[libc++][format] Add __format_arg_store.

[Mordante]

Summary

This implements the struct __format_arg_store and its dependencies:

• the class basic_format_arg,
• the class basic_format_args,
• the class basic_format_context,
• the function make_format_args,
• the function wmake_format_args,
• the function visit_format_arg,
• several Standard required typedefs.

The following parts will be implemented in a later patch:

• the child class basic_format_arg::handle,
• the function basic_format_arg::basic_format_arg(const T* p).

The following extension has been implemented:

• the class basic_format_arg supports __[u]int128_t on platform where libc++ supports 128 bit integrals.

Implements parts of:

• P0645 Text Formatting

Completes:

• LWG3371 visit_format_arg and make_format_args are not hidden friends
• LWG3542 basic_format_arg mishandles basic_string_view with custom traits

Note https://mordante.github.io/blog/2021/06/05/format.html gives a bit more information about the goals and non-goals of this initial patch series.

[Mordante]

Rebased on main
Polishing
implement a real output_iterator concept, main has the require prerequisite.

[Mordante]

Attempt to fix the build.

[Mordante]

libcxx/include/module.modulemap

554

Work-around. Needs to be removed when the modules work properly in main.

[Mordante]

Fixes an bug in the format_arg constructor for string_view.
Improve the format_arg constructor for string by addressing LWG3542.

Note the paper status will be updated once the C++2b LWG issues have been updated.

[vitaut]

"the function make_format_args," appears twice in the summary. I guess the second one was supposed to be "make_wformat_args".

libcxx/include/__format/format_arg.h

18

Do we have to include all of functional for invoke?

70

Visiting an empty argument should pass monostate to the visitor, not throw an exception.

101

Why store a C string as void* and cast? I suggest storing as const char_type* instead.

117

Initializing __bool is unnecessary because the value is never accessed if type is __none. Probably doesn't matters in practice but there is no harm is doing less unnecessary work.

libcxx/include/__format/format_context.h

52–63

basic_format_context is not supposed to be constructed by users so these ctors better be private.

93

create -> created

94

allow -> allows

95–96

Lazy creation is fine but using optional is suboptimal since locale is basically a pointer.

libcxx/include/format

18

It is no longer considering, the paper has been accepted.

241–245

This is identical to make_format_args so it just adds unnecessary indirection and a ton of template instantiation. I suggest removing.

libcxx/test/std/utilities/format/format.arguments/format.arg.store/class.pass.cpp

29

This better be parameterized on char type, not context (and the same throughout the tests).

[Mordante]

Thanks for the review!

In D103357#2819377, @vitaut wrote:

"the function make_format_args," appears twice in the summary. I guess the second one was supposed to be "make_wformat_args".

Good catch.

libcxx/include/__format/format_arg.h

18

It seems we do.

70

I thought this was an allowed optimization, but that seems not to be the case.
Adding monostate will require including <variant>. At the moment we're busy modularizing our headers, but that unveils some clang issues. So I'll include the entire header with a TODO.
This change will break some other patches; I'll update them later.

117

I recall I originally had it like that, but that caused some issues. However testing again it seems not to be required (anymore).

libcxx/include/__format/format_context.h

52–63

Not being able construct it makes it hard to test the implementation. So I prefer to keep them public.
MSVC STL also implements their constructors as public.

95–96

True, want to look at a better solution later, hence the TODO. I initially wanted to focus on getting something working and optimize later.

libcxx/include/format

18

:-) (It was valid at the time of writing, I probably need to update the status of P2216 at more places.)

libcxx/test/std/utilities/format/format.arguments/format.arg.store/class.pass.cpp

29

Nice suggestion, that makes the callers a bit less verbose.

[Mordante]

Addresses review comments.
Update module map.
Update P2216 status; it will be part of C++23 and backported to C++20.

[Mordante]

Tabs -> space.

[vitaut]

LGTM but this will need to be reviewed by someone familiar with libc++ of course =).

libcxx/include/__format/format_arg.h

70

Microsoft STL pulled out monostate into a separate header to avoid including variant.

[Mordante]

Tab -> space.

[Mordante]

In D103357#2829269, @vitaut wrote:

LGTM but this will need to be reviewed by someone familiar with libc++ of course =).

Thanks for the review. Yeah the final approval will be done by someone more familiar with libc++.

libcxx/include/__format/format_arg.h

70

I plan to do the same. I added a todo at line 20.

[Mordante]

Forward declare visit_format_arg. Test whether this fixes the modular build.

[Mordante]

Revert last change.
Included more headers and the test passes locally.

[Mordante]

Adds some missing includes. Hopefully that fixes the GCC builds.

[Mordante]

Disable the failing test on GCC.

[Mordante]

Disable test gcc-10.
Attempt to fix the modular build.

[Mordante]

Fixes for the build. The test pass now locally both with a normal and a modular build.
Thanks to @ldionne and @cjdb for their input.

[Mordante]

Rebase to trigger CI, the previous build failed due to a flacky test.
Use <__utility/forward.h> instead of <utility> in <__format/format_fwd.h>.

[Mordante]

Rebased.
Use new monostate header.

[vitaut]

libcxx/include/__format/format_arg.h

233–235

An overload for [const] void* is missing.

239–240

In general basic_format_args are not comparable because they can refer to type-erased user-defined objects/formatters that can't be compared. If this is for testing I suggest moving to tests.

libcxx/include/__format/format_context.h

64

You could expose __uglified public "factory" functions for testing. MSVC is not a great example, they were doing a bunch of things wrong, I would reported this one if I had time.

libcxx/include/__format/format_fwd.h

2

What is the purpose of the format_fwd.h header?

libcxx/include/format

238

This could be a built-in array and the dependency on <array> removed. There is no advantage to using std::array here.

libcxx/test/std/utilities/format/format.arguments/format.arg.store/class.pass.cpp

36–39

These checks seem overspecified especially considering that the representation will have to change to implement one of the TODOs. I'd recommend doing checks by converting __format_arg_store into basic_format_args and using the public API. Same elsewhere.

Another advantage of going through basic_format_args is that it will mimic the actual use.

libcxx/test/std/utilities/format/format.arguments/format.arg/visit_format_arg.pass.cpp

88–109

All of this will have to change when we add char*_t support. I think we should static_assert in basic_format_context or elsewhere that CharT is char or wchar_t for now since other char type are not usable with <format> anyway and these conversions are morally wrong.

132

Create an alias for std::conditional_t<sizeof(long) == sizeof(int), int, long long> to avoid copy-paste?

198–217

ditto

281–291

Would be nice to add a test for basic_string_view with traits and basic_string with traits and an allocator.

libcxx/test/std/utilities/format/format.formatter/format.context/arg.pass.cpp

30

nit: I'd call it output or result to distinguish from string above.

[Mordante]

Rebased
Addresses most review comments
Uses new XFAIL regex for apple platforms
Minor cleanups

[Mordante]

Thanks for the review. I'll some more time to look at your last suggestion.

libcxx/include/__format/format_arg.h

233–235

That's the TODO below. I haven't added proper pointer formatting yet. (In hindsight I should also have removed the nullptr constructor for now.

239–240

It's for testing purposes. However when I originally added this I hadn't implemented the visitor yet. So I can used the visitor in the tests and remove this altogether.

libcxx/include/__format/format_fwd.h

2

Adding some declarations for types that need to know the other types exist.
(And slightly abused to declare some parts of the not yet implemented iterator library. That will be removed once libc++ implements these parts of the C++20 iterator library.)

libcxx/include/format

238

I did some investigation, but changing this to a built-in array was not trivial. Especially since sizeof...(_Args) can be zero. I still need to implement the required size optimizations for basic_format_args. So I prefer to tackle these two at the same time.
For now I added a TODO in the code.

libcxx/test/std/utilities/format/format.arguments/format.arg.store/class.pass.cpp

36–39

I think that's a valid point. Most of this test is "identical" to the test libcxx/test/std/utilities/format/format.arguments/format.args/ctor.pass.cpp. For now I changed the test to use libc++ specific asserts.

@ldionne What's your opinion of testing exposition only classes? I think the tests were great to have during development, but I agree with @vitaut that it would be better to test this class indirectly by using the public interface.

libcxx/test/std/utilities/format/format.arguments/format.arg/visit_format_arg.pass.cpp

88–109

For some place where it's easy I already test for char*_t support. I think it would make more sense to restrict that in the formatters. There the Standard is explicit about the supported types. I'll add a note to my todo list to update the formatters.

281–291

I really like this suggestion!

[Mordante]

Now really rebase.

[Mordante]

libcxx/include/__format/format_context.h

64

I thought MSVC was heavily based on libfmt.

I did some investigations and your suggestion seems to work.
When I make the constructor private I'll need a "factory" or a friend for the internal parts of <format>.
So I'll go for a "factory" so the unit tests and internal implementation can share the same function.

[Mordante]

Make basic_format_context's constructor private and add a helper for its users.
Mark unit tests using the helper as libc++ specific.
Disabled a failing unit tests that fails on GCC.
Make the module headers private.

[Mordante]

Fixes localization build.
Rebased, hopefully fixes Apple builds.

[Mordante]

Rebase to fix the GCC build.

[vitaut]

LGTM

[vitaut]

libcxx/include/__format/format_context.h

64

I thought MSVC was heavily based on libfmt.

It was loosely based on {fmt} with important pieces removed for some reason (but partly reintroduced now via PRs and issues).

[Mordante]

Guard unit tests for implementation details

[Mordante]

libcxx/include/__format/format_arg.h

67

s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/ here and all other occurrences in this patch.

