libcxx/docs/FeatureTestMacroTable.rst

   1 .. _FeatureTestMacroTable:
   2
   3 ==========================
   4 Feature Test Macro Support
   5 ==========================
   6
   7 .. contents::
   8    :local:
   9
  10 Overview
  11 ========
  12
  13 This file documents the feature test macros currently supported by libc++.
  14
  15 .. _feature-status:
  16
  17 Status
  18 ======
  19
  20 .. table:: Current Status
  21     :name: feature-status-table
  22     :widths: auto
  23
  24     =================================================== =================
  25     Macro Name                                          Value
  26     =================================================== =================
  27     **C++ 14**
  28     ---------------------------------------------------------------------
  29     ``__cpp_lib_chrono_udls``                           ``201304L``
  30     --------------------------------------------------- -----------------
  31     ``__cpp_lib_complex_udls``                          ``201309L``
  32     --------------------------------------------------- -----------------
  33     ``__cpp_lib_exchange_function``                     ``201304L``
  34     --------------------------------------------------- -----------------
  35     ``__cpp_lib_generic_associative_lookup``            ``201304L``
  36     --------------------------------------------------- -----------------
  37     ``__cpp_lib_integer_sequence``                      ``201304L``
  38     --------------------------------------------------- -----------------
  39     ``__cpp_lib_integral_constant_callable``            ``201304L``
  40     --------------------------------------------------- -----------------
  41     ``__cpp_lib_is_final``                              ``201402L``
  42     --------------------------------------------------- -----------------
  43     ``__cpp_lib_is_null_pointer``                       ``201309L``
  44     --------------------------------------------------- -----------------
  45     ``__cpp_lib_make_reverse_iterator``                 ``201402L``
  46     --------------------------------------------------- -----------------
  47     ``__cpp_lib_make_unique``                           ``201304L``
  48     --------------------------------------------------- -----------------
  49     ``__cpp_lib_null_iterators``                        ``201304L``
  50     --------------------------------------------------- -----------------
  51     ``__cpp_lib_quoted_string_io``                      ``201304L``
  52     --------------------------------------------------- -----------------
  53     ``__cpp_lib_result_of_sfinae``                      ``201210L``
  54     --------------------------------------------------- -----------------
  55     ``__cpp_lib_robust_nonmodifying_seq_ops``           ``201304L``
  56     --------------------------------------------------- -----------------
  57     ``__cpp_lib_shared_timed_mutex``                    ``201402L``
  58     --------------------------------------------------- -----------------
  59     ``__cpp_lib_string_udls``                           ``201304L``
  60     --------------------------------------------------- -----------------
  61     ``__cpp_lib_transformation_trait_aliases``          ``201304L``
  62     --------------------------------------------------- -----------------
  63     ``__cpp_lib_transparent_operators``                 ``201210L``
  64     --------------------------------------------------- -----------------
  65     ``__cpp_lib_tuple_element_t``                       ``201402L``
  66     --------------------------------------------------- -----------------
  67     ``__cpp_lib_tuples_by_type``                        ``201304L``
  68     --------------------------------------------------- -----------------
  69     **C++ 17**
  70     ---------------------------------------------------------------------
  71     ``__cpp_lib_addressof_constexpr``                   ``201603L``
  72     --------------------------------------------------- -----------------
  73     ``__cpp_lib_allocator_traits_is_always_equal``      ``201411L``
  74     --------------------------------------------------- -----------------
  75     ``__cpp_lib_any``                                   ``201606L``
  76     --------------------------------------------------- -----------------
  77     ``__cpp_lib_apply``                                 ``201603L``
  78     --------------------------------------------------- -----------------
  79     ``__cpp_lib_array_constexpr``                       ``201603L``
  80     --------------------------------------------------- -----------------
  81     ``__cpp_lib_as_const``                              ``201510L``
  82     --------------------------------------------------- -----------------
  83     ``__cpp_lib_atomic_is_always_lock_free``            ``201603L``
  84     --------------------------------------------------- -----------------
  85     ``__cpp_lib_bool_constant``                         ``201505L``
  86     --------------------------------------------------- -----------------
  87     ``__cpp_lib_boyer_moore_searcher``                  ``201603L``
  88     --------------------------------------------------- -----------------
  89     ``__cpp_lib_byte``                                  ``201603L``
  90     --------------------------------------------------- -----------------
  91     ``__cpp_lib_chrono``                                ``201611L``
