Freestanding Feature-Test Macros and Implementation-Defined Extensions

Document number: P2198R0
Date: 2020-07-12
Reply-to: Ben Craig <ben dot craig at gmail dot com>
Audience: Library Evolution Working Group

Change history

R0

Branching from P1641R3 and P1642R3.
Freestanding feature-test macros now required to be present in hosted.
Added examples and use cases.
Elaborating on how an implementation can extend the bare minimum of freestanding.

Introduction

Users of freestanding implementations would like to be able to detect which library facilities are available for use. With the C++20 feature-test macros, all the feature-test macros are required to be defined in both freestanding and hosted implementations, even when the associated hosted facilities are not available. This paper will make the presence of feature-test macros for hosted facilities implementation-defined. This paper will also bump the version number on all library feature-test macros, so that extensions can be detected.

This paper also clarifies that freestanding implementations need not stop at the bare minimum support for freestanding. Freestanding implementations may provide additional facilities from the standard library, so long as those facilities meet the associated hosted requirements.

Motivation and Design

Beyond the bare minimum

This paper grants implementations the freedom to include more than just the facilities that are marked //freestanding. There will be platforms where, for example, floating point is both available and frequently used (e.g. GPU environments). On those platforms, it is desirable for implementations to provide many of the facilities in the <cmath> and <complex> headers. We must be careful here though, as we don't want to allow divergence of implementation, and we want to be able to add facilities to the required subset of freestanding. To permit both vendor extension and future freestanding growth, this paper will require that additionally included facilities must meet the same requirements as for a hosted implementation, with a few exceptions.

Freestanding implementations may include additional macros and namespace scoped entities, beyond what is required in a minimal freestanding implementation. A freestanding implementation does not need to provide the entirety of a header. If a freestanding implementation supplies an additional class, the entirety of the class must be present and meet all the hosted requirements. This prevents some useful vendor extensions, notably at()-less array, value()-less optional, and similarly partial string_view and bitset. This does not prevent us from standardizing a partial class in the future, though it does prevent us from getting usage experience in a conforming implementation.

Freestanding implementations may make the use of any non-freestanding, namespace scoped function or function template ill-formed (e.g. generally by marking it =delete). The intent is to allow the freestanding implementation to control overload resolution, so that a library that works with a freestanding implementation will have the same semantics in a hosted implementation.

Users will be able to rely on the freestanding portions of the standard library. The freestanding portions have portable semantics, and are required to be present. Users may then decide to rely on implementation-defined additions. WG21 will be able to add hosted entities to the freestanding subset without little fear of breaking implementation-defined additions, because the implementation-defined additions needed to meet the hosted requirements.

Library feature-test macro bump

C++20 requires that all implementations (freestanding and hosted) define all of the library feature-test macros in the <version> header, even the feature-test macros that correspond to facilities not required to be present in freestanding implementations. This means that those feature-test macros provide misleading results on freestanding implementations. This isn't just a theoretical problem. Existing implementations are already deploying <version> headers that report support for std::filesystem facilities, std::chrono facilities, and many others, even though those feature-test macros indicate support for features that require the support of an operating system.

In order for users of freestanding implementations to be able to detect extensions of freestanding, the users need a way of distinguishing the C++20 macro requirements from an accurate expression of extension. This paper will bump all library macro versions, so that users can distinguish between these cases.

Freestanding feature-test macros

The following macros are now required to be present in freestanding implementations. The other library feature test macros in the <version> header are not required to be present on freestanding implementations. The corresponding facilities were either required to be present in C++20, or are added in P1642.

