[libc++] [P0325] Implement to_array from LFTS with updates.

[curdeius]

Summary

This patch implements https://wg21.link/P0325.
Please mind that at it is my first contribution to libc++, so I may have forgotten to abide to some conventions.

[zoecarver]

Thanks for the patch @curdeius! It looks great.

I also have an implementation here, D66103. We can use either (and maybe both sets of tests).

libcxx/include/array

501

I don't think you need the noexcept.

libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp

15

Is this needed?

26

You can get this with #include "MoveOnly.h".

libcxx/test/std/containers/sequences/array/array.creation/to_array.pass.cpp

23

You can use ASSERT_SAME_TYPE (from test_macros).

[curdeius]

Oh, I didn't know it, I only searched for to_array in master branch before starting.
How should I proceed? I let you take out of my patch what can be useful?
I see that you didn't touch feature test macros at all for instance. And as you said, you might add some tests I added and you seem not having (e.g. fail tests).

[ldionne]

Once you guys have figured out how you want to proceed, please say it here and abandon one of the two revisions. I'll review the one you decide to keep.

[zoecarver]

Let's use this patch. Both so that you can get something committed and because it has the feature test macros. Feel free to pull anything you want from my implementation.

Once we have more stuff in GitHub these things will happen less often (I hope).

[curdeius]

• Clean up to_array implementation.
• Add tests from D66103. Clean up.

[curdeius]

• Fix typos.

[curdeius]

Updated with tests from D66103. Answered issues from comments.

Sorry for the delay.

[curdeius]

I'm still not sure what to do about noexcept for to_array.
libstdc++ adds noexcept: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/array#L361
MSVC STL does not add noexcept: https://github.com/microsoft/STL/blob/master/stl/inc/array#L429
It seems to be a low-hanging fruit to add noexcept(is_nothrow_copy_constructible_v<_Tp>) / noexcept(is_nothrow_move_constructible_v<_Tp>).
What do you think?

[ldionne]

libcxx/include/array

499

You can use constexpr directly here (and elsewhere), since this is only enabled when _LIBCPP_STD_VER > 17.

499

Please declare the return type of the function here, since otherwise decltype(to_array(...)) will need to instantiate the function body in order to get the return type. It also makes for better code documentation, since the return type of to_array is definitely part of the public API.

504

Did they really mean is_constructible_v<T, T&> in the paper? They say:

Mandates: is_array_v<T> is false and is_constructible_v<T, T&> is true.

It seems like they should have said this instead?

Mandates: is_array_v<T> is false and is_copy_constructible_v<T> is true.

Regardless, I think it would be better if you used is_constructible_v<T, T&> here, since that's what the paper says.

@lichray , why did you write the wording that way? Any reason?

libcxx/test/std/containers/sequences/array/array.creation/to_array.pass.cpp

11

Can you please add the synopsis of the exact functions you're testing (so the two overloads of to_array)?

[lichray]

libcxx/include/array

504

LWG raised this, probably because someone realized that built-in array initializer and std::array aggregate initialization allow types that "copy-constructs" from mutable references, so it seems not so motivated to ban those in to_array.

libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp

26

Not sure what is tested here. to_array works with move-only elements (as you have tested in to_array.pass.cpp), but certainly won't work with anything that is not a built-in array :)

[ldionne]

libcxx/include/array

504

Thanks a lot for the context. Ugh, bad design IMO, since now we have a subtle difference with anything that requires the usual copy-constructibility. But oh well. @curdeius let's match what the paper says.

[curdeius]

• Fix constexpr and return type.
• Fix tests.

[curdeius]

• Add synopsis.

[curdeius]

Adressed comments.

libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp

26

Oops, fixed it.

[curdeius]

Fix patch diff.

[lichray]

LGTM.

IIRC implementations can make functions "more" noexcept. If so then noexcept is welcome.

[curdeius]

• Readd noexcept.
• Fix message.

[curdeius]

• Fix typo in comment.