  92     --------------------------------------------------- -----------------
  93     ``__cpp_lib_clamp``                                 ``201603L``
  94     --------------------------------------------------- -----------------
  95     ``__cpp_lib_enable_shared_from_this``               ``201603L``
  96     --------------------------------------------------- -----------------
  97     ``__cpp_lib_execution``                             *unimplemented*
  98     --------------------------------------------------- -----------------
  99     ``__cpp_lib_filesystem``                            ``201703L``
 100     --------------------------------------------------- -----------------
 101     ``__cpp_lib_gcd_lcm``                               ``201606L``
 102     --------------------------------------------------- -----------------
 103     ``__cpp_lib_hardware_interference_size``            ``201703L``
 104     --------------------------------------------------- -----------------
 105     ``__cpp_lib_has_unique_object_representations``     ``201606L``
 106     --------------------------------------------------- -----------------
 107     ``__cpp_lib_hypot``                                 ``201603L``
 108     --------------------------------------------------- -----------------
 109     ``__cpp_lib_incomplete_container_elements``         ``201505L``
 110     --------------------------------------------------- -----------------
 111     ``__cpp_lib_invoke``                                ``201411L``
 112     --------------------------------------------------- -----------------
 113     ``__cpp_lib_is_aggregate``                          ``201703L``
 114     --------------------------------------------------- -----------------
 115     ``__cpp_lib_is_invocable``                          ``201703L``
 116     --------------------------------------------------- -----------------
 117     ``__cpp_lib_is_swappable``                          ``201603L``
 118     --------------------------------------------------- -----------------
 119     ``__cpp_lib_launder``                               ``201606L``
 120     --------------------------------------------------- -----------------
 121     ``__cpp_lib_logical_traits``                        ``201510L``
 122     --------------------------------------------------- -----------------
 123     ``__cpp_lib_make_from_tuple``                       ``201606L``
 124     --------------------------------------------------- -----------------
 125     ``__cpp_lib_map_try_emplace``                       ``201411L``
 126     --------------------------------------------------- -----------------
 127     ``__cpp_lib_math_special_functions``                *unimplemented*
 128     --------------------------------------------------- -----------------
 129     ``__cpp_lib_memory_resource``                       ``201603L``
 130     --------------------------------------------------- -----------------
 131     ``__cpp_lib_node_extract``                          ``201606L``
 132     --------------------------------------------------- -----------------
 133     ``__cpp_lib_nonmember_container_access``            ``201411L``
 134     --------------------------------------------------- -----------------
 135     ``__cpp_lib_not_fn``                                ``201603L``
 136     --------------------------------------------------- -----------------
 137     ``__cpp_lib_optional``                              ``201606L``
 138     --------------------------------------------------- -----------------
 139     ``__cpp_lib_parallel_algorithm``                    *unimplemented*
 140     --------------------------------------------------- -----------------
 141     ``__cpp_lib_raw_memory_algorithms``                 ``201606L``
 142     --------------------------------------------------- -----------------
 143     ``__cpp_lib_sample``                                ``201603L``
 144     --------------------------------------------------- -----------------
 145     ``__cpp_lib_scoped_lock``                           ``201703L``
 146     --------------------------------------------------- -----------------
 147     ``__cpp_lib_shared_mutex``                          ``201505L``
 148     --------------------------------------------------- -----------------
 149     ``__cpp_lib_shared_ptr_arrays``                     ``201611L``
 150     --------------------------------------------------- -----------------
 151     ``__cpp_lib_shared_ptr_weak_type``                  ``201606L``
 152     --------------------------------------------------- -----------------
 153     ``__cpp_lib_string_view``                           ``201606L``
 154     --------------------------------------------------- -----------------
 155     ``__cpp_lib_to_chars``                              *unimplemented*
 156     --------------------------------------------------- -----------------
 157     ``__cpp_lib_transparent_operators``                 ``201510L``
 158     --------------------------------------------------- -----------------
 159     ``__cpp_lib_type_trait_variable_templates``         ``201510L``
 160     --------------------------------------------------- -----------------
 161     ``__cpp_lib_uncaught_exceptions``                   ``201411L``
 162     --------------------------------------------------- -----------------
 163     ``__cpp_lib_unordered_map_try_emplace``             ``201411L``