__cpp_lib_addressof_constexpr
__cpp_lib_allocator_traits_is_always_equal
__cpp_lib_apply
__cpp_lib_as_const
__cpp_lib_assume_aligned
__cpp_lib_atomic_flag_test
__cpp_lib_atomic_float
__cpp_lib_atomic_is_always_lock_free
__cpp_lib_atomic_ref
__cpp_lib_atomic_value_initialization
__cpp_lib_atomic_wait
__cpp_lib_bind_front
__cpp_lib_bit_cast
__cpp_lib_bitops
__cpp_lib_bool_constant
__cpp_lib_bounded_array_traits
__cpp_lib_byte
__cpp_lib_char8_t
__cpp_lib_concepts
__cpp_lib_constexpr_functional
__cpp_lib_constexpr_iterator
__cpp_lib_constexpr_memory
__cpp_lib_constexpr_tuple
__cpp_lib_constexpr_utility
__cpp_lib_destroying_delete
__cpp_lib_endian
__cpp_lib_exchange_function
__cpp_lib_hardware_interference_size
__cpp_lib_has_unique_object_representations
__cpp_lib_int_pow2
__cpp_lib_integer_sequence
__cpp_lib_integral_constant_callable
__cpp_lib_invoke
__cpp_lib_is_aggregate
__cpp_lib_is_constant_evaluated
__cpp_lib_is_final
__cpp_lib_is_invocable
__cpp_lib_is_layout_compatible
__cpp_lib_is_nothrow_convertible
__cpp_lib_is_null_pointer
__cpp_lib_is_pointer_interconvertible
__cpp_lib_is_swappable
__cpp_lib_launder
__cpp_lib_logical_traits
__cpp_lib_make_from_tuple
__cpp_lib_make_reverse_iterator
__cpp_lib_nonmember_container_access
__cpp_lib_not_fn
__cpp_lib_null_iterators
__cpp_lib_remove_cvref
__cpp_lib_result_of_sfinae
__cpp_lib_source_location
__cpp_lib_ssize
__cpp_lib_three_way_comparison
__cpp_lib_to_address
__cpp_lib_transformation_trait_aliases
__cpp_lib_transparent_operators
__cpp_lib_tuple_element_t
__cpp_lib_tuples_by_type
__cpp_lib_type_identity
__cpp_lib_type_trait_variable_templates
__cpp_lib_uncaught_exceptions
__cpp_lib_unwrap_ref
__cpp_lib_void_t

New feature-test macros

Users of freestanding implementations will want to know whether including a formerly hosted-only header will work or not. Users of freestanding implementations will also what to know if all the facilities that are required to be in freestanding have been made available yet. This is a concern for highly portable libraries, and for users that need to support old and new compilers.

These feature-test macros are provided at a per-header granularity. This enables implementations to advertise new capabilities more easily than a single feature-test macro for the entirety of this paper. This also follows the precedent set by the __cpp_lib_constexpr_* macros.

If new, pre-C++20 functionality is added to the freestanding subset of C++, then the respective feature-test macro for the header should be bumped. If the functionality is new in C++23 or later, then alternative approaches should be taken. These alternative approaches are discussed in the examples section.

Name	Header
`__cpp_lib_freestanding_utility`	`<utility>`
`__cpp_lib_freestanding_tuple`	`<tuple>`
`__cpp_lib_freestanding_ratio`	`<ratio>`
`__cpp_lib_freestanding_memory`	`<memory>`
`__cpp_lib_freestanding_functional`	`<functional>`
`__cpp_lib_freestanding_iterator`	`<iterator>`
`__cpp_lib_freestanding_ranges`	`<ranges>`

Partial macro coverage

The following, existing feature-test macros cover some features that I am making freestanding, and some features that I am not requiring to be freestanding. These feature-test macros won't be required in freestanding, as they could cause substantial confusion when the hosted parts of those features aren't available. The per-header __cpp_lib_freestanding_* macros should provide a suitable replacement in freestanding environments.

__cpp_lib_boyer_moore_searcher: default_searcher is in, other searchers require the heap.
__cpp_lib_constexpr_dynamic_alloc: constexprification of various memory algorithms is in, anything dealing with std::allocator is out
__cpp_lib_ranges: stream iterators are out
__cpp_lib_raw_memory_algorithms: ExecutionPolicy overloads are out

Feature-test macro policy recommendations

This paper patches up many of the historical problems with freestanding and feature-test macros. The following are the guidelines I recommend to keep feature-test macros useful for freestanding in the future.

