[RFC][libc++] Reduce the size of translation units

[Mordante]

Summary

The C++ standard library adds new features in every C++ version. To
implement these new features libc++ adds more includes to a header. This
means the number of transitive includes grows with newer language version.

The growing size is an issue for users. This change is motivated by a
report of the Chromium team where properly enabling C++23's
std::formatter<std::vector<bool>::reference> had a significant impact on
their build time.

In D149543 Hans Wennborg reported the following

In Chromium we noticed that this almost doubled the preprocessed size
of <vector>, from ca 1.6 MB to 3.2 MB. Since it's a widely included
header, that results in ca 8 GB (2.5%) of extra code to compile during a
full build.

This is an experiment to see how libc++ can reduce this size by only
"enabling" headers per language version. This is mainly done at the level
of the granularized includes, since they often have features for one or a
limited set of minimum language versions.

The compare header is an exception, its inclusion is often mandated by the
standard so this header is entirely guarded by C++20 or newer.

At the moment the removed transitive includes are not conditionally
restored like we usually do; this is intentional for the RFC. It gives a
good view what changes.

For this patch only vector is adjusted and mainly based on what I recall
being based on a language version.

I did some testing with the size of a preprocessed file that contains

#include <vector>

I used the following command

clang++ -E test.cpp -nostdinc++ -I <build>/include/c++/v1" -I -std=$std |wc -l

Version	Before	After
---------+--------+-------
C++03	53153	38721
C++11	55665	40914
C++14	56709	41896
C++17	60591	44957
C++20	75905	75863
C++23	61867	61828
C++26	61867	61828

The interesting value is C++23 (before) there the size drops a lot. In
C++23 some transitive includes are removed unconditionally. These can be
removed with a build option. The Chromimum team uses this option. This is
done by the following command

clang++ -E test.cpp -nostdinc++ -I <build>/include/c++/v1" -I -std=$std -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES |wc -l

Version	Before	After
---------+--------+-------
C++03	45883	18482
C++11	48366	19352
C++14	49352	19745
C++17	51590	22331
C++20	61259	61217
C++23	61867	61828
C++26	61867	61828

[Mordante]

libcxx/include/__format/formatter.h

14

Note <vector> includes this in C++20 even when it's only used in C++23. So in <vector> we could make this include optional.

[EricWF]

I think guarded includes is a really bad idea. I think it's inviting us to create include cycles that are only present in certain dialects & configurationsn.
I would much rather pay the size increase than do this.

[ldionne]

I was really excited about this when I saw the numbers initially. However, the benefits of this patch seem to go down very drastically after C++20, so I am left a bit unsure whether it's worth making this wide-ranging change in libc++. Indeed, we want to push our users to newer standard versions -- for example Clang now defaults to C++17, and eventually will default to C++20. In light of this, I am not sure it is worth shuffling around so much code for a benefit that will go away. Instead, I think the most reliable way of solving compilation time issues is going to be for users to adopt modules. I know this is a really frustrating answer to a lot of people, but concretely that's what WG21 is pushing people towards. For example, when they specify that <vector> should include <format>, they are thinking "yeah users will use modules anyway", otherwise they would make different design choices. So basically I think we might be fighting against the current if we try to optimize build times this way instead.

[Mordante]

@EricWF @ldionne Thanks for the feedback. This was intended as an RFC and an experiment to see what's possible.
Personally I was also not convinced this is worth the effort and I have not heard from the Chromium team since.

So the patch has served its purpose and I'll abandon it.

libcxx/include/__algorithm/lexicographical_compare_three_way.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___ALGORITHM_LEXICOGRAPHICAL_COMPARE_THREE_WAY_H
 #define _LIBCPP___ALGORITHM_LEXICOGRAPHICAL_COMPARE_THREE_WAY_H
 
