Improve std::from_range detection for compilers not defining __cpp_lib_containers_ranges

Author: igaztanaga

include/boost/container/detail/range_utils.hpp

@@ -26,20 +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.
 //!
-//!   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.
+//!   When the standard library provides ::std::from_range_t (see
+//!   BOOST_CONTAINER_DETAIL_STD_FROM_RANGE_SUPPORT in std_fwd.hpp), 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)
+#if defined(BOOST_CONTAINER_DETAIL_STD_FROM_RANGE_SUPPORT)
 
    template <int Dummy = 0>
    struct std_from_range_holder
@@ -55,7 +56,7 @@ typedef const ::std::from_range_t & from_range_t;
 //! A instance of type from_range_t
 static from_range_t from_range = *std_from_range_holder<>::dummy;
 
-#else //!defined(__cpp_lib_containers_ranges)
+#else //!defined(BOOST_CONTAINER_DETAIL_STD_FROM_RANGE_SUPPORT)
 
 //The standard library does not provide ::std::from_range_t, so there is nothing
 //to interoperate with: from_range_t is a standalone type.
@@ -64,7 +65,7 @@ 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)
+#endif   //defined(BOOST_CONTAINER_DETAIL_STD_FROM_RANGE_SUPPORT)
 
 #else    //BOOST_CONTAINER_DOXYGEN_INVOKED
 

include/boost/container/detail/std_fwd.hpp

@@ -25,9 +25,8 @@
 
 #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).
+//<version> is needed to reliably query the C++23 library feature-test macros
+//for ::std::from_range_t / P1206R7. It is one of the lightest standard headers.
 #if defined(__has_include)
 #  if __has_include(<version>)
 #     include <version>
@@ -36,6 +35,22 @@
 #  include <version>
 #endif
 
+// Whether ::std::from_range_t tag is provided by the standard library.
+//
+// The official feature-test macro for std::from_range_t is
+// __cpp_lib_containers_ranges. However, std::from_range_t lives in <ranges>
+// next to std::ranges::to, whose specification uses the tag, so any library
+// that provides std::ranges::to necessarily provides std::from_range_t too.
+//
+// Some standard libraries (e.g. libc++ 17/18) ship std::ranges::to and
+// std::from_range_t -- defining __cpp_lib_ranges_to_container -- before they
+// complete the container support and define __cpp_lib_containers_ranges.
+// In that window only __cpp_lib_ranges_to_container is defined even though the
+// tag is already present.
+#if defined(__cpp_lib_ranges_to_container) || defined(__cpp_lib_containers_ranges)
+#  define BOOST_CONTAINER_DETAIL_STD_FROM_RANGE_SUPPORT
+#endif
+
 #include <boost/move/detail/std_ns_begin.hpp>
 BOOST_MOVE_STD_NS_BEG
 
@@ -70,8 +85,8 @@ 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)
+//provides it; see BOOST_CONTAINER_DETAIL_STD_FROM_RANGE_SUPPORT above.
+#if defined(BOOST_CONTAINER_DETAIL_STD_FROM_RANGE_SUPPORT)
 struct from_range_t;
 #endif