[ldionne]

Except for the nitpick, LGTM.

libcxx/include/array

501

I'll be very nit-picky and ask you to not use auto -> T return types, because we don't use it much elsewhere. I know it's valid, I know it's better style when used throughout a codebase, but I think consistency wins here.

[curdeius]

• Make return type consistent.

[curdeius]

libcxx/include/array

501

Done.

[curdeius]

Just as a side-note, @ldionne, I have commit access, so I can land the patches, but do as you prefer (when the patch gets accepted) :).

[curdeius]

What's the usual way of proceeding, should I land the patch or wait for Louis' approval?

[mclow.lists]

In D69882#1749338, @curdeius wrote:

What's the usual way of proceeding, should I land the patch or wait for Louis' approval?

Wait for Louis (or myself, or Eric). Thanks!

[curdeius]

Ping. :)

[ldionne]

Sorry, this got lost in my inbox. Go ahead and commit it!

[davezarzycki]

This breaks the test suite when built "Release" without assertions enabled (Fedora 31 on x86_64). Can we please revert this?
``
FAIL: libc++ :: std/con:q:qtainers/sequences/array/array.creation/to_array.fail.cpp (53333 of 61086)

• TEST 'libc++ :: std/containers/sequences/array/array.creation/to_array.fail.cpp' FAILED ****

Command: ['/usr/bin/clang++', '-o', '/dev/null', '-x', 'c++', '/home/dave/s/lp/libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp', '-c', '-v', '-ftemplate-depth=270', '-fsyntax-only', '-Xclang', '-verify', '-Xclang', '-verify-ignore-unexpected=note', '-ferror-limit=1024', '-Werror=thread-safety', '-std=c++2a', '-include', '/home/dave/s/lp/libcxx/test/support/nasty_macros.h', '-nostdinc++', '-I/home/dave/s/lp/libcxx/include', '-I/tmp/_update_lc/r/projects/libcxx/include/c++build', '-DSTDC_FORMAT_MACROS', '-DSTDC_LIMIT_MACROS', '-D__STDC_CONSTANT_MACROS', '-I/home/dave/s/lp/libcxx/test/support', '-DLIBCXX_FILESYSTEM_STATIC_TEST_ROOT="/home/dave/s/lp/libcxx/test/std/input.output/filesystems/Inputs/static_test_env"', '-DLIBCXX_FILESYSTEM_DYNAMIC_TEST_ROOT="/tmp/_update_lc/r/projects/libcxx/test/filesystem/Output/dynamic_env"', '-DLIBCXX_FILESYSTEM_DYNAMIC_TEST_HELPER="/usr/bin/python /home/dave/s/lp/libcxx/test/support/filesystem_dynamic_test_helper.py"', '-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER', '-Wall', '-Wextra', '-Werror', '-Wuser-defined-warnings', '-Wshadow', '-Wno-unused-command-line-argument', '-Wno-attributes', '-Wno-pessimizing-move', '-Wno-c++11-extensions', '-Wno-user-defined-literals', '-Wno-noexcept-type', '-Wsign-compare', '-Wunused-variable', '-Wunused-parameter', '-Wunreachable-code', '-Wno-error=user-defined-warnings', '-c']
Exit Code: 1

Standard Error:

clang version 9.0.0 (Fedora 9.0.0-1.fc31)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/9
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/9
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/9
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
"/usr/bin/clang-9" -cc1 -triple x86_64-unknown-linux-gnu -fsyntax-only -disable-free -disable-llvm-verifier -discard-value-names -main-file-name to_array.fail.cpp -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -v -nostdinc++ -resource-dir /usr/lib64/clang/9.0.0 -include /home/dave/s/lp/libcxx/test/support/nasty_macros.h -I /home/dave/s/lp/libcxx/include -I /tmp/_update_lc/r/projects/libcxx/include/c++build -D STDC_FORMAT_MACROS -D STDC_LIMIT_MACROS -D __STDC_CONSTANT_MACROS -I /home/dave/s/lp/libcxx/test/support -D "LIBCXX_FILESYSTEM_STATIC_TEST_ROOT=\"/home/dave/s/lp/libcxx/test/std/input.output/filesystems/Inputs/static_test_env\"" -D "LIBCXX_FILESYSTEM_DYNAMIC_TEST_ROOT=\"/tmp/_update_lc/r/projects/libcxx/test/filesystem/Output/dynamic_env\"" -D "LIBCXX_FILESYSTEM_DYNAMIC_TEST_HELPER=\"/usr/bin/python /home/dave/s/lp/libcxx/test/support/filesystem_dynamic_test_helper.py\"" -D _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -internal-isystem /usr/local/include -internal-isystem /usr/lib64/clang/9.0.0/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -Werror=thread-safety -Wall -Wextra -Werror -Wuser-defined-warnings -Wshadow -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-c++11-extensions -Wno-user-defined-literals -Wno-noexcept-type -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-error=user-defined-warnings -std=c++2a -fdeprecated-macro -fdebug-compilation-dir /tmp/_update_lc/r -ftemplate-depth 270 -ferror-limit 1024 -fmessage-length 0 -fno-implicit-modules -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -verify -verify-ignore-unexpected=note -faddrsig -x c++ /home/dave/s/lp/libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp
clang -cc1 version 9.0.0 based upon LLVM 9.0.0 default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
/home/dave/s/lp/libcxx/include
/tmp/_update_lc/r/projects/libcxx/include/c++build
/home/dave/s/lp/libcxx/test/support
/usr/local/include
/usr/lib64/clang/9.0.0/include
/usr/include
End of search list.
error: 'error' diagnostics expected but not seen:

File /home/dave/s/lp/libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp Line 20: here
File /home/dave/s/lp/libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp Line 25: here
File /home/dave/s/lp/libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp Line 30: here

error: 'error' diagnostics seen but not expected:

File /home/dave/s/lp/libcxx/include/array Line 499: static_assert failed due to requirement '!is_array_v<char [6]>' "[array.creation]/1: to_array does not accept multidimensional arrays."
File /home/dave/s/lp/libcxx/include/array Line 502: static_assert failed due to requirement 'is_constructible_v<char [6], char (&)[6]>' "[array.creation]/1: to_array requires copy constructible elements."
File /home/dave/s/lp/libcxx/include/array Line 487: cannot initialize an array element of type 'char' with an lvalue of type 'char [6]'
File /home/dave/s/lp/libcxx/include/array Line 487: cannot initialize an array element of type 'char' with an lvalue of type 'char [6]'
File /home/dave/s/lp/libcxx/include/array Line 487: cannot initialize an array element of type 'char' with an lvalue of type 'char [6]'
File /home/dave/s/lp/libcxx/include/array Line 487: suggest braces around initialization of subobject
File /home/dave/s/lp/libcxx/include/array Line 502: static_assert failed due to requirement 'is_constructible_v<MoveOnly, MoveOnly &>' "[array.creation]/1: to_array requires copy constructible elements."
File /home/dave/s/lp/libcxx/include/array Line 487: calling a private constructor of class 'MoveOnly'
File /home/dave/s/lp/libcxx/include/array Line 514: static_assert failed due to requirement 'is_move_constructible_v<const MoveOnly>' "[array.creation]/4: to_array requires move constructible elements."
Line 493: calling a private constructor of class 'MoveOnly'

13 errors generated.

Expected compilation using verify to pass!

---

Testing: 0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..

10 warning(s) in tests

Testing Time: 140.79s

---

Failing Tests (1):

  libc++ :: std/containers/sequences/array/array.creation/to_array.fail.cpp

Expected Passes    : 44195
Expected Failures  : 99
Unsupported Tests  : 16791
Unexpected Failures: 1

FAILED: CMakeFiles/check-all

[curdeius]

Reverted.