[• Quuxplusone]

libcxx/include/__format/format_context.h

60

Re your comment on D106567 — It would be cleaner here to write "one function declaration per function signature," i.e. get rid of the defaulted function argument ( https://quuxplusone.github.io/blog/2020/04/18/default-function-arguments-are-the-devil ), and then I bet you wouldn't run into any modules issues with the 2-argument overload (because it wouldn't mention optional in its signature) nor the 3-argument overload (because it wouldn't mention optional in its signature, either). The next step would be to give the two things not-the-same name.
I actually talk about the "ooh, I'll take an optional that defaults to nullopt!" antipattern in Back to Basics: Algebraic Data Types (CppCon 2020), and obviously I've talked about When to Give Two Things the Same Name (CppCon 2021) as well. :)

[Mordante]

etarget for LLVM 14.
s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/
Guard better against ADL
Move internal unit tests
Generate private module tests

[Mordante]

Update the private module tests.
These failed due to local merge conflict files.

[Mordante]

libcxx/include/__format/format_context.h

60

I prefer the current way. This function used to be a constructor before the class' constructor was made private. This was done to prevent users from directly creating basic_format_context objects. I read your blog post before and I partly agree with it, however default arguments are a tool. In this case I feel the code looks better with a default argument instead of having two functions.

I agree this would be bad when the function would take several bool arguments with a defaulted value. IMO a function with multiple bool arguments often is already a code smell by itself.

[ldionne]

libcxx/docs/Status/Format.rst

42–44 ↗

(On Diff #361307)

I'm not sure this is worth mentioning at all, since we're going to implement https://wg21.link/P2216 and users will never notice. Also, it'll have been implemented by the time we ship <format> for the first time.

libcxx/include/__format/format_arg.h

38

We can remove this now. LLVM 13 has branched, and the oldest compiler we support should support concepts unless I'm mistaken. There will be a similar cleanup in the <ranges> code we merged already.

Note: I need to validate that the oldest compiler we support has support for concepts - I think that would be AppleClang 12, but I'm not sure all versions of it support concepts.

145

Why aren't we using a variant here?

@vitaut I'm being told you originally requested that Mark moves from variant to this union, was there a reason? I'd like to avoid going back and forth on this issue.

175–178

Suggestion to reduce the number of overloads:

template <class _Tp>
  requires (__libcpp_signed_integer<_Tp> || __libcpp_unsigned_integer<_Tp>) // might be a better way to do this
_LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(_Tp __v) noexcept
{
  if constexpr (sizeof(_Tp) <= sizeof(int)) { }
  else if constexpr (...) { }
  // etc...
}

Basically, this single constructor should handle all the branches in http://eel.is/c++draft/format#arg-5.

221

IMO I'd rather have a longer line than a funky line break here.

Suggestion: Put _LIBCPP_HIDE_FROM_ABI on a line above, like so:

_LIBCPP_HIDE_FROM_ABI
explicit basic_format_arg(...);

This is what we do in most places. As we just talked about, if this creates pain because of clang-format and rebasing, don't waste your time on this. It's just a suggestion.

libcxx/include/__format/format_args.h

57

Is this [[nodiscard]] really adding anything? Non blocking.

IMO, using nodiscard everywhere because C++ has the wrong default here just adds syntactic overhead and reduces the impact of seeing [[nodiscard]] on a method that could truly be used incorrectly. I think we should stick to putting it on key methods where we really want to diagnose if the user discards the result, because that would most likely be a bug (e.g. anything called empty, something like lock_guard, etc).

libcxx/include/__format/format_context.h

123

Maybe those constructors should all be explicit?

libcxx/include/__format/format_fwd.h

40

This has been implemented now.

libcxx/include/format

13–20

This is not required. We're not shipping the library until it's stable anyway, with the mechanism you added.

libcxx/test/libcxx/utilities/format/format.formatter/format.context/locale.pass.cpp

40 ↗

(On Diff #361307)

Would it be possible to move those tests into libcxx/test/std if we added a function that creates a basic_format_context to the test suite? Different implementations could hook into that function (in the test suite) to tell the test suite how to create a basic_format_context. Do you think that would make sense and be feasible?

I'm trying to see how we can test those parts of the API which are kind of implementation details of the library, but at the same time they're defined in the spec.

libcxx/test/support/test_basic_format_arg.h

15 ↗

(On Diff #361307)

Please drop [[nodiscard]] in the tests unless it adds a clear benefit (e.g. a method that could meaningfully be misused).

[vitaut]

libcxx/include/__format/format_arg.h

145

Not using variant allows optimizing the representation for the common case of a small number of formatting arguments: http://eel.is/c++draft/format#args-note-1

Implementations should optimize the representation of basic_­format_­args for a small number of formatting arguments.
[Note 1: For example, by storing indices of type alternatives separately from values and packing the former. — end note]

[• Quuxplusone]

libcxx/include/__format/format_arg.h

145

This open-coded __arg_t __type_ member takes only 1 byte; libc++'s <variant> would use a field __variant_index_t __index_ which takes 4 bytes unless _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION is set.
See D40210.
The Right Answer here might depend on how close we think we are to "ABI v2 by default". Currently that looks like "never". ;)

However however, my understanding of @vitaut's comment is that this code still is not implementing the recommended practice. The recommended practice is that basic_format_args shouldn't store an array of basic_format_arg {union, char} structs at all; it should store a struct of arrays (one array of unions and one array of chars).

I don't think there's any motivation to optimize the layout of basic_format_arg singular. Using variant would be fine, except that it would introduce an extra dependency on <variant> and probably harm compile times.

libcxx/include/__format/format_args.h

63

This is the array-of-structs that @vitaut pointed to. It should be rewritten as a struct-of-arrays.

[vitaut]

libcxx/include/__format/format_arg.h

145

My understanding is that the optimization is planned for a follow-up diff in which case switch to variant would have to be undone. cc @Mordante

[Mordante]

libcxx/docs/Status/Format.rst

42–44 ↗

(On Diff #361307)

This was based on the original plan to have an incomplete version in LLVM 13. But after our discussion last week I agree this is no longer useful. I'll remove the entire remark.

libcxx/include/__format/format_arg.h

38

I just tested with a quick hack (D107066) and it seems not all CI nodes support concepts.
https://buildkite.com/llvm-project/libcxx-ci/builds/4579

145

Yes the goal is to first have a working implementation where the observable behaviour matches the Standard.
Then this working implementation can be improved. This makes it a lot easier to detect regressions and measure the improvements.
There is a TODO FMT so I won't forget about it.

175–178

I experimented with this approach and I wasn't happy with the result. If feels odd to first require __libcpp_signed_integer || __libcpp_unsigned_integer and then in the code test which of the two has been used. Adding char_type and bool made to code even less readable. Especially when char_type is wchar_t it accepts char, but it doesn't allow narrowing the other way around.
So I reduced this set to 4 constructors

• bool unmodified
• char_type unmodified
• __libcpp_signed_integer all in one overload and then use the if constexpr(sizeof(_Tp)...
• __libcpp_unsigned_integer like above

That IMO leads to a better balance between removing overloads and having readable code.

221

I tested it. Making this change manually will be reverted by clang-format.

libcxx/test/libcxx/utilities/format/format.formatter/format.context/locale.pass.cpp

40 ↗

(On Diff #361307)

It's very feasible I just need 2 wrapper functions that call std::__format_context_create; one with a std::locale and one without that argument not; basically the same as the std::format* functions.

Whether it makes sense, that depends on whether or not other vendors will supply patches for their hooks.

[Mordante]

Rebased and updated to work against main.
Addresses review comments.

[ldionne]

I read the comments and while I am not a big fan of reimplementing std::variant, I understand why it's needed (unless we want to break std::variant's ABI, which we don't). Also, this needs to make progress so I'm approving it -- worst case I can request some post-commit changes.

[Mordante]

Rebased to give it a CI run test all still is fine.
Remove tests for gcc-10 since it's no longer supported.

[Mordante]

Retry to start CI.

[Mordante]

In D103357#2975190, @ldionne wrote:

I read the comments and while I am not a big fan of reimplementing std::variant, I understand why it's needed (unless we want to break std::variant's ABI, which we don't). Also, this needs to make progress so I'm approving it -- worst case I can request some post-commit changes.

Thanks!

A note on the std::variant; breaking the ABI won't help. The tags and unions should be stored separately, something like:

std::array<tag, 4> tags;
std::array<unions, 4> unions;

And the note mentions packing the unions, so something like:

std::array<tag, 4> tags;
std::tuple<t0, t1, t2, t3> packed_unions;

(This is an example, I haven't investigated the solution I want to use.)

[Mordante]

Seems Buildkite became unstuck, retry run the CI.

libcxx/docs/Cxx2aStatusIssuesStatus.csv

 "`3358 <https://wg21.link/LWG3358>`__","|sect|\ [span.cons] is mistaken that ``to_address``\  can throw","Prague","",""
 "`3359 <https://wg21.link/LWG3359>`__","``<chrono>``\  leap second support should allow for negative leap seconds","Prague","",""
 "`3360 <https://wg21.link/LWG3360>`__","``three_way_comparable_with``\  is inconsistent with similar concepts","Prague","",""
 "`3362 <https://wg21.link/LWG3362>`__","Strike ``stop_source``\ 's ``operator!=``\ ","Prague","",""
 "`3363 <https://wg21.link/LWG3363>`__","``drop_while_view``\  should opt-out of ``sized_range``\ ","Prague","",""
 "`3364 <https://wg21.link/LWG3364>`__","Initialize data members of ranges and their iterators","Prague","",""
 "`3367 <https://wg21.link/LWG3367>`__","Integer-class conversions should not throw","Prague","",""
 "`3369 <https://wg21.link/LWG3369>`__","``span``\ 's deduction-guide for built-in arrays doesn't work","Prague","",""
-"`3371 <https://wg21.link/LWG3371>`__","``visit_format_arg``\  and ``make_format_args``\  are not hidden friends","Prague","",""
+"`3371 <https://wg21.link/LWG3371>`__","``visit_format_arg``\  and ``make_format_args``\  are not hidden friends","Prague","|Complete|","13.0"
 "`3372 <https://wg21.link/LWG3372>`__","``vformat_to``\  should not try to deduce ``Out``\  twice","Prague","",""
 "`3373 <https://wg21.link/LWG3373>`__","``{to,from}_chars_result``\  and ``format_to_n_result``\  need the  ""we really mean what we say"" wording","Prague","",""
 "`3374 <https://wg21.link/LWG3374>`__","P0653 + P1006 should have made the other ``std::to_address``\  overload ``constexpr``\ ","Prague","|Complete|","12.0"
 "`3375 <https://wg21.link/LWG3375>`__","``decay``\  in ``viewable_range``\  should be ``remove_cvref``\ ","Prague","",""
 "`3377 <https://wg21.link/LWG3377>`__","``elements_view::iterator``\  befriends a specialization of itself","Prague","",""
 "`3379 <https://wg21.link/LWG3379>`__","""``safe``\ "" in several library names is misleading","Prague","|In Progress|",""
 "`3380 <https://wg21.link/LWG3380>`__","``common_type``\  and comparison categories","Prague","",""
 "`3381 <https://wg21.link/LWG3381>`__","``begin``\  and ``data``\  must agree for ``contiguous_range``\ ","Prague","",""