-#include <__algorithm/min.h>
-#include <__algorithm/three_way_comp_ref_type.h>
-#include <__compare/compare_three_way.h>
-#include <__compare/ordering.h>
-#include <__concepts/arithmetic.h>
-#include <__config>
-#include <__iterator/iterator_traits.h>
-#include <__type_traits/common_type.h>
-#include <__type_traits/is_copy_constructible.h>
-#include <__utility/move.h>
+#include <__config>
+#if _LIBCPP_STD_VER >= 20
+
+#  include <__algorithm/min.h>
+#  include <__algorithm/three_way_comp_ref_type.h>
+#  include <__compare/compare_three_way.h>
+#  include <__compare/ordering.h>
+#  include <__concepts/arithmetic.h>
+#  include <__config>
+#  include <__iterator/iterator_traits.h>
+#  include <__type_traits/common_type.h>
+#  include <__type_traits/is_copy_constructible.h>
+#  include <__utility/move.h>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 20
-
 // Fast path for random access iterators which computes the number of loop iterations up-front and
 // then skips the iterator comparisons inside the loop.
 template <class _InputIterator1, class _InputIterator2, class _Cmp>
 _LIBCPP_HIDE_FROM_ABI constexpr auto __lexicographical_compare_three_way_fast_path(
     _InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 __first2, _InputIterator2 __last2, _Cmp& __comp)
     -> decltype(__comp(*__first1, *__first2)) {
   static_assert(
       signed_integral<__iter_diff_t<_InputIterator1>>, "Using a non-integral difference_type is undefined behavior.");

 
 template <class _InputIterator1, class _InputIterator2>
 _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr auto lexicographical_compare_three_way(
     _InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 __first2, _InputIterator2 __last2) {
   return std::lexicographical_compare_three_way(
       std::move(__first1), std::move(__last1), std::move(__first2), std::move(__last2), std::compare_three_way());
 }
 
-#endif // _LIBCPP_STD_VER >= 20
-
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
+#endif // _LIBCPP_STD_VER >= 20
 #endif // _LIBCPP___ALGORITHM_LEXICOGRAPHICAL_COMPARE_THREE_WAY_H

libcxx/include/__concepts/same_as.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___CONCEPTS_SAME_AS_H
 #define _LIBCPP___CONCEPTS_SAME_AS_H
 
 #include <__config>
-#include <__type_traits/is_same.h>
+#if _LIBCPP_STD_VER >= 20
+
+#  include <__type_traits/is_same.h>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 20
-
 // [concept.same]
 
 template <class _Tp, class _Up>
 concept __same_as_impl = _IsSame<_Tp, _Up>::value;
 
 template <class _Tp, class _Up>
 concept same_as = __same_as_impl<_Tp, _Up> && __same_as_impl<_Up, _Tp>;
 
-#endif // _LIBCPP_STD_VER >= 20
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 20
+
 #endif // _LIBCPP___CONCEPTS_SAME_AS_H

libcxx/include/__format/enable_insertable.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___FORMAT_ENABLE_INSERTABLE_H
 #define _LIBCPP___FORMAT_ENABLE_INSERTABLE_H
 
 #include <__config>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#if _LIBCPP_STD_VER >= 20
+
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 20
-
 namespace __format {
 
 /// Opt-in to enable \ref __insertable for a \a _Container.
 template <class _Container>
 inline constexpr bool __enable_insertable = false;
 
 } // namespace __format
 
-#endif //_LIBCPP_STD_VER >= 20
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif //_LIBCPP_STD_VER >= 20
+
 #endif // _LIBCPP___FORMAT_ENABLE_INSERTABLE_H

libcxx/include/__format/formatter.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___FORMAT_FORMATTER_H
 #define _LIBCPP___FORMAT_FORMATTER_H
 
-#include <__availability>
 #include <__config>
-#include <__format/format_fwd.h>
+#if _LIBCPP_STD_VER >= 20
+
+#  include <__availability>
+#  include <__format/format_fwd.h>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 20
-
 /// The default formatter template.
 ///
 /// [format.formatter.spec]/5
 /// If F is a disabled specialization of formatter, these values are false:
 /// - is_default_constructible_v<F>,
 /// - is_copy_constructible_v<F>,
 /// - is_move_constructible_v<F>,
 /// - is_copy_assignable<F>, and

 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr void __set_debug_format(_Tp& __formatter) {
   if constexpr (requires { __formatter.set_debug_format(); })
     __formatter.set_debug_format();
 }
 
 #  endif // _LIBCPP_STD_VER >= 23
-#endif   // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 20
 #endif // _LIBCPP___FORMAT_FORMATTER_H

libcxx/include/__format/formatter_bool.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___FORMAT_FORMATTER_BOOL_H
 #define _LIBCPP___FORMAT_FORMATTER_BOOL_H
 