[curdeius]

I've fixed the expected error mesages and relanded it in commit 5e7017273f40a48453ebb758e54123e31f751806 (https://reviews.llvm.org/rGe93e58c6c40a365bc9275dd8a8242598343cc835).

libcxx/docs/FeatureTestMacroTable.rst

.. table:: Current Status
     ------------------------------------------------- -----------------
     ``__cpp_lib_is_constant_evaluated``               ``201811L``      
     ------------------------------------------------- -----------------
     ``__cpp_lib_list_remove_return_type``             *unimplemented*  
     ------------------------------------------------- -----------------
     ``__cpp_lib_ranges``                              *unimplemented*  
     ------------------------------------------------- -----------------
     ``__cpp_lib_three_way_comparison``                *unimplemented*  
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_to_array``                            ``201907L``      
     ================================================= =================
 
 

libcxx/include/array

 get(const array<_Tp, _Size>&& __a) _NOEXCEPT
 {
     static_assert(_Ip < _Size, "Index out of bounds in std::get<> (const std::array &&)");
     return _VSTD::move(__a.__elems_[_Ip]);
 }
 
 #endif  // !_LIBCPP_CXX03_LANG
 
+#if _LIBCPP_STD_VER > 17
+
+template <typename _Tp, size_t _Size, size_t... _Index>
+_LIBCPP_INLINE_VISIBILITY constexpr array<remove_cv_t<_Tp>, _Size>
+__to_array_lvalue_impl(_Tp (&__arr)[_Size], index_sequence<_Index...>) {
+  return {{__arr[_Index]...}};
+}
+
+template <typename _Tp, size_t _Size, size_t... _Index>
+_LIBCPP_INLINE_VISIBILITY constexpr array<remove_cv_t<_Tp>, _Size>
+__to_array_rvalue_impl(_Tp(&&__arr)[_Size], index_sequence<_Index...>) {
+  return {{_VSTD::move(__arr[_Index])...}};
+}
+
+template <typename _Tp, size_t _Size>
+_LIBCPP_INLINE_VISIBILITY constexpr array<remove_cv_t<_Tp>, _Size>
+to_array(_Tp (&__arr)[_Size]) noexcept(is_nothrow_constructible_v<_Tp, _Tp&>) {
+  static_assert(
+      !is_array_v<_Tp>,
+      "[array.creation]/1: to_array does not accept multidimensional arrays.");
+  static_assert(
+      is_constructible_v<_Tp, _Tp&>,
+      "[array.creation]/1: to_array requires copy constructible elements.");
+  return __to_array_lvalue_impl(__arr, make_index_sequence<_Size>());
+}
+
+template <typename _Tp, size_t _Size>
+_LIBCPP_INLINE_VISIBILITY constexpr array<remove_cv_t<_Tp>, _Size>
+to_array(_Tp(&&__arr)[_Size]) noexcept(is_nothrow_move_constructible_v<_Tp>) {
+  static_assert(
+      !is_array_v<_Tp>,
+      "[array.creation]/4: to_array does not accept multidimensional arrays.");
+  static_assert(
+      is_move_constructible_v<_Tp>,
+      "[array.creation]/4: to_array requires move constructible elements.");
+  return __to_array_rvalue_impl(_VSTD::move(__arr),
+                                make_index_sequence<_Size>());
+}
+
+#endif // _LIBCPP_STD_VER > 17
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_ARRAY

libcxx/include/version

 __cpp_lib_scoped_lock                                   201703L <mutex>
 __cpp_lib_shared_mutex                                  201505L <shared_mutex>
 __cpp_lib_shared_ptr_arrays                             201611L <memory>
 __cpp_lib_shared_ptr_weak_type                          201606L <memory>
 __cpp_lib_shared_timed_mutex                            201402L <shared_mutex>
 __cpp_lib_string_udls                                   201304L <string>
 __cpp_lib_string_view                                   201606L <string> <string_view>
 __cpp_lib_three_way_comparison                          201711L <compare>
+__cpp_lib_to_array                                      201907L <array>
 __cpp_lib_to_chars                                      201611L <utility>
 __cpp_lib_transformation_trait_aliases                  201304L <type_traits>
 __cpp_lib_transparent_operators                         201510L <functional>
                                                         201210L // C++14
 __cpp_lib_tuple_element_t                               201402L <tuple>
 __cpp_lib_tuples_by_type                                201304L <utility> <tuple>
 __cpp_lib_type_trait_variable_templates                 201510L <type_traits>
 __cpp_lib_uncaught_exceptions                           201411L <exception>