libcxx/include/CMakeLists.txt

 set(files
   __availability
   __bit_reference
   __bits
   __bsd_locale_defaults.h
   __bsd_locale_fallbacks.h
   __config
   __debug
   __errc
+  __format/format_arg.h
+  __format/format_args.h
+  __format/format_context.h
   __format/format_error.h
+  __format/format_fwd.h
   __format/format_parse_context.h
   __function_like.h
   __functional_03
   __functional_base
   __functional_base_03
   __hash_table
   __iterator/advance.h
   __iterator/concepts.h

libcxx/include/__format/format_arg.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FORMAT_FORMAT_ARG_H
+#define _LIBCPP___FORMAT_FORMAT_ARG_H
+
+#include <__config>
+#include <__format/format_error.h>
+#include <__format/format_fwd.h>
+#include <concepts>
+#include <functional>
+#include <string>
+#include <string_view>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+// TODO FMT Remove this once we require compilers with proper C++20 support.
+// If the compiler has no concepts support, the format header will be disabled.
+// Without concepts support enable_if needs to be used and that too much effort
+// to support compilers with partial C++20 support.
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS) &&                                       \
+    !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+namespace __format {
+/** The type stored in @ref basic_format_arg. */
+enum class _LIBCPP_ENUM_VIS __arg_t : uint8_t {
+  __none,
+  __bool,
+  __char_type,
+  __int,
+  __long_long,
+#ifndef _LIBCPP_HAS_NO_INT128
+  __i128,
+#endif
+  __unsigned,
+  __unsigned_long_long,
+#ifndef _LIBCPP_HAS_NO_INT128
+  __u128,
+#endif
+  __float,
+  __double,
+  __long_double,
+  __const_char_type_ptr,
+  __string_view,
+  __ptr
+};
+} // namespace __format
+
+template <class _Visitor, class _Context>
+_LIBCPP_INLINE_VISIBILITY _LIBCPP_AVAILABILITY_FORMAT decltype(auto)
+visit_format_arg(_Visitor&& __vis, basic_format_arg<_Context> __arg) {
+  switch (__arg.__type_) {
+  case __format::__arg_t::__none:
+    __throw_format_error("Argument index out of bounds");
+  case __format::__arg_t::__bool:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__bool);
+  case __format::__arg_t::__char_type:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__char_type);
+  case __format::__arg_t::__int:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__int);
+  case __format::__arg_t::__long_long:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__long_long);
+#ifndef _LIBCPP_HAS_NO_INT128
+  case __format::__arg_t::__i128:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__i128);
+#endif
+  case __format::__arg_t::__unsigned:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__unsigned);
+  case __format::__arg_t::__unsigned_long_long:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis),
+                         __arg.__unsigned_long_long);
+#ifndef _LIBCPP_HAS_NO_INT128
+  case __format::__arg_t::__u128:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__u128);
+#endif
+  case __format::__arg_t::__float:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__float);
+  case __format::__arg_t::__double:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__double);
+  case __format::__arg_t::__long_double:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__long_double);
+  case __format::__arg_t::__const_char_type_ptr:
+    return _VSTD::invoke(
+        _VSTD::forward<_Visitor>(__vis),
+        static_cast<const typename _Context::char_type*>(__arg.__ptr));
+  case __format::__arg_t::__string_view:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__string_view);
+  case __format::__arg_t::__ptr:
+    return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__ptr);
+  }
+  _LIBCPP_UNREACHABLE();
+}
+
+template <class _Context>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT basic_format_arg {
+public:
+  // TODO FMT Define the handle class.
+  class handle;
+
+  _LIBCPP_INLINE_VISIBILITY basic_format_arg() noexcept
+      : __bool{false}, __type_{__format::__arg_t::__none} {}
+
+  _LIBCPP_INLINE_VISIBILITY explicit operator bool() const noexcept {
+    return __type_ != __format::__arg_t::__none;
+  }
+
+private:
+  using char_type = typename _Context::char_type;
+
+  // TODO FMT Implement constrain [format.arg]/4
+  // Constraints: The template specialization
+  //   typename Context::template formatter_type<T>
+  // meets the Formatter requirements ([formatter.requirements]).  The extent
+  // to which an implementation determines that the specialization meets the
+  // Formatter requirements is unspecified, except that as a minimum the
+  // expression
+  //   typename Context::template formatter_type<T>()
+  //    .format(declval<const T&>(), declval<Context&>())
+  // shall be well-formed when treated as an unevaluated operand.
+
+  template <class _Ctx, class... _Args>
+  _LIBCPP_INLINE_VISIBILITY
+      _LIBCPP_AVAILABILITY_FORMAT friend __format_arg_store<_Ctx, _Args...>
+      _VSTD::__make_format_args(const _Args&...);
+
+  template <class _Visitor, class _Ctx>
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_AVAILABILITY_FORMAT friend decltype(auto)
+  _VSTD::visit_format_arg(_Visitor&& __vis, basic_format_arg<_Ctx> __arg);
+
+  union {
+    bool __bool;
+    char_type __char_type;
+    int __int;
+    unsigned __unsigned;
+    long long __long_long;
+    unsigned long long __unsigned_long_long;
+#ifndef _LIBCPP_HAS_NO_INT128
+    __int128_t __i128;
+    __uint128_t __u128;
+#endif
+    float __float;
+    double __double;
+    long double __long_double;
+    const void* __ptr;
+    basic_string_view<char_type> __string_view;
+    // TODO FMT Add the handle.
+  };
+  __format::__arg_t __type_;
+
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(bool __v) noexcept
+      : __bool(__v), __type_(__format::__arg_t::__bool) {}
+
+  template <class _Tp>
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(_Tp __v) noexcept
+      requires(same_as<_Tp, char_type> ||
+               (same_as<_Tp, char> && same_as<char_type, wchar_t>))
+      : __char_type(__v), __type_(__format::__arg_t::__char_type) {}
+
+  template <__libcpp_signed_integer _Tp>
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(_Tp __v) noexcept
+      requires(sizeof(_Tp) <= sizeof(int))
+      : __int(__v), __type_(__format::__arg_t::__int) {}
+
+  template <__libcpp_unsigned_integer _Tp>
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(_Tp __v) noexcept
+      requires(sizeof(_Tp) <= sizeof(unsigned))
+      : __unsigned(__v), __type_(__format::__arg_t::__unsigned) {}
+
+  template <__libcpp_signed_integer _Tp>
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(_Tp __v) noexcept
+      requires(sizeof(_Tp) > sizeof(int))
+      : __long_long(__v), __type_(__format::__arg_t::__long_long) {}
+
+  template <__libcpp_unsigned_integer _Tp>
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(_Tp __v) noexcept
+      requires(sizeof(_Tp) > sizeof(unsigned))
+      : __unsigned_long_long(static_cast<unsigned long long>(__v)),
+        __type_(__format::__arg_t::__unsigned_long_long) {}
+
+#ifndef _LIBCPP_HAS_NO_INT128
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(__int128_t __v) noexcept
+      : __i128(__v), __type_(__format::__arg_t::__i128) {}
+
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(__uint128_t __v) noexcept
+      : __u128(__v), __type_(__format::__arg_t::__u128) {}
+#endif
+
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(float __v) noexcept
+      : __float(__v), __type_(__format::__arg_t::__float) {}
+
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(double __v) noexcept
+      : __double(__v), __type_(__format::__arg_t::__double) {}
+
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(long double __v) noexcept
+      : __long_double(__v), __type_(__format::__arg_t::__long_double) {}
+
+  // Note not a 'noexcept' function.
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(const char_type* __s)
+      : __ptr(__s), __type_(__format::__arg_t::__const_char_type_ptr) {
+    _LIBCPP_ASSERT(__s, "Used a nullptr argument to initialize a C-string");
+  }
+
+  template <class _Traits>
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(
+      basic_string_view<char_type, _Traits> __s) noexcept
+      : __string_view{__s}, __type_(__format::__arg_t::__string_view) {}
+
+  template <class _Traits, class _Allocator>
+  _LIBCPP_INLINE_VISIBILITY explicit basic_format_arg(
+      const basic_string<char_type, _Traits, _Allocator>& __s) noexcept
+      : __string_view{__s}, __type_(__format::__arg_t::__string_view) {}
+
+  _LIBCPP_INLINE_VISIBILITY
+  explicit basic_format_arg(nullptr_t) noexcept
+      : __ptr(nullptr), __type_(__format::__arg_t::__ptr) {}
+
+  // TODO FMT Implement the const _Tp* constructor.
+
+  friend bool operator==(const basic_format_arg& __lhs,
+                         const basic_format_arg& __rhs) {
+    if (__lhs.__type_ != __rhs.__type_)
+      return false;
+    switch (__lhs.__type_) {
+    case __format::__arg_t::__none:
+      return true;
+    case __format::__arg_t::__bool:
+      return __lhs.__bool == __rhs.__bool;
+    case __format::__arg_t::__char_type:
+      return __lhs.__char_type == __rhs.__char_type;
+    case __format::__arg_t::__int:
+      return __lhs.__int == __rhs.__int;
+    case __format::__arg_t::__long_long:
+      return __lhs.__long_long == __rhs.__long_long;
+#ifndef _LIBCPP_HAS_NO_INT128
+    case __format::__arg_t::__i128:
+      return __lhs.__i128 == __rhs.__i128;
+#endif
+    case __format::__arg_t::__unsigned:
+      return __lhs.__unsigned == __rhs.__unsigned;
+    case __format::__arg_t::__unsigned_long_long:
+      return __lhs.__unsigned_long_long == __rhs.__unsigned_long_long;
+#ifndef _LIBCPP_HAS_NO_INT128
+    case __format::__arg_t::__u128:
+      return __lhs.__u128 == __rhs.__u128;
+#endif
+    case __format::__arg_t::__float:
+      return __lhs.__float == __rhs.__float;
+    case __format::__arg_t::__double:
+      return __lhs.__double == __rhs.__double;
+    case __format::__arg_t::__long_double:
+      return __lhs.__long_double == __rhs.__long_double;
+    case __format::__arg_t::__const_char_type_ptr:
+      // Note this isn't efficient since the string is evaluated twice, but it
+      // works regardless of the type of char_type. User code shouldn't use
+      // this function, since it isn't promised to be available.
+      return basic_string_view(static_cast<const char_type*>(__lhs.__ptr)) ==
+             basic_string_view(static_cast<const char_type*>(__rhs.__ptr));
+    case __format::__arg_t::__string_view:
+      return __lhs.__string_view == __rhs.__string_view;
+    case __format::__arg_t::__ptr:
+      return __lhs.__ptr == __rhs.__ptr;
+    }
+    _LIBCPP_UNREACHABLE();
+  }
+};
+
+#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+#endif //_LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___FORMAT_FORMAT_ARG_H