-#include <__algorithm/copy.h>
-#include <__assert>
-#include <__availability>
-#include <__config>
-#include <__format/concepts.h>
-#include <__format/format_parse_context.h>
-#include <__format/formatter.h>
-#include <__format/formatter_integral.h>
-#include <__format/parser_std_format_spec.h>
-#include <__utility/unreachable.h>
+#include <__config>
+#if _LIBCPP_STD_VER >= 20
+
+#  include <__algorithm/copy.h>
+#  include <__assert>
+#  include <__availability>
+#  include <__format/concepts.h>
+#  include <__format/format_parse_context.h>
+#  include <__format/formatter.h>
+#  include <__format/formatter_integral.h>
+#  include <__format/parser_std_format_spec.h>
+#  include <__utility/unreachable.h>
 
-#ifndef _LIBCPP_HAS_NO_LOCALIZATION
-#  include <locale>
-#endif
+#  ifndef _LIBCPP_HAS_NO_LOCALIZATION
+#    include <locale>
+#  endif
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 20
-
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS formatter<bool, _CharT> {
 public:
   template <class _ParseContext>
   _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
     typename _ParseContext::iterator __result = __parser_.__parse(__ctx, __format_spec::__fields_integral);
     __format_spec::__process_parsed_bool(__parser_, "a bool");
     return __result;
default:
       _LIBCPP_ASSERT_UNCATEGORIZED(false, "The parse function should have validated the type");
       __libcpp_unreachable();
     }
   }
 
   __format_spec::__parser<_CharT> __parser_;
 };
 
-#endif //_LIBCPP_STD_VER >= 20
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif //_LIBCPP_STD_VER >= 20
+
 #endif // _LIBCPP___FORMAT_FORMATTER_BOOL_H

libcxx/include/__memory_resource/polymorphic_allocator.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___MEMORY_RESOURCE_POLYMORPHIC_ALLOCATOR_H
 #define _LIBCPP___MEMORY_RESOURCE_POLYMORPHIC_ALLOCATOR_H
 
-#include <__assert>
-#include <__availability>
-#include <__config>
-#include <__memory_resource/memory_resource.h>
-#include <__utility/exception_guard.h>
-#include <cstddef>
-#include <limits>
-#include <new>
-#include <stdexcept>
-#include <tuple>
+#include <__config>
+#if _LIBCPP_STD_VER >= 17
+
+#  include <__assert>
+#  include <__availability>
+#  include <__memory_resource/memory_resource.h>
+#  include <__utility/exception_guard.h>
+#  include <cstddef>
+#  include <limits>
+#  include <new>
+#  include <stdexcept>
+#  include <tuple>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
-#if _LIBCPP_STD_VER >= 17
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace pmr {
 
 // [mem.poly.allocator.class]
 
 template <class _ValueType
 #  if _LIBCPP_STD_VER >= 20

 }
 
 #  endif
 
 } // namespace pmr
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // _LIBCPP_STD_VER >= 17
-
 _LIBCPP_POP_MACROS
 
+#endif // _LIBCPP_STD_VER >= 17
+
 #endif // _LIBCPP___MEMORY_RESOURCE_POLYMORPHIC_ALLOCATOR_H

libcxx/include/__ranges/access.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___RANGES_ACCESS_H
 #define _LIBCPP___RANGES_ACCESS_H
 
-#include <__concepts/class_or_enum.h>
 #include <__config>
-#include <__iterator/concepts.h>
-#include <__iterator/readable_traits.h>
-#include <__ranges/enable_borrowed_range.h>
-#include <__type_traits/decay.h>
-#include <__type_traits/is_reference.h>
-#include <__type_traits/remove_cvref.h>
-#include <__type_traits/remove_reference.h>
-#include <__utility/auto_cast.h>
-#include <__utility/declval.h>
-#include <cstddef>
+#if _LIBCPP_STD_VER >= 20
+
+#  include <__concepts/class_or_enum.h>
+#  include <__iterator/concepts.h>
+#  include <__iterator/readable_traits.h>
+#  include <__ranges/enable_borrowed_range.h>
+#  include <__type_traits/decay.h>
+#  include <__type_traits/is_reference.h>
+#  include <__type_traits/remove_cvref.h>
+#  include <__type_traits/remove_reference.h>
+#  include <__utility/auto_cast.h>
+#  include <__utility/declval.h>
+#  include <cstddef>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 20
-
 namespace ranges {
   template <class _Tp>
   concept __can_borrow =
     is_lvalue_reference_v<_Tp> || enable_borrowed_range<remove_cvref_t<_Tp>>;
 } // namespace ranges
 
 // [range.access.begin]
 
namespace __cend {
   };
 } // namespace __cend
 
 inline namespace __cpo {
   inline constexpr auto cend = __cend::__fn{};
 } // namespace __cpo
 } // namespace ranges
 