 # define __cpp_lib_erase_if                             201811L
 // # define __cpp_lib_generic_unordered_lookup             201811L
 # define __cpp_lib_interpolate                          201902L
 # if !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
 #   define __cpp_lib_is_constant_evaluated              201811L
 # endif
 // # define __cpp_lib_list_remove_return_type              201806L
 // # define __cpp_lib_ranges                               201811L
+# define __cpp_lib_to_array                                201907L
 // # define __cpp_lib_three_way_comparison                 201711L
 #endif
 
 #endif // _LIBCPP_VERSIONH

libcxx/test/std/containers/sequences/array/array.creation/to_array.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <array>
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+#include <array>
+
+#include "test_macros.h"
+#include "MoveOnly.h"
+
+int main(int, char**) {
+  {
+    char source[3][6] = {"hi", "world"};
+    std::to_array(source); // expected-error {{here}}
+  }
+
+  {
+    MoveOnly mo[] = {MoveOnly{3}};
+    std::to_array(mo); // expected-error {{here}}
+  }
+
+  {
+    const MoveOnly cmo[] = {MoveOnly{3}};
+    std::to_array(std::move(cmo)); // expected-error {{here}}
+  }
+
+  return 0;
+}

libcxx/test/std/containers/sequences/array/array.creation/to_array.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <array>
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+// template <typename T, size_t Size>
+// constexpr auto to_array(T (&arr)[Size])
+//    -> array<remove_cv_t<T>, Size>;
+
+// template <typename T, size_t Size>
+// constexpr auto to_array(T (&&arr)[Size])
+//    -> array<remove_cv_t<T>, Size>;
+
+#include <array>
+#include <cassert>
+
+#include "test_macros.h"
+#include "MoveOnly.h"
+
+int main(int, char**) {
+  //  Test deduced type.
+  {
+    auto arr = std::to_array({1, 2, 3});
+    ASSERT_SAME_TYPE(decltype(arr), std::array<int, 3>);
+    assert(arr[0] == 1);
+    assert(arr[1] == 2);
+    assert(arr[2] == 3);
+  }
+
+  {
+    const long l1 = 42;
+    auto arr = std::to_array({1L, 4L, 9L, l1});
+    ASSERT_SAME_TYPE(decltype(arr)::value_type, long);
+    static_assert(arr.size() == 4, "");
+    assert(arr[0] == 1);
+    assert(arr[1] == 4);
+    assert(arr[2] == 9);
+    assert(arr[3] == l1);
+  }
+
+  {
+    auto arr = std::to_array("meow");
+    ASSERT_SAME_TYPE(decltype(arr), std::array<char, 5>);
+    assert(arr[0] == 'm');
+    assert(arr[1] == 'e');
+    assert(arr[2] == 'o');
+    assert(arr[3] == 'w');
+    assert(arr[4] == '\0');
+  }
+
+  {
+    double source[3] = {4.0, 5.0, 6.0};
+    auto arr = std::to_array(source);
+    ASSERT_SAME_TYPE(decltype(arr), std::array<double, 3>);
+    assert(arr[0] == 4.0);
+    assert(arr[1] == 5.0);
+    assert(arr[2] == 6.0);
+  }
+
+  {
+    double source[3] = {4.0, 5.0, 6.0};
+    auto arr = std::to_array(std::move(source));
+    ASSERT_SAME_TYPE(decltype(arr), std::array<double, 3>);
+    assert(arr[0] == 4.0);
+    assert(arr[1] == 5.0);
+    assert(arr[2] == 6.0);
+  }
+
+  {
+    MoveOnly source[] = {MoveOnly{0}, MoveOnly{1}, MoveOnly{2}};
+
+    auto arr = std::to_array(std::move(source));
+    ASSERT_SAME_TYPE(decltype(arr), std::array<MoveOnly, 3>);
+    for (int i = 0; i < 3; ++i)
+      assert(arr[i].get() == i && source[i].get() == 0);
+  }
+
+  // Test C99 compound literal.
+  {
+    auto arr = std::to_array((int[]){3, 4});
+    ASSERT_SAME_TYPE(decltype(arr), std::array<int, 2>);
+    assert(arr[0] == 3);
+    assert(arr[1] == 4);
+  }
+
+  //  Test explicit type.
+  {
+    auto arr = std::to_array<long>({1, 2, 3});
+    ASSERT_SAME_TYPE(decltype(arr), std::array<long, 3>);
+    assert(arr[0] == 1);
+    assert(arr[1] == 2);
+    assert(arr[2] == 3);
+  }
+
+  {
+    struct A {
+      int a;
+      double b;
+    };
+
+    auto arr = std::to_array<A>({{3, .1}});
+    ASSERT_SAME_TYPE(decltype(arr), std::array<A, 1>);
+    assert(arr[0].a == 3);
+    assert(arr[0].b == .1);
+  }
+
+  // Test constexpr.
+  {
+    constexpr std::array<int, 3> arr = std::to_array({1, 2, 3});
+    static_assert(arr[0] == 1);
+    static_assert(arr[1] == 2);
+    static_assert(arr[2] == 3);
+  }
+
+  return 0;
+}