libcxx/include/__format/format_args.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FORMAT_FORMAT_ARGS_H
+#define _LIBCPP___FORMAT_FORMAT_ARGS_H
+
+#include <__config>
+#include <__format/format_fwd.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+// TODO FMT Remove this once we require compilers with proper C++20 support.
+// If the compiler has no concepts support, the format header will be disabled.
+// Without concepts support enable_if needs to be used and that too much effort
+// to support compilers with partial C++20 support.
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS) &&                                       \
+    !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+template <class _Context>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT basic_format_args {
+public:
+  // TODO FMT Implement [format.args]/5
+  // [Note 1: Implementations are encouraged to optimize the representation of
+  // basic_format_args for small number of formatting arguments by storing
+  // indices of type alternatives separately from values and packing the
+  // former. - end note]
+  _LIBCPP_INLINE_VISIBILITY basic_format_args() noexcept = default;
+
+  template <class... _Args>
+  _LIBCPP_INLINE_VISIBILITY basic_format_args(
+      const __format_arg_store<_Context, _Args...>& __store) noexcept
+      : __size_(sizeof...(_Args)), __data_(__store.__args.data()) {}
+
+  _LIBCPP_INLINE_VISIBILITY
+  basic_format_arg<_Context> get(size_t __id) const noexcept {
+    return __id < __size_ ? __data_[__id] : basic_format_arg<_Context>();
+  }
+
+  [[nodiscard]] _LIBCPP_INLINE_VISIBILITY size_t __size() const noexcept {
+    return __size_;
+  }
+
+private:
+  size_t __size_{0};
+  const basic_format_arg<_Context>* __data_{nullptr};
+};
+
+#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+#endif //_LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___FORMAT_FORMAT_ARGS_H

libcxx/include/__format/format_context.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FORMAT_FORMAT_CONTEXT_H
+#define _LIBCPP___FORMAT_FORMAT_CONTEXT_H
+
+#include <__config>
+#include <__format/format_fwd.h>
+#include <concepts>
+#include <iterator>
+
+#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+#include <locale>
+#include <optional>
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+// TODO FMT Remove this once we require compilers with proper C++20 support.
+// If the compiler has no concepts support, the format header will be disabled.
+// Without concepts support enable_if needs to be used and that too much effort
+// to support compilers with partial C++20 support.
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS) &&                                       \
+    !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+template <class _OutIt, class _CharT>
+requires __output_iterator<_OutIt, const _CharT&>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT basic_format_context {
+public:
+  using iterator = _OutIt;
+  using char_type = _CharT;
+  template <class _Tp>
+  using formatter_type = formatter<_Tp, _CharT>;
+
+#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+  // Note: the Standard doesn't specify the required constructors.
+  _LIBCPP_INLINE_VISIBILITY
+  basic_format_context(_OutIt __out_it,
+                       basic_format_args<basic_format_context> __args,
+                       optional<_VSTD::locale>&& __loc = nullopt)
+      : __out_it_(_VSTD::move(__out_it)), __args_(__args),
+        __loc_(_VSTD::move(__loc)) {}
+#else
+  _LIBCPP_INLINE_VISIBILITY
+  basic_format_context(_OutIt __out_it,
+                       basic_format_args<basic_format_context> __args)
+      : __out_it_(_VSTD::move(__out_it)), __args_(__args) {}
+#endif
+
+  basic_format_context(const basic_format_context&) = delete;
+  basic_format_context& operator=(const basic_format_context&) = delete;
+
+  _LIBCPP_INLINE_VISIBILITY basic_format_arg<basic_format_context>
+  arg(size_t __id) const {
+    return __args_.get(__id);
+  }
+#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+  _LIBCPP_INLINE_VISIBILITY _VSTD::locale locale() {
+    if (!__loc_)
+      __loc_ = _VSTD::locale();
+    return *__loc_;
+  }
+#endif
+  _LIBCPP_INLINE_VISIBILITY iterator out() { return __out_it_; }
+  _LIBCPP_INLINE_VISIBILITY void advance_to(iterator __it) { __out_it_ = __it; }
+
+private:
+  iterator __out_it_;
+  basic_format_args<basic_format_context> __args_;
+#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+  // The Standard doesn't specify how the locale is stored.
+  // [format.context]/6
+  // std::locale locale();
+  //   Returns: The locale passed to the formatting function if the latter
+  //   takes one, and std::locale() otherwise.
+  // This is done by storing the locale of the constructor in this optional. If
+  // locale() is called and the optional has no value the value will be create.
+  // This allow the implementation to lazily create the locale.
+  // TODO FMT Validate whether lazy creation is the best solution.
+  optional<_VSTD::locale> __loc_;
+#endif
+};
+
+// TODO FMT Implement [format.context]/4
+// [Note 1: For a given type charT, implementations are encouraged to provide a
+// single instantiation of basic_format_context for appending to
+// basic_string<charT>, vector<charT>, or any other container with contiguous
+// storage by wrapping those in temporary objects with a uniform interface
+// (such as a span<charT>) and polymorphic reallocation. - end note]
+
+using format_context = basic_format_context<back_insert_iterator<string>, char>;
+using wformat_context =
+    basic_format_context<back_insert_iterator<wstring>, wchar_t>;
+
+#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+#endif //_LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___FORMAT_FORMAT_CONTEXT_H

libcxx/include/__format/format_fwd.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FORMAT_FORMAT_FWD_H
+#define _LIBCPP___FORMAT_FORMAT_FWD_H
+
+#include <__config>
+#include <__iterator/concepts.h>
+#include <utility>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+// TODO FMT Remove this once we require compilers with proper C++20 support.
+// If the compiler has no concepts support, the format header will be disabled.
+// Without concepts support enable_if needs to be used and that too much effort
+// to support compilers with partial C++20 support.
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS) &&                                       \
+    !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+// Currently libc++ hasn't implemented output_iterator. Use our own version
+// that follows the wording of [iterator.concept.output]/1.
+// TODO FMT Switch to the real concepts when they are available.
+template <class _Ip, class _Tp>
+concept __output_iterator = input_or_output_iterator<_Ip> &&
+    indirectly_writable<_Ip, _Tp> && requires(_Ip __i, _Tp&& __t) {
+  *__i++ = _VSTD::forward<_Tp>(__t);
+};
+
+// Forward declarations.
+
+template <class _Context>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT basic_format_arg;
+
+template <class _Context, class... _Args>
+struct _LIBCPP_TEMPLATE_VIS __format_arg_store;
+
+template <class _Ctx, class... _Args>
+_LIBCPP_INLINE_VISIBILITY __format_arg_store<_Ctx, _Args...>
+__make_format_args(const _Args&...);
+
+template <class _Tp, class _CharT = char>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter;
+
+#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+#endif //_LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___FORMAT_FORMAT_FWD_H

libcxx/include/concepts

 // [concept.equiv]
 template<class _Rp, class _Tp, class _Up>
 concept equivalence_relation = relation<_Rp, _Tp, _Up>;
 
 // [concept.strictweakorder]
 template<class _Rp, class _Tp, class _Up>
 concept strict_weak_order = relation<_Rp, _Tp, _Up>;
 
+// Concept helpers for the internal type traits for the fundamental types.
+
+template <class _Tp>
+concept __libcpp_unsigned_integer = __libcpp_is_unsigned_integer<_Tp>::value;
+template <class _Tp>
+concept __libcpp_signed_integer = __libcpp_is_signed_integer<_Tp>::value;
+template <class _Tp>
+concept __libcpp_floating_point = __libcpp_is_floating_point<_Tp>::value;
+
 #endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_CONCEPTS

libcxx/include/format

 // -*- C++ -*-
 //===--------------------------- format -----------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP_FORMAT
 #define _LIBCPP_FORMAT
 