-#endif // _LIBCPP_STD_VER >= 20
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 20
+
 #endif // _LIBCPP___RANGES_ACCESS_H

libcxx/include/__ranges/concepts.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___RANGES_CONCEPTS_H
 #define _LIBCPP___RANGES_CONCEPTS_H
 
-#include <__concepts/constructible.h>
-#include <__concepts/movable.h>
-#include <__concepts/same_as.h>
-#include <__config>
-#include <__iterator/concepts.h>
-#include <__iterator/incrementable_traits.h>
-#include <__iterator/iter_move.h>
-#include <__iterator/iterator_traits.h>
-#include <__iterator/readable_traits.h>
-#include <__ranges/access.h>
-#include <__ranges/data.h>
-#include <__ranges/enable_borrowed_range.h>
-#include <__ranges/enable_view.h>
-#include <__ranges/size.h>
-#include <__type_traits/add_pointer.h>
-#include <__type_traits/is_reference.h>
-#include <__type_traits/remove_cvref.h>
-#include <__type_traits/remove_reference.h>
-#include <__utility/declval.h>
-#include <initializer_list>
+#include <__config>
+#if _LIBCPP_STD_VER >= 20
+
+#  include <__concepts/constructible.h>
+#  include <__concepts/movable.h>
+#  include <__concepts/same_as.h>
+#  include <__iterator/concepts.h>
+#  include <__iterator/incrementable_traits.h>
+#  include <__iterator/iter_move.h>
+#  include <__iterator/iterator_traits.h>
+#  include <__iterator/readable_traits.h>
+#  include <__ranges/access.h>
+#  include <__ranges/data.h>
+#  include <__ranges/enable_borrowed_range.h>
+#  include <__ranges/enable_view.h>
+#  include <__ranges/size.h>
+#  include <__type_traits/add_pointer.h>
+#  include <__type_traits/is_reference.h>
+#  include <__type_traits/remove_cvref.h>
+#  include <__type_traits/remove_reference.h>
+#  include <__utility/declval.h>
+#  include <initializer_list>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 20
-
 namespace ranges {
 
   // [range.range]
 
   template <class _Tp>
   concept range = requires(_Tp& __t) {
     ranges::begin(__t); // sometimes equality-preserving
     ranges::end(__t);
concept viewable_range =
     range<_Tp> &&
     ((view<remove_cvref_t<_Tp>> && constructible_from<remove_cvref_t<_Tp>, _Tp>) ||
      (!view<remove_cvref_t<_Tp>> &&
       (is_lvalue_reference_v<_Tp> ||
        (movable<remove_reference_t<_Tp>> && !__is_std_initializer_list<remove_cvref_t<_Tp>>))));
 
 } // namespace ranges
 
-#endif // _LIBCPP_STD_VER >= 20
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 20
+
 #endif // _LIBCPP___RANGES_CONCEPTS_H

libcxx/include/__ranges/container_compatible_range.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___RANGES_CONTAINER_COMPATIBLE_RANGE_H
 #define _LIBCPP___RANGES_CONTAINER_COMPATIBLE_RANGE_H
 
-#include <__concepts/convertible_to.h>
 #include <__config>
-#include <__ranges/concepts.h>
+#if _LIBCPP_STD_VER >= 23
+
+#  include <__concepts/convertible_to.h>
+#  include <__ranges/concepts.h>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 23
-
 template <class _Range, class _Tp>
 concept _ContainerCompatibleRange =
     ranges::input_range<_Range> && convertible_to<ranges::range_reference_t<_Range>, _Tp>;
 
-#endif // _LIBCPP_STD_VER >= 23
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 23
+
 #endif // _LIBCPP___RANGES_CONTAINER_COMPATIBLE_RANGE_H

libcxx/include/__ranges/from_range.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___RANGES_FROM_RANGE_H
 #define _LIBCPP___RANGES_FROM_RANGE_H
 
 #include <__config>
+#if _LIBCPP_STD_VER >= 23
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 23
-
 struct from_range_t {
   explicit from_range_t() = default;
 };
 
 inline constexpr from_range_t from_range{};
 
-#endif // _LIBCPP_STD_VER >= 23
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 23
+
 #endif // _LIBCPP___RANGES_FROM_RANGE_H

libcxx/include/__ranges/size.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___RANGES_SIZE_H
 #define _LIBCPP___RANGES_SIZE_H
 
