
without the iterator_adaptor template.

include/boost/iterator/detail/if_default.hpp

@@ -0,0 +1,41 @@
+/*
+ * 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_DETAIL_IF_DEFAULT_HPP_INCLUDED_
+#define BOOST_ITERATOR_DETAIL_IF_DEFAULT_HPP_INCLUDED_
+
+#include <boost/core/use_default.hpp>
+
+namespace boost {
+namespace iterators {
+namespace detail {
+
+// If T is use_default, return Default, otherwise - Nondefault.
+// By default, Nondefault is T, which means
+// the metafunction can be called with just two parameters
+// and in that case will return either T or Default.
+template< typename T, typename Default, typename Nondefault = T >
+struct if_default
+{
+    using type = Nondefault;
+};
+
+template< typename Default, typename Nondefault >
+struct if_default< use_default, Default, Nondefault >
+{
+    using type = Default;
+};
+
+template< typename T, typename Default, typename Nondefault = T >
+using if_default_t = typename if_default< T, Default, Nondefault >::type;
+
+} // namespace detail
+} // namespace iterators
+} // namespace boost
+
+#endif // BOOST_ITERATOR_DETAIL_IF_DEFAULT_HPP_INCLUDED_