+// TODO FMT Remove this once the library is complete.
+/*
+ * WARNING
+ * The header is work in progress and not complete yet. The ABI isn't stable.
+ * Partly due to the lack of recommended optimizations, partly since the
+ * committee is considering the following paper to be retroactively be applied
+ * to C++20.
+ * https://wg21.link/P2216 std::format improvements
+ */
+
 /*
 
 namespace std {
+  // [format.context], class template basic_format_context
+  template<class Out, class charT>
+  class basic_format_context {
+    basic_format_args<basic_format_context> args_;      // exposition only
+    Out out_;                                           // exposition only
+
+  public:
+    using iterator = Out;
+    using char_type = charT;
+    template<class T> using formatter_type = formatter<T, charT>;
+
+    basic_format_arg<basic_format_context> arg(size_t id) const;
+    std::locale locale();
+
+    iterator out();
+    void advance_to(iterator it);
+  };
+  using format_context = basic_format_context<unspecified, char>;
+  using wformat_context = basic_format_context<unspecified, wchar_t>;
+
+  // [format.args], class template basic_format_args
+  template<class Context>
+  class basic_format_args {
+    size_t size_;                               // exposition only
+    const basic_format_arg<Context>* data_;     // exposition only
+
+  public:
+    basic_format_args() noexcept;
+
+    template<class... Args>
+      basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;
+
+    basic_format_arg<Context> get(size_t i) const noexcept;
+  };
+  using format_args = basic_format_args<format_context>;
+  using wformat_args = basic_format_args<wformat_context>;
+
+
+  template<class Out, class charT>
+    using format_args_t = basic_format_args<basic_format_context<Out, charT>>;
+
+  // [format.parse.ctx], class template basic_format_parse_context
+  template<class charT>
+  class basic_format_parse_context {
+  public:
+    using char_type = charT;
+    using const_iterator = typename basic_string_view<charT>::const_iterator;
+    using iterator = const_iterator;
+
+  private:
+    iterator begin_;                                    // exposition only
+    iterator end_;                                      // exposition only
+    enum indexing { unknown, manual, automatic };       // exposition only
+    indexing indexing_;                                 // exposition only
+    size_t next_arg_id_;                                // exposition only
+    size_t num_args_;                                   // exposition only
+
+  public:
+    constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt,
+                                                  size_t num_args = 0) noexcept;
+    basic_format_parse_context(const basic_format_parse_context&) = delete;
+    basic_format_parse_context& operator=(const basic_format_parse_context&) = delete;
+
+    constexpr const_iterator begin() const noexcept;
+    constexpr const_iterator end() const noexcept;
+    constexpr void advance_to(const_iterator it);
+
+    constexpr size_t next_arg_id();
+    constexpr void check_arg_id(size_t id);
+  };
+  using format_parse_context = basic_format_parse_context<char>;
+  using wformat_parse_context = basic_format_parse_context<wchar_t>;
+
+  // [format.arguments], arguments
+  // [format.arg], class template basic_format_arg
+  template<class Context>
+  class basic_format_arg {
+  public:
+    class handle;
+
+  private:
+    using char_type = typename Context::char_type;                              // exposition only
+
+    variant<monostate, bool, char_type,
+            int, unsigned int, long long int, unsigned long long int,
+            float, double, long double,
+            const char_type*, basic_string_view<char_type>,
+            const void*, handle> value;                                         // exposition only
+
+    template<class T> explicit basic_format_arg(const T& v) noexcept;           // exposition only
+    explicit basic_format_arg(float n) noexcept;                                // exposition only
+    explicit basic_format_arg(double n) noexcept;                               // exposition only
+    explicit basic_format_arg(long double n) noexcept;                          // exposition only
+    explicit basic_format_arg(const char_type* s);                              // exposition only
+
+    template<class traits>
+      explicit basic_format_arg(
+        basic_string_view<char_type, traits> s) noexcept;                       // exposition only
+
+    template<class traits, class Allocator>
+      explicit basic_format_arg(
+        const basic_string<char_type, traits, Allocator>& s) noexcept;          // exposition only
+
+    explicit basic_format_arg(nullptr_t) noexcept;                              // exposition only
+
+    template<class T>
+      explicit basic_format_arg(const T* p) noexcept;                           // exposition only
+
+  public:
+    basic_format_arg() noexcept;
+
+    explicit operator bool() const noexcept;
+  };
+
+  template<class Visitor, class Context>
+    see below visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);
+
+  // [format.arg.store], class template format-arg-store
+  template<class Context, class... Args>
+  struct format-arg-store {      // exposition only
+    array<basic_format_arg<Context>, sizeof...(Args)> args;
+  };
+
+  template<class Context = format_context, class... Args>
+    format-arg-store<Context, Args...>
+      make_format_args(const Args&... args);
+  template<class... Args>
+    format-arg-store<wformat_context, Args...>
+      make_wformat_args(const Args&... args);
+
   // [format.error], class format_error
   class format_error : public runtime_error {
   public:
     explicit format_error(const string& what_arg);
     explicit format_error(const char* what_arg);
   };
 
   // [format.parse.ctx], class template basic_format_parse_context
namespace std {
   };
   using format_parse_context = basic_format_parse_context<char>;
   using wformat_parse_context = basic_format_parse_context<wchar_t>;
 }
 
 */
 
 #include <__config>
+#include <__format/format_arg.h>
+#include <__format/format_args.h>
+#include <__format/format_context.h>
 #include <__format/format_error.h>
 #include <__format/format_parse_context.h>
+#include <array>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
+#pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
+// TODO FMT Remove this once we require compilers with proper C++20 support.
+// If the compiler has no concepts support, the format header will be disabled.
+// Without concepts support enable_if needs to be used and that too much effort
+// to support compilers with partial C++20 support.
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS) &&                                       \
+    !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
+
+// TODO FMT Evaluate which templates should be external templates. This
+// improves the efficiency of the header. However since the header is still
+// under heavy development and not all classes are stable it makes no sense
+// to do this optimization now.
+
+using format_args = basic_format_args<format_context>;
+using wformat_args = basic_format_args<wformat_context>;
+
+template <class _OutIt, class _CharT>
+using format_args_t = basic_format_args<basic_format_context<_OutIt, _CharT>>;
+
+template <class _Context, class... _Args>
+struct _LIBCPP_TEMPLATE_VIS __format_arg_store {
+  array<basic_format_arg<_Context>, sizeof...(_Args)> __args;
+};
+
+template <class _Context, class... _Args>
+_LIBCPP_INLINE_VISIBILITY __format_arg_store<_Context, _Args...>
+__make_format_args(const _Args&... __args) {
+  return {basic_format_arg<_Context>(__args)...};
+}
+
+template <class _Context = format_context, class... _Args>
+_LIBCPP_INLINE_VISIBILITY __format_arg_store<_Context, _Args...>
+make_format_args(const _Args&... __args) {
+  return __make_format_args<_Context>(__args...);
+}
+
+template <class... _Args>
+_LIBCPP_INLINE_VISIBILITY __format_arg_store<wformat_context, _Args...>
+make_wformat_args(const _Args&... __args) {
+  return __make_format_args<wformat_context>(__args...);
+}
+
+#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_FORMAT

libcxx/include/module.modulemap

module std [system] {
   module __node_handle { header "__node_handle" export * }
   module __split_buffer { header "__split_buffer" export * }
   module __std_stream { header "__std_stream" export * }
   module __string { header "__string" export * }
   module __threading_support { header "__threading_support" export * }
   module __tree { header "__tree" export * }
   module __tuple { header "__tuple" export * }
   module __undef_macros { header "__undef_macros" export * }
+  module __iterator_iterator_traits { header "__iterator/iterator_traits.h" export * }
+  module __iterator_concepts { header "__iterator/concepts.h" export * }
+  module __iterator_incrementable_traits { header "__iterator/incrementable_traits.h" export * }
 
   module experimental {
     requires cplusplus11
 
     module algorithm {
       header "experimental/algorithm"
       export *
     }

libcxx/include/type_traits

 template <class _Tp>
 _LIBCPP_INLINE_VAR _LIBCPP_CONSTEXPR bool is_integral_v
     = is_integral<_Tp>::value;
 #endif
 
 #endif // __has_keyword(__is_integral)
 
 // __libcpp_is_signed_integer, __libcpp_is_unsigned_integer
+// <concepts> implements __libcpp_signed_integer, __libcpp_unsigned_integer
 
 // [basic.fundamental] defines five standard signed integer types;
 // __int128_t is an extended signed integer type.
 // The signed and unsigned integer types, plus bool and the
 // five types with "char" in their name, compose the "integral" types.
 
 template <class _Tp> struct __libcpp_is_signed_integer : public false_type {};
 template <> struct __libcpp_is_signed_integer<signed char>      : public true_type {};

 template <> struct __libcpp_is_unsigned_integer<unsigned int>       : public true_type {};
 template <> struct __libcpp_is_unsigned_integer<unsigned long>      : public true_type {};
 template <> struct __libcpp_is_unsigned_integer<unsigned long long> : public true_type {};
 #ifndef _LIBCPP_HAS_NO_INT128
 template <> struct __libcpp_is_unsigned_integer<__uint128_t>        : public true_type {};
 #endif
 