-#include <__concepts/arithmetic.h>
-#include <__concepts/class_or_enum.h>
-#include <__config>
-#include <__iterator/concepts.h>
-#include <__iterator/iterator_traits.h>
-#include <__ranges/access.h>
-#include <__type_traits/decay.h>
-#include <__type_traits/make_signed.h>
-#include <__type_traits/make_unsigned.h>
-#include <__type_traits/remove_cvref.h>
-#include <__utility/auto_cast.h>
-#include <__utility/declval.h>
-#include <cstddef>
+#include <__config>
+#if _LIBCPP_STD_VER >= 20
+
+#  include <__concepts/arithmetic.h>
+#  include <__concepts/class_or_enum.h>
+#  include <__iterator/concepts.h>
+#  include <__iterator/iterator_traits.h>
+#  include <__ranges/access.h>
+#  include <__type_traits/decay.h>
+#  include <__type_traits/make_signed.h>
+#  include <__type_traits/make_unsigned.h>
+#  include <__type_traits/remove_cvref.h>
+#  include <__utility/auto_cast.h>
+#  include <__utility/declval.h>
+#  include <cstddef>
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 20
-
 namespace ranges {
   template<class>
   inline constexpr bool disable_sized_range = false;
 } // namespace ranges
 
 // [range.prim.size]
 
 namespace ranges {

 };
 } // namespace __ssize
 
 inline namespace __cpo {
   inline constexpr auto ssize = __ssize::__fn{};
 } // namespace __cpo
 } // namespace ranges
 
-#endif // _LIBCPP_STD_VER >= 20
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 20
+
 #endif // _LIBCPP___RANGES_SIZE_H

libcxx/include/compare

public:
     friend constexpr bool operator>=(unspecified, strong_ordering v) noexcept;
     friend constexpr strong_ordering operator<=>(strong_ordering v, unspecified) noexcept;
     friend constexpr strong_ordering operator<=>(unspecified, strong_ordering v) noexcept;
   };
 }
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
+#include <__config>
+#include <version>
+#if _LIBCPP_STD_VER >= 20
+
 #include <__compare/common_comparison_category.h>
 #include <__compare/compare_partial_order_fallback.h>
 #include <__compare/compare_strong_order_fallback.h>
 #include <__compare/compare_three_way.h>
 #include <__compare/compare_three_way_result.h>
 #include <__compare/compare_weak_order_fallback.h>
 #include <__compare/is_eq.h>
 #include <__compare/ordering.h>
 #include <__compare/partial_order.h>
 #include <__compare/strong_order.h>
 #include <__compare/synth_three_way.h>
 #include <__compare/three_way_comparable.h>
 #include <__compare/weak_order.h>
-#include <__config>
-#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
+#endif // _LIBCPP_STD_VER >= 20
+
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <type_traits>
 #endif
 
 #endif // _LIBCPP_COMPARE

libcxx/test/libcxx/transitive_includes/cxx03.csv

 codecvt limits
 codecvt mutex
 codecvt new
 codecvt stdexcept
 codecvt string
 codecvt type_traits
 codecvt typeinfo
 codecvt version
-compare cmath
-compare cstddef
-compare cstdint
-compare limits
 compare type_traits
 compare version
 complex cmath
 complex iosfwd
 complex sstream
 complex stdexcept
 complex type_traits
 complex version

 deque cwchar
 deque functional
 deque initializer_list
 deque iosfwd
 deque iterator
 deque limits
 deque new
 deque stdexcept
-deque tuple
 deque type_traits
 deque typeinfo
 deque version
 exception cstddef
 exception cstdlib
 exception type_traits
 exception version
 execution cstddef

 forward_list cstdlib
 forward_list cstring
 forward_list functional
 forward_list initializer_list
 forward_list iosfwd
 forward_list iterator
 forward_list limits
 forward_list new
-forward_list stdexcept
-forward_list tuple
 forward_list type_traits
 forward_list typeinfo
 forward_list version
 fstream atomic
 fstream cctype
 fstream clocale
 fstream concepts
 fstream cstddef

 list cstdlib
 list cstring
 list functional
 list initializer_list
 list iosfwd
 list iterator
 list limits
 list new
-list stdexcept
-list tuple
 list type_traits
 list typeinfo
 list version
 locale atomic
 locale cctype
 locale cerrno
 locale clocale
 locale concepts

 memory stdexcept
 memory tuple
 memory type_traits
 memory typeinfo
 memory utility
 memory version
 memory_resource cstddef
 memory_resource cstdint
-memory_resource limits
 memory_resource mutex
-memory_resource new
-memory_resource stdexcept
-memory_resource tuple
 memory_resource version
 mutex atomic
 mutex cerrno
 mutex concepts
 mutex cstddef
 mutex cstdint
 mutex cstdlib
 mutex cstring