If a paper contains some facilities that are freestanding, and some that are hosted, then the paper needs at least two feature test macros. __cpp_lib_foo will cover the full paper in the hosted case, and __cpp_lib_freestanding_foo will cover the freestanding portions.
If a paper adds facilities to the hosted implementation and has a feature-test macro, and a later paper adds the entirety of the paper to freestanding, then bump the version of the original feature-test macro.
If a paper adds facilities to the hosted implementation and has a feature-test macro, and a later paper adds a portion of the paper to freestanding, then add a new freestanding macro for the feature, of the form __cpp_lib_freestanding_foo.

Examples

The following feature detection examples should ...

... work on freestanding, hosted, and with implementation-defined freestanding extensions.
... work with old standards and new standards.
... provide a conservative "false" answer when working with an old standard.
... not cause warnings for the use of an undefined macro, like with -Wundef.

Detect C++20 (or older) feature that is not required in freestanding

Is filesystem::copy available?

#if __STDC_HOSTED__ && defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L
// hosted success path
#elif defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 202007L
// freestanding extension path
#else
// fallback path
#endif

Detect C++20 (or older) feature that is now entirely required in freestanding

Is ssize available?

#if __STDC_HOSTED__ && defined(__cpp_lib_ssize) && __cpp_lib_ssize >= 201902L
// hosted success path
#elif defined(__cpp_lib_ssize) && __cpp_lib_ssize >= 202007L
// freestanding path
#else
// fallback path
#endif

Detect C++20 (or older) feature that is now partially required in freestanding

Is the non-parallel version of uninitialized_default_construct available?

#if defined(__cpp_lib_raw_memory_algorithms) && __cpp_lib_raw_memory_algorithms >= 201606L
// hosted success path and freestanding extension path
#elif defined(__cpp_lib_freestanding_memory) && __cpp_lib_freestanding_memory >= 202007L
// freestanding path
#else
// fallback path
#endif

Detect pre-feature-test-macro feature that is required in freestanding

Is tuple available?

#if defined(__cpp_lib_freestanding_tuple) && __cpp_lib_freestanding_tuple >= 202007L
// freestanding and future hosted success path
#elif __STDC_HOSTED__
// flakey hosted success path.  Assume tuple works here
#else
// fallback path
#endif

Detect pre-feature-test-macro feature that is not required in freestanding

Is thread available?

// No good answers here.  Currently calling this out of scope.

Detect C++23 feature that is entirely freestanding in the initial paper

#if defined(__cpp_lib_always_freestanding_feature) && __cpp_lib_always_freestanding_feature >= 202202L
// hosted and freestanding success path
#else
// fallback path
#endif

Detect C++23 feature that is hosted in the initial paper, but made entirely freestanding later (possibly in another version of C++)

For this category of features, the recommendation will be to bump the integer constant in the version test macro.

#if defined(__cpp_lib_eventually_freestanding_feature)
    #if __cpp_lib_eventually_freestanding_feature >= 202702L
    // freestanding success path.  Will also trigger for new hosted toolchains
    #elif __STDC_HOSTED__ && __cpp_lib_eventually_freestanding_feature >= 202102L
    // Interim hosted success path
    #else
    // Interim freestanding fallback path
    #endif
#else
// fallback path
#endif

// Alternative that will work 99% of the time
#if defined(__cpp_lib_eventually_freestanding_feature)
// freestanding and hosted success, probably
#else
// fallback path
#endif

Detect freestanding part of C++23 feature that is partially freestanding in the initial paper

For this category of features, the recommendation will be to have two macros, one for the freestanding portion, and one for the entire paper. The macros should follow the convention __cpp_lib_foo for the full paper, and __cpp_lib_freestanding_foo for the freestanding portion.
Is the freestanding portion of feature foo available?

#if defined(__cpp_lib_freestanding_foo) && __cpp_lib_freestanding_foo >= 202202L
// hosted and freestanding success path
#else
// fallback path
#endif

Detect freestanding part of C++23 feature that is made freestanding after the initial paper (possibly in a different C++ release)

For this category of features, the recommendation will be to add a new freestanding macro for the feature, following the convention of __cpp_lib_freestanding_bar for the freestanding portion.
Is the freestanding portion of feature bar available?

