Only forward-declare from_range_t if __cpp_lib_containers_ranges is defined. Define al alternative otherwise.

Author: igaztanaga

include/boost/container/detail/range_utils.hpp

@@ -26,17 +26,21 @@
 //!   Defines boost::container::from_range_t and the boost::container::from_range
 //!   tag, used to disambiguate the range-based members of the containers.
 //!
-//!   Following the same technique as boost::container::piecewise_construct_t and
-//!   boost::container::allocator_arg_t, from_range_t is a reference to the
-//!   standard ::std::from_range_t (only forward declared in <std_fwd.hpp>), so
-//!   the tag is the very same type as the standard one and interoperates with
-//!   ::std::from_range, yet the heavyweight <ranges> header is not required.
+//!   When the standard library provides ::std::from_range_t (feature-test macro
+//!   __cpp_lib_containers_ranges), from_range_t is a reference to it -following
+//!   the same technique as boost::container::piecewise_construct_t and
+//!   boost::container::allocator_arg_t- so the tag is the very same type as the
+//!   standard one and interoperates with ::std::from_range, yet the heavyweight
+//!   <ranges> header is not required. Otherwise, there is no standard type to
+//!   refer to and from_range_t is defined as a standalone type.
 
 namespace boost {
 namespace container {
 
 #ifndef BOOST_CONTAINER_DOXYGEN_INVOKED
 
+#if defined(__cpp_lib_containers_ranges)
+
    template <int Dummy = 0>
    struct std_from_range_holder
    {
@@ -48,18 +52,31 @@ namespace container {
 
 typedef const ::std::from_range_t & from_range_t;
 
-#else
+//! A instance of type from_range_t
+static from_range_t from_range = *std_from_range_holder<>::dummy;
+
+#else //!defined(__cpp_lib_containers_ranges)
+
+//The standard library does not provide ::std::from_range_t, so there is nothing
+//to interoperate with: from_range_t is a standalone type.
+struct from_range_t {};
+
+//! A instance of type from_range_t
+static const from_range_t from_range = from_range_t();
+
+#endif   //defined(__cpp_lib_containers_ranges)
+
+#else    //BOOST_CONTAINER_DOXYGEN_INVOKED
 
 //! The from_range_t struct is an empty structure type used as a unique type to
 //! disambiguate the constructors and member functions that build or replace the
 //! contents of a container from a container-compatible range.
 typedef unspecified from_range_t;
 
-#endif   //#ifndef BOOST_CONTAINER_DOXYGEN_INVOKED
+//! A instance of type from_range_t
+static from_range_t from_range;
 
-//! A instance of type
-//! from_range_t
-static from_range_t from_range = BOOST_CONTAINER_DOC1ST(unspecified, *std_from_range_holder<>::dummy);
+#endif   //#ifndef BOOST_CONTAINER_DOXYGEN_INVOKED
 
 ///@cond
 

include/boost/container/detail/std_fwd.hpp

@@ -25,6 +25,17 @@
 
 #include <cstddef>
 
+//<version> is needed to reliably query the __cpp_lib_containers_ranges
+//feature-test macro, which tells whether the standard library provides
+//::std::from_range_t. It is one of the lightest standard headers (only macros).
+#if defined(__has_include)
+#  if __has_include(<version>)
+#     include <version>
+#  endif
+#elif BOOST_CXX_VERSION >= 202002L
+#  include <version>
+#endif
+
 #include <boost/move/detail/std_ns_begin.hpp>
 BOOST_MOVE_STD_NS_BEG
 
@@ -58,7 +69,11 @@ struct piecewise_construct_t;
 template <class Ptr>
 struct pointer_traits;
 
+//Only forward declare ::std::from_range_t when the standard library actually
+//provides it; otherwise there is no such type to refer to.
+#if defined(__cpp_lib_containers_ranges)
 struct from_range_t;
+#endif
 
 BOOST_MOVE_STD_NS_END
 #include <boost/move/detail/std_ns_end.hpp>