 set cstdlib
 set functional
 set initializer_list
 set iterator
 set limits
 set new
 set optional
 set stdexcept
-set tuple
 set type_traits
 set version
 shared_mutex cerrno
 shared_mutex cstddef
 shared_mutex ctime
 shared_mutex limits
 shared_mutex ratio
 shared_mutex stdexcept
 shared_mutex string
 shared_mutex system_error
 shared_mutex type_traits
 shared_mutex version
 source_location cstdint
 source_location version
 span array
 span concepts
 span cstddef
 span functional
-span initializer_list
 span iterator
 span limits
 span type_traits
 span version
 sstream cstddef
 sstream istream
 sstream ostream
 sstream string

 stack type_traits
 stack version
 stdexcept cstdlib
 stdexcept exception
 stdexcept iosfwd
 stop_token atomic
 stop_token cstddef
 stop_token cstdint
+stop_token cstring
+stop_token ctime
 stop_token limits
+stop_token ratio
 stop_token thread
+stop_token type_traits
 stop_token version
 streambuf cstdint
 streambuf ios
 streambuf iosfwd
 streambuf version
 string algorithm
 string climits
 string compare
 string concepts
 string cstddef
 string cstdint
 string cstdio
 string cstdlib
 string cstring
 string cwchar
 string initializer_list
 string iosfwd
 string iterator
 string limits
 string new
 string stdexcept
 string string_view
-string tuple
 string type_traits
 string typeinfo
 string utility
 string version
 string_view algorithm
 string_view compare
 string_view concepts
 string_view cstddef

 unordered_set cstdlib
 unordered_set cstring
 unordered_set functional
 unordered_set initializer_list
 unordered_set iterator
 unordered_set limits
 unordered_set new
 unordered_set optional
-unordered_set stdexcept
-unordered_set tuple
 unordered_set type_traits
 unordered_set version
 utility compare
 utility cstddef
 utility cstdlib
 utility initializer_list
 utility iosfwd
 utility limits

 variant limits
 variant new
 variant tuple
 variant type_traits
 variant typeinfo
 variant utility
 variant version
 vector algorithm
-vector array
 vector atomic
-vector cerrno
 vector climits
 vector compare
 vector concepts
 vector cstddef
 vector cstdint
 vector cstdlib
 vector cstring
 vector cwchar
 vector initializer_list
 vector iosfwd
 vector limits
-vector locale
 vector new
 vector stdexcept
-vector string
-vector string_view
-vector tuple
 vector type_traits
 vector typeinfo
 vector utility
 vector version

libcxx/test/libcxx/transitive_includes/cxx11.csv

 codecvt limits
 codecvt mutex
 codecvt new
 codecvt stdexcept
 codecvt string
 codecvt type_traits
 codecvt typeinfo
 codecvt version
-compare cmath
-compare cstddef
-compare cstdint
-compare limits
 compare type_traits
 compare version
 complex cmath
 complex iosfwd
 complex sstream
 complex stdexcept
 complex type_traits
 complex version

 deque cwchar
 deque functional
 deque initializer_list
 deque iosfwd
 deque iterator
 deque limits
 deque new
 deque stdexcept
-deque tuple
 deque type_traits
 deque typeinfo
 deque version
 exception cstddef
 exception cstdlib
 exception type_traits
 exception version
 execution cstddef

 forward_list cstdlib
 forward_list cstring
 forward_list functional
 forward_list initializer_list
 forward_list iosfwd
 forward_list iterator
 forward_list limits
 forward_list new
-forward_list stdexcept
-forward_list tuple
 forward_list type_traits
 forward_list typeinfo
 forward_list version
 fstream atomic
 fstream cctype
 fstream clocale
 fstream concepts
 fstream cstddef

 list cstdlib
 list cstring
 list functional
 list initializer_list
 list iosfwd
 list iterator
 list limits
 list new
-list stdexcept
-list tuple
 list type_traits
 list typeinfo
 list version
 locale atomic
 locale cctype
 locale cerrno
 locale clocale
 locale concepts

 memory stdexcept
 memory tuple
 memory type_traits
 memory typeinfo
 memory utility
 memory version
 memory_resource cstddef
 memory_resource cstdint
-memory_resource limits
 memory_resource mutex
-memory_resource new
-memory_resource stdexcept
-memory_resource tuple
 memory_resource version
 mutex atomic
 mutex cerrno
 mutex concepts
 mutex cstddef
 mutex cstdint
 mutex cstdlib
 mutex cstring