 // is_floating_point
+// <concepts> implements __libcpp_floating_point
 
 template <class _Tp> struct __libcpp_is_floating_point              : public false_type {};
 template <>          struct __libcpp_is_floating_point<float>       : public true_type {};
 template <>          struct __libcpp_is_floating_point<double>      : public true_type {};
 template <>          struct __libcpp_is_floating_point<long double> : public true_type {};
 
 template <class _Tp> struct _LIBCPP_TEMPLATE_VIS is_floating_point
     : public __libcpp_is_floating_point<typename remove_cv<_Tp>::type> {};

libcxx/test/std/utilities/format/format.arguments/format.arg.store/class.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// template<class Context, class... Args>
+// struct format-arg-store {      // exposition only
+//   array<basic_format_arg<Context>, sizeof...(Args)> args;
+// };
+//
+// Note more testing is done in the unit test for:
+// template<class Visitor, class Context>
+//   see below visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);
+
+#include <format>
+#include <cassert>
+#include <type_traits>
+
+#include "test_macros.h"
+
+template <class Context>
+void test() {
+  {
+    auto store = std::make_format_args<Context>();
+    static_assert(
+        std::is_same_v<decltype(store), std::__format_arg_store<Context>>);
+    static_assert(
+        std::is_same_v<decltype(store.__args),
+                       std::array<std::basic_format_arg<Context>, 0>>);
+    assert(store.__args.size() == 0);
+  }
+  {
+    auto store = std::make_format_args<Context>(1);
+    static_assert(
+        std::is_same_v<decltype(store), std::__format_arg_store<Context, int>>);
+    static_assert(
+        std::is_same_v<decltype(store.__args),
+                       std::array<std::basic_format_arg<Context>, 1>>);
+    assert(store.__args.size() == 1);
+  }
+  {
+    auto store = std::make_format_args<Context>(1, 'c');
+    static_assert(std::is_same_v<decltype(store),
+                                 std::__format_arg_store<Context, int, char>>);
+    static_assert(
+        std::is_same_v<decltype(store.__args),
+                       std::array<std::basic_format_arg<Context>, 2>>);
+    assert(store.__args.size() == 2);
+  }
+  {
+    auto store = std::make_format_args<Context>(1, 'c', nullptr);
+    static_assert(
+        std::is_same_v<decltype(store),
+                       std::__format_arg_store<Context, int, char, nullptr_t>>);
+    static_assert(
+        std::is_same_v<decltype(store.__args),
+                       std::array<std::basic_format_arg<Context>, 3>>);
+    assert(store.__args.size() == 3);
+  }
+}
+
+void test() {
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char>>, char>>();
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>();
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.arguments/format.arg.store/make_format_args.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// template<class Context = format_context, class... Args>
+// format-arg-store<Context, Args...> make_format_args(const Args&... args);
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  using Context = std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char>>, char>;
+
+  auto value = std::make_format_args(42, nullptr, false, 1.0);
+  auto expected =
+      std::make_format_args<Context, int, std::nullptr_t, bool, double>(
+          42, nullptr, false, 1.0);
+
+  assert(value.__args.size() == 4);
+  assert(expected.__args.size() == value.__args.size());
+
+  for (size_t i = 0, e = value.__args.size(); i != e; ++i)
+    assert(value.__args[i] == expected.__args[i]);
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.arguments/format.arg.store/make_format_args.sh.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// Validate it works regardless of the signedness of `char`.
+// RUN: %{cxx} %{flags} %{compile_flags} -fsigned-char -fsyntax-only %s
+// RUN: %{cxx} %{flags} %{compile_flags} -funsigned-char -fsyntax-only %s
+
+// <format>
+
+// [format.arg]/5.2
+// - otherwise, if T is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(v);
+
+#include <format>
+
+void test() {
+  std::make_format_args<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>('c');
+}