#if defined(__cpp_lib_bar) && __cpp_lib_bar >= 202202L
// Old hosted toolchain path and freestanding extension path
#elif defined(__cpp_lib_freestanding_bar) && __cpp_lib_freestanding_bar >= 202702L
// freestanding success path.  Will also trigger for new hosted toolchains
#else
// fallback path
#endif

Rejected alternatives

Allow partial classes in conforming vendor extensions to freestanding

Allowing partial classes gives implementers a great deal of freedom to experiment and to provide extensions. It also permits abuse by the implementers. On the positive side, an implementation could provide std::array without at. This helps replace C facilities with better facilities from C++. On the negative side, an implementation could =delete the copy constructor. Removing the wrong functions would also mean that a class may not satisfy the same concepts in hosted vs. freestanding.

Update old feature-test macros to indicate freestanding status

Rather than have a __cpp_lib_freestanding_ratio (for example), we could instead update __cpp_lib_chrono. Users could test the value of __cpp_lib_chrono to determine if it is freestanding safe or not.

There are at least two big flaws with this approach.

First, not all facilities (particularly older ones) have existing feature-test macros to bump (e.g. unique_ptr). Users would still like to be able to detect the availability of the features in freestanding.

Second, implementers don't always implement features in the same order that they are added to the working draft. If a new feature were added to the <chrono> header that necessitated a feature-test macro bump, and an implementer addressed that feature before making the <ratio> header freestanding safe, then the implementer would either need to stick with the pre-freestanding feature-test macro version, or provide a misleading feature-test macro version.

Per-freestanding paper feature-test macro granularity

Rather than introduce a feature-test macro per header in this paper, I could instead introduce one feature-test macro... perhaps __cpp_lib_freestanding. Each paper that adds old facilities to freestanding could then introduce it's own macro, or bump the old one.

This approach can work, but it restricts the order in which implementers can meaningfully implement freestanding features. All of the old paper needs to be done before any of the old paper's progress can be advertised. All of the old paper needs to be done before advertising any newer papers.

Per-facility feature-test macro granularity

This paper could choose to add a feature-test macro for __cpp_lib_freestanding_unique_ptr, __cpp_lib_freestanding_pair, and __cpp_lib_freestanding_iterator_categories. There would be some value to users in that they could express exactly what it is they need, and see if that very specific facility is available.

However, this approach is an implementation hassle, and prone to endless wg21 debates on how to partition and name the facilities. Grouping facilities by header provides a natural partitioning and naming scheme.

Rely on `__has_include`

Some have suggested using __has_include to detect whether the <tuple> header (for example) is usable on a particular freestanding implementation. This doesn't work in practice for multiple reasons.

The first example is the <iterator> header. In older versions of Visual Studio, a user could attempt to #include <iterator>. The streaming iterators in the header result in compiler errors. We need feature-test macros to indicate whether including the header is well-formed.

The second example is headers that do standard versions checks inside. The libstdc++ implementation of the <ranges> header is mostly empty if the language version is less than C++20. __has_include will still report the header as present though.

Don't bump all the library feature-test macros

This makes it even more difficult to detect extensions to freestanding. Existing implementations report support for __cpp_lib_filesystem when the -ffreestanding flag is passed, even though it is unlikely that filesystem is supported on such platforms. The C++20 standard currently requires this behavior.

Make `__cpp_lib_freestanding_*` macros freestanding only

This complicates the client feature-test code for features that were marked freestanding in their initial papers. It doesn't make any of the client feature-test code any simpler. It also violates the principle that freestanding should be a subset of hosted.

Define `__cpp_lib_freestanding_*` macros to 0 in hosted

Users of the freestanding macros would still need to account for the case where the macro isn't present. Defining the macro to zero doesn't provide any information beyond what __STDC_HOSTED__ provides.

Wording

Additions to [compliance]

Please append the following paragraphs to [compliance].

4

The hosted library facilities are the set of facilities described in this document that are required for hosted implementations, but not required for freestanding implementations.

A freestanding implementation provides a (possibly empty) implementation-defined subset of the hosted library facilities.

Unless otherwise specified, the requirements on each macro and namespace scoped entity provided in this way shall be the same as the corresponding requirements in a hosted implementation.

5

A freestanding implementation provides deleted definitions for a (possibly empty) implementation-defined subset of the namespace scoped functions and function templates from the hosted library facilities.