 164     --------------------------------------------------- -----------------
 165     ``__cpp_lib_variant``                               ``202102L``
 166     --------------------------------------------------- -----------------
 167     ``__cpp_lib_void_t``                                ``201411L``
 168     --------------------------------------------------- -----------------
 169     **C++ 20**
 170     ---------------------------------------------------------------------
 171     ``__cpp_lib_array_constexpr``                       ``201811L``
 172     --------------------------------------------------- -----------------
 173     ``__cpp_lib_assume_aligned``                        ``201811L``
 174     --------------------------------------------------- -----------------
 175     ``__cpp_lib_atomic_flag_test``                      ``201907L``
 176     --------------------------------------------------- -----------------
 177     ``__cpp_lib_atomic_float``                          *unimplemented*
 178     --------------------------------------------------- -----------------
 179     ``__cpp_lib_atomic_lock_free_type_aliases``         ``201907L``
 180     --------------------------------------------------- -----------------
 181     ``__cpp_lib_atomic_ref``                            *unimplemented*
 182     --------------------------------------------------- -----------------
 183     ``__cpp_lib_atomic_shared_ptr``                     *unimplemented*
 184     --------------------------------------------------- -----------------
 185     ``__cpp_lib_atomic_value_initialization``           ``201911L``
 186     --------------------------------------------------- -----------------
 187     ``__cpp_lib_atomic_wait``                           ``201907L``
 188     --------------------------------------------------- -----------------
 189     ``__cpp_lib_barrier``                               ``201907L``
 190     --------------------------------------------------- -----------------
 191     ``__cpp_lib_bind_front``                            ``201907L``
 192     --------------------------------------------------- -----------------
 193     ``__cpp_lib_bit_cast``                              ``201806L``
 194     --------------------------------------------------- -----------------
 195     ``__cpp_lib_bitops``                                ``201907L``
 196     --------------------------------------------------- -----------------
 197     ``__cpp_lib_bounded_array_traits``                  ``201902L``
 198     --------------------------------------------------- -----------------
 199     ``__cpp_lib_char8_t``                               ``201907L``
 200     --------------------------------------------------- -----------------
 201     ``__cpp_lib_concepts``                              ``202002L``
 202     --------------------------------------------------- -----------------
 203     ``__cpp_lib_constexpr_algorithms``                  ``201806L``
 204     --------------------------------------------------- -----------------
 205     ``__cpp_lib_constexpr_complex``                     ``201711L``
 206     --------------------------------------------------- -----------------
 207     ``__cpp_lib_constexpr_dynamic_alloc``               ``201907L``
 208     --------------------------------------------------- -----------------
 209     ``__cpp_lib_constexpr_functional``                  ``201907L``
 210     --------------------------------------------------- -----------------
 211     ``__cpp_lib_constexpr_iterator``                    ``201811L``
 212     --------------------------------------------------- -----------------
 213     ``__cpp_lib_constexpr_memory``                      ``201811L``
 214     --------------------------------------------------- -----------------
 215     ``__cpp_lib_constexpr_numeric``                     ``201911L``
 216     --------------------------------------------------- -----------------
 217     ``__cpp_lib_constexpr_string``                      ``201907L``
 218     --------------------------------------------------- -----------------
 219     ``__cpp_lib_constexpr_string_view``                 ``201811L``
 220     --------------------------------------------------- -----------------
 221     ``__cpp_lib_constexpr_tuple``                       ``201811L``
 222     --------------------------------------------------- -----------------
 223     ``__cpp_lib_constexpr_utility``                     ``201811L``
 224     --------------------------------------------------- -----------------
 225     ``__cpp_lib_constexpr_vector``                      ``201907L``
 226     --------------------------------------------------- -----------------
 227     ``__cpp_lib_coroutine``                             ``201902L``
 228     --------------------------------------------------- -----------------
 229     ``__cpp_lib_destroying_delete``                     ``201806L``
 230     --------------------------------------------------- -----------------
 231     ``__cpp_lib_endian``                                ``201907L``
 232     --------------------------------------------------- -----------------
 233     ``__cpp_lib_erase_if``                              ``202002L``
 234     --------------------------------------------------- -----------------
 235     ``__cpp_lib_execution``                             *unimplemented*
 236     --------------------------------------------------- -----------------
 237     ``__cpp_lib_format``                                *unimplemented*