libcxx/test/std/utilities/format/format.arguments/format.arg.store/make_wformat_args.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// template<class... Args>
+//   format-arg-store<wformat_context, Args...>
+//   make_wformat_args(const Args&... args);
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  using Context = std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>;
+
+  auto value = std::make_wformat_args(42, nullptr, false, 1.0);
+  auto expected =
+      std::make_format_args<Context, int, std::nullptr_t, bool, double>(
+          42, nullptr, false, 1.0);
+
+  assert(value.__args.size() == 4);
+  assert(expected.__args.size() == value.__args.size());
+
+  for (size_t i = 0, e = value.__args.size(); i != e; ++i)
+    assert(value.__args[i] == expected.__args[i]);
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.arguments/format.arg/ctor.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// basic_format_arg() noexcept;
+
+// The class has several exposition only private constructors. These are tested
+// in visit_format_arg.pass.cpp
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class Context>
+void test() {
+  ASSERT_NOEXCEPT(std::basic_format_arg<Context>{});
+
+  std::basic_format_arg<Context> format_arg{};
+  assert(!format_arg);
+}
+
+void test() {
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char>>, char>>();
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>();
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>>();
+#endif
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char16_t>>, char16_t>>();
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char32_t>>, char32_t>>();
+#endif
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.arguments/format.arg/operator_bool.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// explicit operator bool() const noexcept
+//
+// Note more testing is done in the unit test for:
+// template<class Visitor, class Context>
+//   see below visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);
+
+#include <format>
+#include <cassert>
+#include <type_traits>
+
+#include "test_macros.h"
+
+void test(const auto& store) {
+  for (const auto& arg : store.__args) {
+    assert(arg);
+    assert(static_cast<bool>(arg));
+  }
+}
+
+template <class Context>
+void test() {
+  {
+    std::basic_format_arg<Context> format_arg{};
+    ASSERT_NOEXCEPT(!format_arg);
+    assert(!format_arg);
+    ASSERT_NOEXCEPT(static_cast<bool>(format_arg));
+    assert(!static_cast<bool>(format_arg));
+  }
+  test(std::make_format_args<Context>());
+  test(std::make_format_args<Context>(1));
+  test(std::make_format_args<Context>(1, 'c'));
+  test(std::make_format_args<Context>(1, 'c', nullptr));
+}
+
+void test() {
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char>>, char>>();
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>();
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.arguments/format.arg/visit_format_arg.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// This test requires the dylib support introduced in D92214.
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <format>
+
+// template<class Visitor, class Context>
+//   see below visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);
+
+#include <format>
+#include <cassert>
+#include <type_traits>
+
+#include "test_macros.h"
+#include "make_string.h"
+
+template <class Context, class To, class From>
+void test(From value) {
+  auto format_args = std::make_format_args<Context>(value);
+  assert(format_args.__args.size() == 1);
+  assert(format_args.__args[0]);
+  auto result = std::visit_format_arg(
+      [v = To(value)](auto&& a) -> To {
+        if constexpr (std::is_same_v<To, std::decay_t<decltype(a)>>) {
+          assert(v == a);
+          return a;
+        } else {
+          assert(false);
+          return 0;
+        }
+      },
+      format_args.__args[0]);
+
+  using ct = std::common_type_t<From, To>;
+  assert(static_cast<ct>(result) == static_cast<ct>(value));
+}
+
+template <class Context>
+void test() {
+  using char_type = typename Context::char_type;
+  std::basic_string<char_type> empty;
+  std::basic_string<char_type> str = MAKE_STRING(char_type, "abc");
+
+  // Test boolean types.
+
+  test<Context, bool>(true);
+  test<Context, bool>(false);
+
+  // Test char_type types.
+
+  test<Context, char_type, char_type>('a');
+  test<Context, char_type, char_type>('z');
+  test<Context, char_type, char_type>('0');
+  test<Context, char_type, char_type>('9');
+
+  // Test char types.
+
+  if (std::is_same_v<char_type, char>) {
+    // char to char -> char
+    test<Context, char_type, char>('a');
+    test<Context, char_type, char>('z');
+    test<Context, char_type, char>('0');
+    test<Context, char_type, char>('9');
+  } else {
+    if (std::is_same_v<char_type, wchar_t>) {
+      // char to wchar_t -> wchar_t
+      test<Context, wchar_t, char>('a');
+      test<Context, wchar_t, char>('z');
+      test<Context, wchar_t, char>('0');
+      test<Context, wchar_t, char>('9');
+    } else if (std::is_signed_v<char>) {
+      // char to char_type -> int
+      // This happens when Context::char_type is a char8_t, char16_t, or
+      // char32_t and char is a signed type.
+      // Note if sizeof(char_type) > sizeof(int) this test fails. If there are
+      // platforms where that occurs extra tests need to be added for char32_t
+      // testing it against a long long.
+      test<Context, int, char>('a');
+      test<Context, int, char>('z');
+      test<Context, int, char>('0');
+      test<Context, int, char>('9');
+    } else {
+      // char to char_type -> unsigned
+      // This happens when Context::char_type is a char8_t, char16_t, or
+      // char32_t and char is an unsigned type.
+      // Note if sizeof(char_type) > sizeof(unsigned) this test fails. If there
+      // are platforms where that occurs extra tests need to be added for
+      // char32_t testing it against an unsigned long long.
+      test<Context, unsigned, char>('a');
+      test<Context, unsigned, char>('z');
+      test<Context, unsigned, char>('0');
+      test<Context, unsigned, char>('9');
+    }
+  }
+
+  // Test signed integer types.
+
+  test<Context, int, signed char>(std::numeric_limits<signed char>::min());
+  test<Context, int, signed char>(0);
+  test<Context, int, signed char>(std::numeric_limits<signed char>::max());
+
+  test<Context, int, short>(std::numeric_limits<short>::min());
+  test<Context, int, short>(std::numeric_limits<signed char>::min());
+  test<Context, int, short>(0);
+  test<Context, int, short>(std::numeric_limits<signed char>::max());
+  test<Context, int, short>(std::numeric_limits<short>::max());
+
+  test<Context, int, int>(std::numeric_limits<int>::min());
+  test<Context, int, int>(std::numeric_limits<short>::min());
+  test<Context, int, int>(std::numeric_limits<signed char>::min());
+  test<Context, int, int>(0);
+  test<Context, int, int>(std::numeric_limits<signed char>::max());
+  test<Context, int, int>(std::numeric_limits<short>::max());
+  test<Context, int, int>(std::numeric_limits<int>::max());
+
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<long>::min());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<int>::min());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<short>::min());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<signed char>::min());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(0);
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<signed char>::max());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<short>::max());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<int>::max());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<long>::max());
+
+  test<Context, long long, long long>(std::numeric_limits<long long>::min());
+  test<Context, long long, long long>(std::numeric_limits<long>::min());
+  test<Context, long long, long long>(std::numeric_limits<int>::min());
+  test<Context, long long, long long>(std::numeric_limits<short>::min());
+  test<Context, long long, long long>(std::numeric_limits<signed char>::min());
+  test<Context, long long, long long>(0);
+  test<Context, long long, long long>(std::numeric_limits<signed char>::max());
+  test<Context, long long, long long>(std::numeric_limits<short>::max());
+  test<Context, long long, long long>(std::numeric_limits<int>::max());
+  test<Context, long long, long long>(std::numeric_limits<long>::max());
+  test<Context, long long, long long>(std::numeric_limits<long long>::max());
+
+#ifndef _LIBCPP_HAS_NO_INT128
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<__int128_t>::min());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<long long>::min());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<long>::min());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<int>::min());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<short>::min());
+  test<Context, __int128_t, __int128_t>(
+      std::numeric_limits<signed char>::min());
+  test<Context, __int128_t, __int128_t>(0);
+  test<Context, __int128_t, __int128_t>(
+      std::numeric_limits<signed char>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<short>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<int>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<long>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<long long>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<__int128_t>::max());
+#endif
+
+  // Test unsigned integer types.
+
+  test<Context, unsigned, unsigned char>(0);
+  test<Context, unsigned, unsigned char>(
+      std::numeric_limits<unsigned char>::max());
+
+  test<Context, unsigned, unsigned short>(0);
+  test<Context, unsigned, unsigned short>(
+      std::numeric_limits<unsigned char>::max());
+  test<Context, unsigned, unsigned short>(
+      std::numeric_limits<unsigned short>::max());
+
+  test<Context, unsigned, unsigned>(0);
+  test<Context, unsigned, unsigned>(std::numeric_limits<unsigned char>::max());
+  test<Context, unsigned, unsigned>(std::numeric_limits<unsigned short>::max());
+  test<Context, unsigned, unsigned>(std::numeric_limits<unsigned>::max());
+
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(0);
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(std::numeric_limits<unsigned char>::max());
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(std::numeric_limits<unsigned short>::max());
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(std::numeric_limits<unsigned>::max());
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(std::numeric_limits<unsigned long>::max());
+
+  test<Context, unsigned long long, unsigned long long>(0);
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned char>::max());
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned short>::max());
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned>::max());
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned long>::max());
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned long long>::max());
+
+#ifndef _LIBCPP_HAS_NO_INT128
+  test<Context, __uint128_t, __uint128_t>(0);
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned char>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned short>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned int>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned long>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned long long>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<__uint128_t>::max());
+#endif
+
+  // Test floating point types.
+
+  test<Context, float, float>(-std::numeric_limits<float>::max());
+  test<Context, float, float>(-std::numeric_limits<float>::min());
+  test<Context, float, float>(-0.0);
+  test<Context, float, float>(0.0);
+  test<Context, float, float>(std::numeric_limits<float>::min());
+  test<Context, float, float>(std::numeric_limits<float>::max());
+
+  test<Context, double, double>(-std::numeric_limits<double>::max());
+  test<Context, double, double>(-std::numeric_limits<double>::min());
+  test<Context, double, double>(-0.0);
+  test<Context, double, double>(0.0);
+  test<Context, double, double>(std::numeric_limits<double>::min());
+  test<Context, double, double>(std::numeric_limits<double>::max());
+
+  test<Context, long double, long double>(
+      -std::numeric_limits<long double>::max());
+  test<Context, long double, long double>(
+      -std::numeric_limits<long double>::min());
+  test<Context, long double, long double>(-0.0);
+  test<Context, long double, long double>(0.0);
+  test<Context, long double, long double>(
+      std::numeric_limits<long double>::min());
+  test<Context, long double, long double>(
+      std::numeric_limits<long double>::max());
+
+  // Test const char_type pointer types.
+
+  test<Context, const char_type*, const char_type*>(empty.c_str());
+  test<Context, const char_type*, const char_type*>(str.c_str());
+
+  // Test string_view types.
+
+  test<Context, std::basic_string_view<char_type>>(
+      std::basic_string_view<char_type>());
+  test<Context, std::basic_string_view<char_type>,
+       std::basic_string_view<char_type>>(empty);
+  test<Context, std::basic_string_view<char_type>,
+       std::basic_string_view<char_type>>(str);
+
+  // Test string types.
+
+  test<Context, std::basic_string_view<char_type>>(
+      std::basic_string<char_type>());
+  test<Context, std::basic_string_view<char_type>,
+       std::basic_string<char_type>>(empty);
+  test<Context, std::basic_string_view<char_type>,
+       std::basic_string<char_type>>(str);
+
+  // Test pointer types.
+
+  test<Context, const void*>(nullptr);
+}
+
+void test() {
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char>>, char>>();
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>();
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.arguments/format.args/ctor.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// basic_format_args() noexcept;
+// template<class... Args>
+//   basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class Context>
+void test() {
+  {
+    ASSERT_NOEXCEPT(std::basic_format_args<Context>{});
+
+    std::basic_format_args<Context> format_args{};
+    assert(!format_args.get(0));
+  }
+  {
+    auto store = std::make_format_args<Context>(1);
+    ASSERT_NOEXCEPT(std::basic_format_args<Context>{store});
+    std::basic_format_args<Context> format_args{store};
+    assert(format_args.get(0));
+    assert(!format_args.get(1));
+  }
+  {
+    auto store = std::make_format_args<Context>(1, 'c');
+    ASSERT_NOEXCEPT(std::basic_format_args<Context>{store});
+    std::basic_format_args<Context> format_args{store};
+    assert(format_args.get(0));
+    assert(format_args.get(1));
+    assert(!format_args.get(2));
+  }
+  {
+    auto store = std::make_format_args<Context>(1, 'c', nullptr);
+    ASSERT_NOEXCEPT(std::basic_format_args<Context>{store});
+    std::basic_format_args<Context> format_args{store};
+    assert(format_args.get(0));
+    assert(format_args.get(1));
+    assert(format_args.get(2));
+    assert(!format_args.get(3));
+  }
+}
+
+void test() {
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char>>, char>>();
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>();
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.arguments/format.args/get.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// This test requires the dylib support introduced in D92214.
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
+// <format>
+
+// basic_format_arg<Context> get(size_t i) const noexcept;
+
+#include <format>
+#include <cassert>
+#include <type_traits>
+
+#include "test_macros.h"
+#include "make_string.h"
+
+template <class Context, class To, class From>
+void test(From value) {
+  auto store = std::make_format_args<Context>(value);
+  const std::basic_format_args<Context> format_args{store};
+
+  std::visit_format_arg(
+      [v = To(value)](auto&& a) {
+        if constexpr (std::is_same_v<To, std::decay_t<decltype(a)>>)
+          assert(v == a);
+        else
+          assert(false);
+      },
+      format_args.get(0));
+}
+
+template <class Context>
+void test() {
+  {
+    const std::basic_format_args<Context> format_args{};
+    ASSERT_NOEXCEPT(format_args.get(0));
+    assert(!format_args.get(0));
+  }
+
+  using char_type = typename Context::char_type;
+  std::basic_string<char_type> empty;
+  std::basic_string<char_type> str = MAKE_STRING(char_type, "abc");
+
+  // Test boolean types.
+
+  test<Context, bool>(true);
+  test<Context, bool>(false);
+
+  // Test char_type types.
+
+  test<Context, char_type, char_type>('a');
+  test<Context, char_type, char_type>('z');
+  test<Context, char_type, char_type>('0');
+  test<Context, char_type, char_type>('9');
+
+  // Test char types.
+
+  if (std::is_same_v<char_type, char>) {
+    // char to char -> char
+    test<Context, char_type, char>('a');
+    test<Context, char_type, char>('z');
+    test<Context, char_type, char>('0');
+    test<Context, char_type, char>('9');
+  } else {
+    if (std::is_same_v<char_type, wchar_t>) {
+      // char to wchar_t -> wchar_t
+      test<Context, wchar_t, char>('a');
+      test<Context, wchar_t, char>('z');
+      test<Context, wchar_t, char>('0');
+      test<Context, wchar_t, char>('9');
+    } else if (std::is_signed_v<char>) {
+      // char to char_type -> int
+      // This happens when Context::char_type is a char8_t, char16_t, or
+      // char32_t and char is a signed type.
+      // Note if sizeof(char_type) > sizeof(int) this test fails. If there are
+      // platforms where that occurs extra tests need to be added for char32_t
+      // testing it against a long long.
+      test<Context, int, char>('a');
+      test<Context, int, char>('z');
+      test<Context, int, char>('0');
+      test<Context, int, char>('9');
+    } else {
+      // char to char_type -> unsigned
+      // This happens when Context::char_type is a char8_t, char16_t, or
+      // char32_t and char is an unsigned type.
+      // Note if sizeof(char_type) > sizeof(unsigned) this test fails. If there
+      // are platforms where that occurs extra tests need to be added for
+      // char32_t testing it against an unsigned long long.
+      test<Context, unsigned, char>('a');
+      test<Context, unsigned, char>('z');
+      test<Context, unsigned, char>('0');
+      test<Context, unsigned, char>('9');
+    }
+  }
+
+  // Test signed integer types.
+
+  test<Context, int, signed char>(std::numeric_limits<signed char>::min());
+  test<Context, int, signed char>(0);
+  test<Context, int, signed char>(std::numeric_limits<signed char>::max());
+
+  test<Context, int, short>(std::numeric_limits<short>::min());
+  test<Context, int, short>(std::numeric_limits<signed char>::min());
+  test<Context, int, short>(0);
+  test<Context, int, short>(std::numeric_limits<signed char>::max());
+  test<Context, int, short>(std::numeric_limits<short>::max());
+
+  test<Context, int, int>(std::numeric_limits<int>::min());
+  test<Context, int, int>(std::numeric_limits<short>::min());
+  test<Context, int, int>(std::numeric_limits<signed char>::min());
+  test<Context, int, int>(0);
+  test<Context, int, int>(std::numeric_limits<signed char>::max());
+  test<Context, int, int>(std::numeric_limits<short>::max());
+  test<Context, int, int>(std::numeric_limits<int>::max());
+
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<long>::min());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<int>::min());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<short>::min());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<signed char>::min());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(0);
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<signed char>::max());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<short>::max());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<int>::max());
+  test<Context, std::conditional_t<sizeof(long) == sizeof(int), int, long long>,
+       long>(std::numeric_limits<long>::max());
+
+  test<Context, long long, long long>(std::numeric_limits<long long>::min());
+  test<Context, long long, long long>(std::numeric_limits<long>::min());
+  test<Context, long long, long long>(std::numeric_limits<int>::min());
+  test<Context, long long, long long>(std::numeric_limits<short>::min());
+  test<Context, long long, long long>(std::numeric_limits<signed char>::min());
+  test<Context, long long, long long>(0);
+  test<Context, long long, long long>(std::numeric_limits<signed char>::max());
+  test<Context, long long, long long>(std::numeric_limits<short>::max());
+  test<Context, long long, long long>(std::numeric_limits<int>::max());
+  test<Context, long long, long long>(std::numeric_limits<long>::max());
+  test<Context, long long, long long>(std::numeric_limits<long long>::max());
+
+#ifndef _LIBCPP_HAS_NO_INT128
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<__int128_t>::min());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<long long>::min());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<long>::min());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<int>::min());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<short>::min());
+  test<Context, __int128_t, __int128_t>(
+      std::numeric_limits<signed char>::min());
+  test<Context, __int128_t, __int128_t>(0);
+  test<Context, __int128_t, __int128_t>(
+      std::numeric_limits<signed char>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<short>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<int>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<long>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<long long>::max());
+  test<Context, __int128_t, __int128_t>(std::numeric_limits<__int128_t>::max());
+#endif
+
+  // Test unsigned integer types.
+
+  test<Context, unsigned, unsigned char>(0);
+  test<Context, unsigned, unsigned char>(
+      std::numeric_limits<unsigned char>::max());
+
+  test<Context, unsigned, unsigned short>(0);
+  test<Context, unsigned, unsigned short>(
+      std::numeric_limits<unsigned char>::max());
+  test<Context, unsigned, unsigned short>(
+      std::numeric_limits<unsigned short>::max());
+
+  test<Context, unsigned, unsigned>(0);
+  test<Context, unsigned, unsigned>(std::numeric_limits<unsigned char>::max());
+  test<Context, unsigned, unsigned>(std::numeric_limits<unsigned short>::max());
+  test<Context, unsigned, unsigned>(std::numeric_limits<unsigned>::max());
+
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(0);
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(std::numeric_limits<unsigned char>::max());
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(std::numeric_limits<unsigned short>::max());
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(std::numeric_limits<unsigned>::max());
+  test<Context,
+       std::conditional_t<sizeof(unsigned long) == sizeof(unsigned), unsigned,
+                          unsigned long long>,
+       unsigned long>(std::numeric_limits<unsigned long>::max());
+
+  test<Context, unsigned long long, unsigned long long>(0);
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned char>::max());
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned short>::max());
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned>::max());
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned long>::max());
+  test<Context, unsigned long long, unsigned long long>(
+      std::numeric_limits<unsigned long long>::max());
+
+#ifndef _LIBCPP_HAS_NO_INT128
+  test<Context, __uint128_t, __uint128_t>(0);
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned char>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned short>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned int>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned long>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<unsigned long long>::max());
+  test<Context, __uint128_t, __uint128_t>(
+      std::numeric_limits<__uint128_t>::max());
+#endif
+
+  // Test floating point types.
+
+  test<Context, float, float>(-std::numeric_limits<float>::max());
+  test<Context, float, float>(-std::numeric_limits<float>::min());
+  test<Context, float, float>(-0.0);
+  test<Context, float, float>(0.0);
+  test<Context, float, float>(std::numeric_limits<float>::min());
+  test<Context, float, float>(std::numeric_limits<float>::max());
+
+  test<Context, double, double>(-std::numeric_limits<double>::max());
+  test<Context, double, double>(-std::numeric_limits<double>::min());
+  test<Context, double, double>(-0.0);
+  test<Context, double, double>(0.0);
+  test<Context, double, double>(std::numeric_limits<double>::min());
+  test<Context, double, double>(std::numeric_limits<double>::max());
+
+  test<Context, long double, long double>(
+      -std::numeric_limits<long double>::max());
+  test<Context, long double, long double>(
+      -std::numeric_limits<long double>::min());
+  test<Context, long double, long double>(-0.0);
+  test<Context, long double, long double>(0.0);
+  test<Context, long double, long double>(
+      std::numeric_limits<long double>::min());
+  test<Context, long double, long double>(
+      std::numeric_limits<long double>::max());
+
+  // Test const char_type pointer types.
+
+  test<Context, const char_type*, const char_type*>(empty.c_str());
+  test<Context, const char_type*, const char_type*>(str.c_str());
+
+  // Test string_view types.
+
+  test<Context, std::basic_string_view<char_type>>(
+      std::basic_string_view<char_type>());
+  test<Context, std::basic_string_view<char_type>,
+       std::basic_string_view<char_type>>(empty);
+  test<Context, std::basic_string_view<char_type>,
+       std::basic_string_view<char_type>>(str);
+
+  // Test string types.
+
+  test<Context, std::basic_string_view<char_type>>(
+      std::basic_string<char_type>());
+  test<Context, std::basic_string_view<char_type>,
+       std::basic_string<char_type>>(empty);
+  test<Context, std::basic_string_view<char_type>,
+       std::basic_string<char_type>>(str);
+
+  // Test pointer types.
+
+  test<Context, const void*>(nullptr);
+}
+
+void test() {
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<char>>, char>>();
+  test<std::basic_format_context<
+      std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>();
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.arguments/format.args/types.compile.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// Namespace std typedefs:
+// using format_args = basic_format_args<format_context>;
+// using wformat_args = basic_format_args<wformat_context>;
+// template<class Out, class charT>
+//   using format_args_t = basic_format_args<basic_format_context<Out, charT>>;
+
+#include <format>
+#include <vector>
+#include <type_traits>
+
+#include "test_macros.h"
+
+static_assert(std::is_same_v<std::format_args,
+                             std::basic_format_args<std::format_context>>);
+static_assert(std::is_same_v<std::wformat_args,
+                             std::basic_format_args<std::wformat_context>>);
+
+static_assert(std::is_same_v<
+              std::format_args_t<std::back_insert_iterator<std::string>, char>,
+              std::basic_format_args<std::basic_format_context<
+                  std::back_insert_iterator<std::string>, char>>>);
+
+static_assert(
+    std::is_same_v<
+        std::format_args_t<std::back_insert_iterator<std::wstring>, wchar_t>,
+        std::basic_format_args<std::basic_format_context<
+            std::back_insert_iterator<std::wstring>, wchar_t>>>);
+
+static_assert(
+    std::is_same_v<
+        std::format_args_t<std::back_insert_iterator<std::vector<char>>, char>,
+        std::basic_format_args<std::basic_format_context<
+            std::back_insert_iterator<std::vector<char>>, char>>>);
+
+// Required for MSVC internal test runner compatibility.
+int main(int, char**) { return 0; }