libcxx/test/std/language.support/support.limits/support.limits.general/array.version.pass.cpp

 // <array>
 
 // Test the feature test macros defined by <array>
 
 /*  Constant                                Value
     __cpp_lib_array_constexpr               201603L [C++17]
     __cpp_lib_constexpr_misc                201811L [C++2a]
     __cpp_lib_nonmember_container_access    201411L [C++17]
+    __cpp_lib_to_array                      201907L [C++2a]
 */
 
 #include <array>
 #include "test_macros.h"
 
 #if TEST_STD_VER < 14
 
 # ifdef __cpp_lib_array_constexpr
 #   error "__cpp_lib_array_constexpr should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_constexpr_misc
 #   error "__cpp_lib_constexpr_misc should not be defined before c++2a"
 # endif
 
 # ifdef __cpp_lib_nonmember_container_access
 #   error "__cpp_lib_nonmember_container_access should not be defined before c++17"
 # endif
 
+# ifdef __cpp_lib_to_array
+#   error "__cpp_lib_to_array should not be defined before c++2a"
+# endif
+
 #elif TEST_STD_VER == 14
 
 # ifdef __cpp_lib_array_constexpr
 #   error "__cpp_lib_array_constexpr should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_constexpr_misc
 #   error "__cpp_lib_constexpr_misc should not be defined before c++2a"
 # endif
 
 # ifdef __cpp_lib_nonmember_container_access
 #   error "__cpp_lib_nonmember_container_access should not be defined before c++17"
 # endif
 
+# ifdef __cpp_lib_to_array
+#   error "__cpp_lib_to_array should not be defined before c++2a"
+# endif
+
 #elif TEST_STD_VER == 17
 
 # ifndef __cpp_lib_array_constexpr
 #   error "__cpp_lib_array_constexpr should be defined in c++17"
 # endif
 # if __cpp_lib_array_constexpr != 201603L
 #   error "__cpp_lib_array_constexpr should have the value 201603L in c++17"
 # endif
 
 # ifdef __cpp_lib_constexpr_misc
 #   error "__cpp_lib_constexpr_misc should not be defined before c++2a"
 # endif
 
 # ifndef __cpp_lib_nonmember_container_access
 #   error "__cpp_lib_nonmember_container_access should be defined in c++17"
 # endif
 # if __cpp_lib_nonmember_container_access != 201411L
 #   error "__cpp_lib_nonmember_container_access should have the value 201411L in c++17"
 # endif
 
+# ifdef __cpp_lib_to_array
+#   error "__cpp_lib_to_array should not be defined before c++2a"
+# endif
+
 #elif TEST_STD_VER > 17
 