 238     --------------------------------------------------- -----------------
 239     ``__cpp_lib_format_uchar``                          ``202311L``
 240     --------------------------------------------------- -----------------
 241     ``__cpp_lib_generic_unordered_lookup``              ``201811L``
 242     --------------------------------------------------- -----------------
 243     ``__cpp_lib_int_pow2``                              ``202002L``
 244     --------------------------------------------------- -----------------
 245     ``__cpp_lib_integer_comparison_functions``          ``202002L``
 246     --------------------------------------------------- -----------------
 247     ``__cpp_lib_interpolate``                           ``201902L``
 248     --------------------------------------------------- -----------------
 249     ``__cpp_lib_is_constant_evaluated``                 ``201811L``
 250     --------------------------------------------------- -----------------
 251     ``__cpp_lib_is_layout_compatible``                  *unimplemented*
 252     --------------------------------------------------- -----------------
 253     ``__cpp_lib_is_nothrow_convertible``                ``201806L``
 254     --------------------------------------------------- -----------------
 255     ``__cpp_lib_is_pointer_interconvertible``           *unimplemented*
 256     --------------------------------------------------- -----------------
 257     ``__cpp_lib_jthread``                               ``201911L``
 258     --------------------------------------------------- -----------------
 259     ``__cpp_lib_latch``                                 ``201907L``
 260     --------------------------------------------------- -----------------
 261     ``__cpp_lib_list_remove_return_type``               ``201806L``
 262     --------------------------------------------------- -----------------
 263     ``__cpp_lib_math_constants``                        ``201907L``
 264     --------------------------------------------------- -----------------
 265     ``__cpp_lib_move_iterator_concept``                 ``202207L``
 266     --------------------------------------------------- -----------------
 267     ``__cpp_lib_polymorphic_allocator``                 ``201902L``
 268     --------------------------------------------------- -----------------
 269     ``__cpp_lib_ranges``                                ``202207L``
 270     --------------------------------------------------- -----------------
 271     ``__cpp_lib_remove_cvref``                          ``201711L``
 272     --------------------------------------------------- -----------------
 273     ``__cpp_lib_semaphore``                             ``201907L``
 274     --------------------------------------------------- -----------------
 275     ``__cpp_lib_shared_ptr_arrays``                     ``201707L``
 276     --------------------------------------------------- -----------------
 277     ``__cpp_lib_shift``                                 ``201806L``
 278     --------------------------------------------------- -----------------
 279     ``__cpp_lib_smart_ptr_for_overwrite``               *unimplemented*
 280     --------------------------------------------------- -----------------
 281     ``__cpp_lib_source_location``                       ``201907L``
 282     --------------------------------------------------- -----------------
 283     ``__cpp_lib_span``                                  ``202002L``
 284     --------------------------------------------------- -----------------
 285     ``__cpp_lib_ssize``                                 ``201902L``
 286     --------------------------------------------------- -----------------
 287     ``__cpp_lib_starts_ends_with``                      ``201711L``
 288     --------------------------------------------------- -----------------
 289     ``__cpp_lib_string_view``                           ``201803L``
 290     --------------------------------------------------- -----------------
 291     ``__cpp_lib_syncbuf``                               ``201803L``
 292     --------------------------------------------------- -----------------
 293     ``__cpp_lib_three_way_comparison``                  *unimplemented*
 294     --------------------------------------------------- -----------------
 295     ``__cpp_lib_to_address``                            ``201711L``
 296     --------------------------------------------------- -----------------
 297     ``__cpp_lib_to_array``                              ``201907L``
 298     --------------------------------------------------- -----------------
 299     ``__cpp_lib_type_identity``                         ``201806L``
 300     --------------------------------------------------- -----------------
 301     ``__cpp_lib_unwrap_ref``                            ``201811L``
 302     --------------------------------------------------- -----------------
 303     **C++ 23**
 304     ---------------------------------------------------------------------
 305     ``__cpp_lib_adaptor_iterator_pair_constructor``     ``202106L``
 306     --------------------------------------------------- -----------------
 307     ``__cpp_lib_allocate_at_least``                     ``202106L``
 308     --------------------------------------------------- -----------------
 309     ``__cpp_lib_associative_heterogeneous_erasure``     *unimplemented*
 310     --------------------------------------------------- -----------------