 set cstdlib
 set functional
 set initializer_list
 set iterator
 set limits
 set new
 set optional
 set stdexcept
-set tuple
 set type_traits
 set version
 shared_mutex cerrno
 shared_mutex cstddef
 shared_mutex ctime
 shared_mutex limits
 shared_mutex ratio
 shared_mutex stdexcept
 shared_mutex string
 shared_mutex system_error
 shared_mutex type_traits
 shared_mutex version
 source_location cstdint
 source_location version
 span array
 span concepts
 span cstddef
 span functional
-span initializer_list
 span iterator
 span limits
 span type_traits
 span version
 sstream cstddef
 sstream istream
 sstream ostream
 sstream string

 stack type_traits
 stack version
 stdexcept cstdlib
 stdexcept exception
 stdexcept iosfwd
 stop_token atomic
 stop_token cstddef
 stop_token cstdint
+stop_token cstring
+stop_token ctime
 stop_token limits
+stop_token ratio
 stop_token thread
+stop_token type_traits
 stop_token version
 streambuf cstdint
 streambuf ios
 streambuf iosfwd
 streambuf version
 string algorithm
 string climits
 string compare
 string concepts
 string cstddef
 string cstdint
 string cstdio
 string cstdlib
 string cstring
 string cwchar
 string initializer_list
 string iosfwd
 string iterator
 string limits
 string new
 string stdexcept
 string string_view
-string tuple
 string type_traits
 string typeinfo
 string utility
 string version
 string_view algorithm
 string_view compare
 string_view concepts
 string_view cstddef

 unordered_set cstdlib
 unordered_set cstring
 unordered_set functional
 unordered_set initializer_list
 unordered_set iterator
 unordered_set limits
 unordered_set new
 unordered_set optional
-unordered_set stdexcept
-unordered_set tuple
 unordered_set type_traits
 unordered_set version
 utility compare
 utility cstddef
 utility cstdlib
 utility initializer_list
 utility iosfwd
 utility limits

 variant limits
 variant new
 variant tuple
 variant type_traits
 variant typeinfo
 variant utility
 variant version
 vector algorithm
-vector array
 vector atomic
-vector cerrno
 vector climits
 vector compare
 vector concepts
 vector cstddef
 vector cstdint
 vector cstdlib
 vector cstring
 vector cwchar
 vector initializer_list
 vector iosfwd
 vector limits
-vector locale
 vector new
 vector stdexcept
-vector string
-vector string_view
-vector tuple
 vector type_traits
 vector typeinfo
 vector utility
 vector version

libcxx/test/libcxx/transitive_includes/cxx14.csv

 codecvt limits
 codecvt mutex
 codecvt new
 codecvt stdexcept
 codecvt string
 codecvt type_traits
 codecvt typeinfo
 codecvt version
-compare cmath
-compare cstddef
-compare cstdint
-compare limits
 compare type_traits
 compare version
 complex cmath
 complex iosfwd
 complex sstream
 complex stdexcept
 complex type_traits
 complex version

 deque cwchar
 deque functional
 deque initializer_list
 deque iosfwd
 deque iterator
 deque limits
 deque new
 deque stdexcept
-deque tuple
 deque type_traits
 deque typeinfo
 deque version
 exception cstddef
 exception cstdlib
 exception type_traits
 exception version
 execution cstddef

 forward_list cstdlib
 forward_list cstring
 forward_list functional
 forward_list initializer_list
 forward_list iosfwd
 forward_list iterator
 forward_list limits
 forward_list new
-forward_list stdexcept
-forward_list tuple
 forward_list type_traits
 forward_list typeinfo
 forward_list version
 fstream atomic
 fstream cctype
 fstream clocale
 fstream concepts
 fstream cstddef

 list cstdlib
 list cstring
 list functional
 list initializer_list
 list iosfwd
 list iterator
 list limits
 list new
-list stdexcept
-list tuple
 list type_traits
 list typeinfo
 list version
 locale atomic
 locale cctype
 locale cerrno
 locale clocale
 locale concepts

 memory stdexcept
 memory tuple
 memory type_traits
 memory typeinfo
 memory utility
 memory version
 memory_resource cstddef
 memory_resource cstdint
-memory_resource limits
 memory_resource mutex
-memory_resource new
-memory_resource stdexcept
-memory_resource tuple
 memory_resource version
 mutex atomic
 mutex cerrno
 mutex concepts
 mutex cstddef
 mutex cstdint
 mutex cstdlib
 mutex cstring