 # ifndef __cpp_lib_array_constexpr
 #   error "__cpp_lib_array_constexpr should be defined in c++2a"
 # endif
 # if __cpp_lib_array_constexpr != 201603L
 #   error "__cpp_lib_array_constexpr should have the value 201603L in c++2a"
 # endif

 
 # ifndef __cpp_lib_nonmember_container_access
 #   error "__cpp_lib_nonmember_container_access should be defined in c++2a"
 # endif
 # if __cpp_lib_nonmember_container_access != 201411L
 #   error "__cpp_lib_nonmember_container_access should have the value 201411L in c++2a"
 # endif
 
+# ifndef __cpp_lib_to_array
+#   error "__cpp_lib_to_array should be defined in c++2a"
+# endif
+# if __cpp_lib_to_array != 201907L
+#   error "__cpp_lib_to_array should have the value 201907L in c++2a"
+# endif
+
 #endif // TEST_STD_VER > 17
 
 int main(int, char**) { return 0; }

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp

/*  Constant                                       Value
     __cpp_lib_scoped_lock                          201703L [C++17]
     __cpp_lib_shared_mutex                         201505L [C++17]
     __cpp_lib_shared_ptr_arrays                    201611L [C++17]
     __cpp_lib_shared_ptr_weak_type                 201606L [C++17]
     __cpp_lib_shared_timed_mutex                   201402L [C++14]
     __cpp_lib_string_udls                          201304L [C++14]
     __cpp_lib_string_view                          201606L [C++17]
     __cpp_lib_three_way_comparison                 201711L [C++2a]
+    __cpp_lib_to_array                             201907L [C++2a]
     __cpp_lib_to_chars                             201611L [C++17]
     __cpp_lib_transformation_trait_aliases         201304L [C++14]
     __cpp_lib_transparent_operators                201210L [C++14]
                                                    201510L [C++17]
     __cpp_lib_tuple_element_t                      201402L [C++14]
     __cpp_lib_tuples_by_type                       201304L [C++14]
     __cpp_lib_type_trait_variable_templates        201510L [C++17]
     __cpp_lib_uncaught_exceptions                  201411L [C++17]

 # ifdef __cpp_lib_string_view
 #   error "__cpp_lib_string_view should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_three_way_comparison
 #   error "__cpp_lib_three_way_comparison should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_to_array
+#   error "__cpp_lib_to_array should not be defined before c++2a"
+# endif
+
 # ifdef __cpp_lib_to_chars
 #   error "__cpp_lib_to_chars should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_transformation_trait_aliases
 #   error "__cpp_lib_transformation_trait_aliases should not be defined before c++14"
 # endif
 

 # ifdef __cpp_lib_string_view
 #   error "__cpp_lib_string_view should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_three_way_comparison
 #   error "__cpp_lib_three_way_comparison should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_to_array
+#   error "__cpp_lib_to_array should not be defined before c++2a"
+# endif
+
 # ifdef __cpp_lib_to_chars
 #   error "__cpp_lib_to_chars should not be defined before c++17"
 # endif
 
 # ifndef __cpp_lib_transformation_trait_aliases
 #   error "__cpp_lib_transformation_trait_aliases should be defined in c++14"
 # endif
 # if __cpp_lib_transformation_trait_aliases != 201304L

 # if __cpp_lib_string_view != 201606L
 #   error "__cpp_lib_string_view should have the value 201606L in c++17"
 # endif
 
 # ifdef __cpp_lib_three_way_comparison
 #   error "__cpp_lib_three_way_comparison should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_to_array
+#   error "__cpp_lib_to_array should not be defined before c++2a"
+# endif
+
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_to_chars
 #     error "__cpp_lib_to_chars should be defined in c++17"
 #   endif
 #   if __cpp_lib_to_chars != 201611L
 #     error "__cpp_lib_to_chars should have the value 201611L in c++17"
 #   endif
 # else // _LIBCPP_VERSION