 311     ``__cpp_lib_bind_back``                             *unimplemented*
 312     --------------------------------------------------- -----------------
 313     ``__cpp_lib_byteswap``                              ``202110L``
 314     --------------------------------------------------- -----------------
 315     ``__cpp_lib_constexpr_bitset``                      ``202207L``
 316     --------------------------------------------------- -----------------
 317     ``__cpp_lib_constexpr_charconv``                    ``202207L``
 318     --------------------------------------------------- -----------------
 319     ``__cpp_lib_constexpr_cmath``                       *unimplemented*
 320     --------------------------------------------------- -----------------
 321     ``__cpp_lib_constexpr_memory``                      ``202202L``
 322     --------------------------------------------------- -----------------
 323     ``__cpp_lib_constexpr_typeinfo``                    ``202106L``
 324     --------------------------------------------------- -----------------
 325     ``__cpp_lib_expected``                              ``202211L``
 326     --------------------------------------------------- -----------------
 327     ``__cpp_lib_format_ranges``                         ``202207L``
 328     --------------------------------------------------- -----------------
 329     ``__cpp_lib_formatters``                            *unimplemented*
 330     --------------------------------------------------- -----------------
 331     ``__cpp_lib_forward_like``                          ``202207L``
 332     --------------------------------------------------- -----------------
 333     ``__cpp_lib_invoke_r``                              ``202106L``
 334     --------------------------------------------------- -----------------
 335     ``__cpp_lib_ios_noreplace``                         ``202207L``
 336     --------------------------------------------------- -----------------
 337     ``__cpp_lib_is_scoped_enum``                        ``202011L``
 338     --------------------------------------------------- -----------------
 339     ``__cpp_lib_mdspan``                                ``202207L``
 340     --------------------------------------------------- -----------------
 341     ``__cpp_lib_move_only_function``                    *unimplemented*
 342     --------------------------------------------------- -----------------
 343     ``__cpp_lib_optional``                              ``202110L``
 344     --------------------------------------------------- -----------------
 345     ``__cpp_lib_out_ptr``                               *unimplemented*
 346     --------------------------------------------------- -----------------
 347     ``__cpp_lib_print``                                 ``202207L``
 348     --------------------------------------------------- -----------------
 349     ``__cpp_lib_ranges_as_const``                       *unimplemented*
 350     --------------------------------------------------- -----------------
 351     ``__cpp_lib_ranges_as_rvalue``                      ``202207L``
 352     --------------------------------------------------- -----------------
 353     ``__cpp_lib_ranges_chunk``                          *unimplemented*
 354     --------------------------------------------------- -----------------
 355     ``__cpp_lib_ranges_chunk_by``                       ``202202L``
 356     --------------------------------------------------- -----------------
 357     ``__cpp_lib_ranges_iota``                           *unimplemented*
 358     --------------------------------------------------- -----------------
 359     ``__cpp_lib_ranges_join_with``                      *unimplemented*
 360     --------------------------------------------------- -----------------
 361     ``__cpp_lib_ranges_repeat``                         ``202207L``
 362     --------------------------------------------------- -----------------
 363     ``__cpp_lib_ranges_slide``                          *unimplemented*
 364     --------------------------------------------------- -----------------
 365     ``__cpp_lib_ranges_starts_ends_with``               *unimplemented*
 366     --------------------------------------------------- -----------------
 367     ``__cpp_lib_ranges_to_container``                   ``202202L``
 368     --------------------------------------------------- -----------------
 369     ``__cpp_lib_ranges_zip``                            *unimplemented*
 370     --------------------------------------------------- -----------------
 371     ``__cpp_lib_reference_from_temporary``              *unimplemented*
 372     --------------------------------------------------- -----------------
 373     ``__cpp_lib_spanstream``                            *unimplemented*
 374     --------------------------------------------------- -----------------
 375     ``__cpp_lib_stacktrace``                            *unimplemented*
 376     --------------------------------------------------- -----------------
 377     ``__cpp_lib_stdatomic_h``                           ``202011L``
 378     --------------------------------------------------- -----------------
 379     ``__cpp_lib_string_contains``                       ``202011L``
 380     --------------------------------------------------- -----------------
 381     ``__cpp_lib_string_resize_and_overwrite``           ``202110L``
 382     --------------------------------------------------- -----------------