 set cstdlib
 set functional
 set initializer_list
 set iterator
 set limits
 set new
 set optional
 set stdexcept
-set tuple
 set type_traits
 set version
 shared_mutex cerrno
 shared_mutex cstddef
 shared_mutex ctime
 shared_mutex limits
 shared_mutex ratio
 shared_mutex stdexcept
 shared_mutex string
 shared_mutex system_error
 shared_mutex type_traits
 shared_mutex version
 source_location cstdint
 source_location version
 span array
 span concepts
 span cstddef
 span functional
-span initializer_list
 span iterator
 span limits
 span type_traits
 span version
 sstream cstddef
 sstream istream
 sstream ostream
 sstream string

 stack type_traits
 stack version
 stdexcept cstdlib
 stdexcept exception
 stdexcept iosfwd
 stop_token atomic
 stop_token cstddef
 stop_token cstdint
+stop_token cstring
+stop_token ctime
 stop_token limits
+stop_token ratio
 stop_token thread
+stop_token type_traits
 stop_token version
 streambuf cstdint
 streambuf ios
 streambuf iosfwd
 streambuf version
 string algorithm
 string climits
 string compare
 string concepts
 string cstddef
 string cstdint
 string cstdio
 string cstdlib
 string cstring
 string cwchar
 string initializer_list
 string iosfwd
 string iterator
 string limits
 string new
 string stdexcept
 string string_view
-string tuple
 string type_traits
 string typeinfo
 string utility
 string version
 string_view algorithm
 string_view compare
 string_view concepts
 string_view cstddef

 unordered_set cstdlib
 unordered_set cstring
 unordered_set functional
 unordered_set initializer_list
 unordered_set iterator
 unordered_set limits
 unordered_set new
 unordered_set optional
-unordered_set stdexcept
-unordered_set tuple
 unordered_set type_traits
 unordered_set version
 utility compare
 utility cstddef
 utility cstdlib
 utility initializer_list
 utility iosfwd
 utility limits

 variant limits
 variant new
 variant tuple
 variant type_traits
 variant typeinfo
 variant utility
 variant version
 vector algorithm
-vector array
 vector atomic
-vector cerrno
 vector climits
 vector compare
 vector concepts
 vector cstddef
 vector cstdint
 vector cstdlib
 vector cstring
 vector cwchar
 vector initializer_list
 vector iosfwd
 vector limits
-vector locale
 vector new
 vector stdexcept
-vector string
-vector string_view
-vector tuple
 vector type_traits
 vector typeinfo
 vector utility
 vector version

libcxx/test/libcxx/transitive_includes/cxx17.csv

 codecvt limits
 codecvt mutex
 codecvt new
 codecvt stdexcept
 codecvt string
 codecvt type_traits
 codecvt typeinfo
 codecvt version
-compare cmath
-compare cstddef
-compare cstdint
-compare limits
 compare type_traits
 compare version
 complex cmath
 complex iosfwd
 complex sstream
 complex stdexcept
 complex type_traits
 complex version

 shared_mutex type_traits
 shared_mutex version
 source_location cstdint
 source_location version
 span array
 span concepts
 span cstddef
 span functional
-span initializer_list
 span iterator
 span limits
 span type_traits
 span version
 sstream cstddef
 sstream istream
 sstream ostream
 sstream string

 stack type_traits
 stack version
 stdexcept cstdlib
 stdexcept exception
 stdexcept iosfwd
 stop_token atomic
 stop_token cstddef
 stop_token cstdint
+stop_token cstring
+stop_token ctime
 stop_token limits
+stop_token ratio
 stop_token thread
+stop_token type_traits
 stop_token version
 streambuf cstdint
 streambuf ios
 streambuf iosfwd
 streambuf version
 string algorithm
 string climits
 string compare

 variant limits
 variant new
 variant tuple
 variant type_traits
 variant typeinfo
 variant utility
 variant version
 vector algorithm
-vector array
 vector atomic
-vector cerrno
 vector climits
 vector compare
 vector concepts
 vector cstddef
 vector cstdint
 vector cstdlib
 vector cstring
 vector cwchar
 vector initializer_list
 vector iosfwd
 vector limits
-vector locale
 vector new
 vector stdexcept
-vector string
-vector string_view
 vector tuple
 vector type_traits
 vector typeinfo
 vector utility
 vector version