[ Note: An implementation may provide a deleted definition so that overload resolution does not silently change when migrating a library from a freestanding implementation to a hosted implementation. -end note]

Change in [version.syn]

Please make the following changes to [version.syn]. Use the current year-based constant instead of "new-val".

#define __cpp_lib_addressof_constexpr               201603Lnew-val // freestanding, also in <memory>
#define __cpp_lib_allocator_traits_is_always_equal  201411Lnew-val
  // freestanding, also in <memory>, <scoped-allocator>, <string>, <deque>, <forward-list>, <list>, <vector>,
  // <map>, <set>, <unordered-map>, <unordered-set>
#define __cpp_lib_any                               201606Lnew-val // also in <any>
#define __cpp_lib_apply                             201603Lnew-val // freestanding, also in <tuple>
#define __cpp_lib_array_constexpr                   201811Lnew-val // also in <iterator>, <array>
#define __cpp_lib_as_const                          201510Lnew-val // freestanding, also in <utility>
#define __cpp_lib_assume_aligned                    201811Lnew-val // freestanding, also in <memory>
#define __cpp_lib_atomic_flag_test                  201907Lnew-val // freestanding, also in <atomic>
#define __cpp_lib_atomic_float                      201711Lnew-val // freestanding, also in <atomic>
#define __cpp_lib_atomic_is_always_lock_free        201603Lnew-val // freestanding, also in <atomic>
#define __cpp_lib_atomic_lock_free_type_aliases     201907Lnew-val // also in <atomic>
#define __cpp_lib_atomic_ref                        201806Lnew-val // freestanding, also in <atomic>
#define __cpp_lib_atomic_shared_ptr                 201711Lnew-val // also in <memory>
#define __cpp_lib_atomic_value_initialization       201911Lnew-val // freestanding, also in <atomic>, <memory>
#define __cpp_lib_atomic_wait                       201907Lnew-val // freestanding, also in <atomic>
#define __cpp_lib_barrier                           201907Lnew-val // also in <barrier>
#define __cpp_lib_bind_front                        201907Lnew-val // freestanding, also in <functional>
#define __cpp_lib_bit_cast                          201806Lnew-val // freestanding, also in <bit>
#define __cpp_lib_bitops                            201907Lnew-val // freestanding, also in <bit>
#define __cpp_lib_bool_constant                     201505Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_bounded_array_traits              201902Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_boyer_moore_searcher              201603Lnew-val // also in <functional>
#define __cpp_lib_byte                              201603Lnew-val // freestanding, also in <cstddef>
#define __cpp_lib_char8_t                           201907Lnew-val
  // freestanding, also in <atomic>, <filesystem>, <istream>, <limits>, <locale>, <ostream>, <string>, <string-view>
#define __cpp_lib_chrono                            201907Lnew-val // also in <chrono>
#define __cpp_lib_chrono_udls                       201304Lnew-val // also in <chrono>
#define __cpp_lib_clamp                             201603Lnew-val // also in <algorithm>
#define __cpp_lib_complex_udls                      201309Lnew-val // also in <complex>
#define __cpp_lib_concepts                          202002Lnew-val // freestanding, also in <concepts>
#define __cpp_lib_constexpr_algorithms              201806Lnew-val // also in <algorithm>
#define __cpp_lib_constexpr_complex                 201711Lnew-val // also in <complex>
#define __cpp_lib_constexpr_dynamic_alloc           201907Lnew-val // also in <memory>
#define __cpp_lib_constexpr_functional              201907Lnew-val // freestanding, also in <functional>
#define __cpp_lib_constexpr_iterator                201811Lnew-val // freestanding, also in <iterator>
#define __cpp_lib_constexpr_memory                  201811Lnew-val // freestanding, also in <memory>
#define __cpp_lib_constexpr_numeric                 201911Lnew-val // also in <numeric>
#define __cpp_lib_constexpr_string                  201907Lnew-val // also in <string>
#define __cpp_lib_constexpr_string_view             201811Lnew-val // also in <string-view>
#define __cpp_lib_constexpr_tuple                   201811Lnew-val // freestanding, also in <tuple>
#define __cpp_lib_constexpr_utility                 201811Lnew-val // freestanding, also in <utility>
#define __cpp_lib_constexpr_vector                  201907Lnew-val // also in <vector>
#define __cpp_lib_coroutine                         201902Lnew-val // also in <coroutine>