 383     ``__cpp_lib_to_string``                             *unimplemented*
 384     --------------------------------------------------- -----------------
 385     ``__cpp_lib_to_underlying``                         ``202102L``
 386     --------------------------------------------------- -----------------
 387     ``__cpp_lib_tuple_like``                            *unimplemented*
 388     --------------------------------------------------- -----------------
 389     ``__cpp_lib_unreachable``                           ``202202L``
 390     --------------------------------------------------- -----------------
 391     **C++ 26**
 392     ---------------------------------------------------------------------
 393     ``__cpp_lib_associative_heterogeneous_insertion``   *unimplemented*
 394     --------------------------------------------------- -----------------
 395     ``__cpp_lib_bind_back``                             *unimplemented*
 396     --------------------------------------------------- -----------------
 397     ``__cpp_lib_bind_front``                            ``202306L``
 398     --------------------------------------------------- -----------------
 399     ``__cpp_lib_bitset``                                ``202306L``
 400     --------------------------------------------------- -----------------
 401     ``__cpp_lib_copyable_function``                     *unimplemented*
 402     --------------------------------------------------- -----------------
 403     ``__cpp_lib_debugging``                             *unimplemented*
 404     --------------------------------------------------- -----------------
 405     ``__cpp_lib_freestanding_algorithm``                *unimplemented*
 406     --------------------------------------------------- -----------------
 407     ``__cpp_lib_freestanding_array``                    *unimplemented*
 408     --------------------------------------------------- -----------------
 409     ``__cpp_lib_freestanding_cstring``                  *unimplemented*
 410     --------------------------------------------------- -----------------
 411     ``__cpp_lib_freestanding_expected``                 *unimplemented*
 412     --------------------------------------------------- -----------------
 413     ``__cpp_lib_freestanding_mdspan``                   *unimplemented*
 414     --------------------------------------------------- -----------------
 415     ``__cpp_lib_freestanding_optional``                 *unimplemented*
 416     --------------------------------------------------- -----------------
 417     ``__cpp_lib_freestanding_string_view``              *unimplemented*
 418     --------------------------------------------------- -----------------
 419     ``__cpp_lib_freestanding_variant``                  *unimplemented*
 420     --------------------------------------------------- -----------------
 421     ``__cpp_lib_fstream_native_handle``                 ``202306L``
 422     --------------------------------------------------- -----------------
 423     ``__cpp_lib_function_ref``                          *unimplemented*
 424     --------------------------------------------------- -----------------
 425     ``__cpp_lib_hazard_pointer``                        *unimplemented*
 426     --------------------------------------------------- -----------------
 427     ``__cpp_lib_linalg``                                *unimplemented*
 428     --------------------------------------------------- -----------------
 429     ``__cpp_lib_out_ptr``                               *unimplemented*
 430     --------------------------------------------------- -----------------
 431     ``__cpp_lib_ratio``                                 ``202306L``
 432     --------------------------------------------------- -----------------
 433     ``__cpp_lib_rcu``                                   *unimplemented*
 434     --------------------------------------------------- -----------------
 435     ``__cpp_lib_saturation_arithmetic``                 *unimplemented*
 436     --------------------------------------------------- -----------------
 437     ``__cpp_lib_smart_ptr_owner_equality``              *unimplemented*
 438     --------------------------------------------------- -----------------
 439     ``__cpp_lib_span_at``                               ``202311L``
 440     --------------------------------------------------- -----------------
 441     ``__cpp_lib_span_initializer_list``                 *unimplemented*
 442     --------------------------------------------------- -----------------
 443     ``__cpp_lib_sstream_from_string_view``              *unimplemented*
 444     --------------------------------------------------- -----------------
 445     ``__cpp_lib_submdspan``                             *unimplemented*
 446     --------------------------------------------------- -----------------
 447     ``__cpp_lib_text_encoding``                         *unimplemented*
 448     --------------------------------------------------- -----------------
 449     ``__cpp_lib_to_chars``                              *unimplemented*
 450     --------------------------------------------------- -----------------
 451     ``__cpp_lib_tuple_like``                            *unimplemented*
 452     --------------------------------------------------- -----------------
 453     ``__cpp_lib_within_lifetime``                       *unimplemented*
 454     =================================================== =================
 455