 #     error "__cpp_lib_three_way_comparison should have the value 201711L in c++2a"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_three_way_comparison
 #     error "__cpp_lib_three_way_comparison should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
+# ifndef __cpp_lib_to_array
+#   error "__cpp_lib_to_array should be defined in c++2a"
+# endif
+# if __cpp_lib_to_array != 201907L
+#   error "__cpp_lib_to_array should have the value 201907L in c++2a"
+# endif
+
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_to_chars
 #     error "__cpp_lib_to_chars should be defined in c++2a"
 #   endif
 #   if __cpp_lib_to_chars != 201611L
 #     error "__cpp_lib_to_chars should have the value 201611L in c++2a"
 #   endif
 # else // _LIBCPP_VERSION

libcxx/utils/generate_feature_test_macro_components.py

feature_test_macros = sorted([ add_version_header(x) for x in [
    "headers": ["numeric"],
    },
   {"name": "__cpp_lib_endian",
    "values": {
      "c++2a": int(201907),
    },
    "headers": ["bit"],
    },
+  {"name": "__cpp_lib_to_array",
+   "values": {
+     "c++2a": 201907L,
+   },
+   "headers": ["array"],
+   },
 ]], key=lambda tc: tc["name"])
 
 def get_std_dialects():
   std_dialects = ['c++14', 'c++17', 'c++2a']
   return list(std_dialects)
 
 def get_first_std(d):
     for s in get_std_dialects():

libcxx/www/cxx2a_status.html

-->
   <tr><td><a href="https://wg21.link/P1357R1">P1357R1</a></td><td>LWG</td><td>Traits for [Un]bounded Arrays</td><td>Kona</td><td>Complete</td><td>9.0</td></tr>
   <tr><td><a href="https://wg21.link/P1458R1">P1458R1</a></td><td>LWG</td><td>Mandating the Standard Library: Clause 16 - Language support library</td><td>Kona</td><td>Complete</td><td>9.0</td></tr>
   <tr><td><a href="https://wg21.link/P1459R1">P1459R1</a></td><td>LWG</td><td>Mandating the Standard Library: Clause 18 - Diagnostics library</td><td>Kona</td><td>Complete</td><td>9.0</td></tr>
   <tr><td><a href="https://wg21.link/P1462R1">P1462R1</a></td><td>LWG</td><td>Mandating the Standard Library: Clause 20 - Strings library</td><td>Kona</td><td>Complete</td><td>9.0</td></tr>
   <tr><td><a href="https://wg21.link/P1463R1">P1463R1</a></td><td>LWG</td><td>Mandating the Standard Library: Clause 21 - Containers library</td><td>Kona</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1464R1">P1464R1</a></td><td>LWG</td><td>Mandating the Standard Library: Clause 22 - Iterators library</td><td>Kona</td><td>Complete</td><td>9.0</td></tr>
 
    <tr><td></td><td></td><td></td><td></td><td></td><td></td></tr>
-  <tr><td><a href="https://wg21.link/P0325">P0325</a></td><td>LWG</td><td>to_array from LFTS with updates</td><td>Cologne</td><td></td><td></td></tr>
+  <tr><td><a href="https://wg21.link/P0325">P0325</a></td><td>LWG</td><td>to_array from LFTS with updates</td><td>Cologne</td><td>Complete</td><td>10.0</td></tr>
   <tr><td><a href="https://wg21.link/P0408">P0408</a></td><td>LWG</td><td>Efficient Access to basic_stringbuf ’s Buffer</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0466">P0466</a></td><td>LWG</td><td>Layout-compatibility and Pointer-interconvertibility Traits</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0553">P0553</a></td><td>LWG</td><td>Bit operations</td><td>Cologne</td><td>Complete</td><td>9.0</td></tr>
   <tr><td><a href="https://wg21.link/P0631">P0631</a></td><td>LWG</td><td>Math Constants</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0645">P0645</a></td><td>LWG</td><td>Text Formatting</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0660">P0660</a></td><td>LWG</td><td>Stop Token and Joining Thread, Rev 10</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0784">P0784</a></td><td>CWG</td><td>More constexpr containers</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0980">P0980</a></td><td>LWG</td><td>Making std::string constexpr</td><td>Cologne</td><td></td><td></td></tr>