#define __cpp_lib_destroying_delete                 201806Lnew-val // freestanding, also in <new>
#define __cpp_lib_enable_shared_from_this           201603Lnew-val // also in <memory>
#define __cpp_lib_endian                            201907Lnew-val // freestanding, also in <bit>
#define __cpp_lib_erase_if                          202002Lnew-val
  // also in <string>, <deque>, <forward-list>, <list>, <vector>, <map>, <set>, <unordered-map>,
  // <unordered-set>
#define __cpp_lib_exchange_function                 201304Lnew-val // freestanding, also in <utility>
#define __cpp_lib_execution                         201902Lnew-val // also in <execution>
#define __cpp_lib_filesystem                        201703Lnew-val // also in <filesystem>
#define __cpp_lib_format                            201907Lnew-val // also in <format>
#define __cpp_lib_freestanding_functional           new-val // freestanding, also in <functional>
#define __cpp_lib_freestanding_iterator             new-val // freestanding, also in <iterator>
#define __cpp_lib_freestanding_memory               new-val // freestanding, also in <memory>
#define __cpp_lib_freestanding_ranges               new-val // freestanding, also in <ranges>
#define __cpp_lib_freestanding_ratio                new-val // freestanding, also in <ratio>
#define __cpp_lib_freestanding_tuple                new-val // freestanding, also in <tuple>
#define __cpp_lib_freestanding_utility              new-val // freestanding, also in <utility>
#define __cpp_lib_gcd_lcm                           201606Lnew-val // also in <numeric>
#define __cpp_lib_generic_associative_lookup        201304Lnew-val // also in <map>, <set>
#define __cpp_lib_generic_unordered_lookup          201811Lnew-val
  // also in <unordered-map>, <unordered-set>
#define __cpp_lib_hardware_interference_size        201703Lnew-val // freestanding, also in <new>
#define __cpp_lib_has_unique_object_representations 201606Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_hypot                             201603Lnew-val // also in <cmath>
#define __cpp_lib_incomplete_container_elements     201505Lnew-val
  // also in <forward-list>, <list>, <vector>
#define __cpp_lib_int_pow2                          202002Lnew-val // freestanding, also in <bit>
#define __cpp_lib_integer_comparison_functions      202002Lnew-val // also in <utility>
#define __cpp_lib_integer_sequence                  201304Lnew-val // freestanding, also in <utility>
#define __cpp_lib_integral_constant_callable        201304Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_interpolate                       201902Lnew-val // also in <cmath>, <numeric>
#define __cpp_lib_invoke                            201411Lnew-val // freestanding, also in <functional>
#define __cpp_lib_is_aggregate                      201703Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_is_constant_evaluated             201811Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_is_final                          201402Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_is_invocable                      201703Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_is_layout_compatible              201907Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_is_nothrow_convertible            201806Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_is_null_pointer                   201309Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_is_pointer_interconvertible       201907Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_is_swappable                      201603Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_jthread                           201911Lnew-val // also in <stop-token>, <thread>
#define __cpp_lib_latch                             201907Lnew-val // also in <latch>
#define __cpp_lib_launder                           201606Lnew-val // freestanding, also in <new>
#define __cpp_lib_list_remove_return_type           201806Lnew-val // also in <forward-list>, <list>
#define __cpp_lib_logical_traits                    201510Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_make_from_tuple                   201606Lnew-val // freestanding, also in <tuple>
#define __cpp_lib_make_reverse_iterator             201402Lnew-val // freestanding, also in <iterator>
#define __cpp_lib_make_unique                       201304Lnew-val // also in <memory>
#define __cpp_lib_map_try_emplace                   201411Lnew-val // also in <map>
#define __cpp_lib_math_constants                    201907Lnew-val // also in <numbers>
#define __cpp_lib_math_special_functions            201603Lnew-val // also in <cmath>
#define __cpp_lib_memory_resource                   201603Lnew-val // also in <memory-resource>
#define __cpp_lib_node_extract                      201606Lnew-val
  // also in <map>, <set>, <unordered-map>, <unordered-set>
#define __cpp_lib_nonmember_container_access        201411Lnew-val
  // freestanding, also in <array>, <deque>, <forward-list>, <iterator>, <list>, <map>, <regex>, <set>, <string>,
  // <unordered-map>, <unordered-set>, <vector>
#define __cpp_lib_not_fn                            201603Lnew-val // freestanding, also in <functional>
#define __cpp_lib_null_iterators                    201304Lnew-val // freestanding, also in <iterator>
#define __cpp_lib_optional                          201606Lnew-val // also in <optional>
#define __cpp_lib_parallel_algorithm                201603Lnew-val // also in <algorithm>, <numeric>
#define __cpp_lib_polymorphic_allocator             201902Lnew-val // also in <memory>
#define __cpp_lib_quoted_string_io                  201304Lnew-val // also in <iomanip>
#define __cpp_lib_ranges                            201911Lnew-val
  // also in <algorithm>, <functional>, <iterator>, <memory>, <ranges>
#define __cpp_lib_raw_memory_algorithms             201606Lnew-val // also in <memory>
#define __cpp_lib_remove_cvref                      201711Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_result_of_sfinae                  201210Lnew-val // freestanding, also in <functional>, <type-traits>
#define __cpp_lib_robust_nonmodifying_seq_ops       201304Lnew-val // also in <algorithm>
#define __cpp_lib_sample                            201603Lnew-val // also in <algorithm>
#define __cpp_lib_scoped_lock                       201703Lnew-val // also in <mutex>
#define __cpp_lib_semaphore                         201907Lnew-val // also in <semaphore>
#define __cpp_lib_shared_mutex                      201505Lnew-val // also in <shared-mutex>
#define __cpp_lib_shared_ptr_arrays                 201707Lnew-val // also in <memory>
#define __cpp_lib_shared_ptr_weak_type              201606Lnew-val // also in <memory>
#define __cpp_lib_shared_timed_mutex                201402Lnew-val // also in <shared-mutex>
#define __cpp_lib_shift                             201806Lnew-val // also in <algorithm>
#define __cpp_lib_smart_ptr_for_overwrite           202002Lnew-val // also in <memory>
#define __cpp_lib_source_location                   201907Lnew-val // freestanding, also in <source-location>
#define __cpp_lib_span                              202002Lnew-val // also in <span>
#define __cpp_lib_ssize                             201902Lnew-val // freestanding, also in <iterator>
#define __cpp_lib_starts_ends_with                  201711Lnew-val // also in <string>, <string-view>
#define __cpp_lib_string_udls                       201304Lnew-val // also in <string>
#define __cpp_lib_string_view                       201803Lnew-val // also in <string>, <string-view>
#define __cpp_lib_syncbuf                           201803Lnew-val // also in <syncstream>
#define __cpp_lib_three_way_comparison              201907Lnew-val // freestanding, also in <compare>
#define __cpp_lib_to_address                        201711Lnew-val // freestanding, also in <memory>
#define __cpp_lib_to_array                          201907Lnew-val // also in <array>
#define __cpp_lib_to_chars                          201611Lnew-val // also in <charconv>
#define __cpp_lib_transformation_trait_aliases      201304Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_transparent_operators             201510Lnew-val // freestanding, also in <memory>, <functional>
#define __cpp_lib_tuple_element_t                   201402Lnew-val // freestanding, also in <tuple>
#define __cpp_lib_tuples_by_type                    201304Lnew-val // freestanding, also in <utility>, <tuple>
#define __cpp_lib_type_identity                     201806Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_type_trait_variable_templates     201510Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_uncaught_exceptions               201411Lnew-val // freestanding, also in <exception>
#define __cpp_lib_unordered_map_try_emplace         201411Lnew-val // also in <unordered-map>
#define __cpp_lib_unwrap_ref                        201811Lnew-val // freestanding, also in <type-traits>
#define __cpp_lib_variant                           201606Lnew-val // also in <variant>
#define __cpp_lib_void_t                            201411Lnew-val // freestanding, also in <type-traits>