libcxx/test/std/utilities/format/format.formatter/format.context/advance_to.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// void advance_to(iterator it);
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class OutIt, class CharT>
+void test(
+    std::basic_format_args<std::basic_format_context<OutIt, CharT>> args) {
+  {
+    std::basic_string<CharT> str[3];
+    std::basic_format_context context{OutIt{str[0]}, args};
+    context.out() = CharT('a');
+    context.advance_to(OutIt{str[1]});
+    context.out() = CharT('b');
+    context.advance_to(OutIt{str[2]});
+    context.out() = CharT('c');
+
+    assert(str[0].size() == 1);
+    assert(str[0].front() == CharT('a'));
+    assert(str[1].size() == 1);
+    assert(str[1].front() == CharT('b'));
+    assert(str[2].size() == 1);
+    assert(str[2].front() == CharT('c'));
+  }
+}
+
+void test() {
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<char>>, char>>()));
+
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>()));
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>>()));
+#endif
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<char16_t>>,
+          char16_t>>()));
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<char32_t>>,
+          char32_t>>()));
+#endif
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.formatter/format.context/arg.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// basic_format_arg<basic_format_context> arg(size_t id) const;
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+#include "make_string.h"
+
+template <class OutIt, class CharT>
+void test() {
+  std::basic_string<CharT> string = MAKE_STRING(CharT, "string");
+  auto store =
+      std::make_format_args<std::basic_format_context<OutIt, CharT>, CharT>(
+          true, CharT('a'), 42, string);
+  std::basic_format_args args = store;
+
+  std::basic_string<CharT> str;
+  {
+    const std::basic_format_context context{OutIt{str}, args};
+    assert(args.__size() == 4);
+    for (size_t i = 0, e = args.__size(); i != e; ++i) {
+      assert(context.arg(i));
+      assert(context.arg(i) == args.get(i));
+    }
+    assert(!context.arg(args.__size()));
+  }
+}
+
+int main(int, char**) {
+  test<std::back_insert_iterator<std::basic_string<char>>, char>();
+  test<std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>();
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  test<std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>();
+#endif
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+  test<std::back_insert_iterator<std::basic_string<char16_t>>, char16_t>();
+  test<std::back_insert_iterator<std::basic_string<char32_t>>, char32_t>();
+#endif
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.formatter/format.context/ctor.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: libcpp-has-no-localization
+
+// REQUIRES: locale.en_US.UTF-8
+// REQUIRES: locale.fr_FR.UTF-8
+
+// <format>
+
+// The Standard does not specifiy a constructor
+// basic_format_context(Out out,
+//                      basic_format_args<basic_format_context> args,
+//                      std::optional<std::::locale>&& loc = std::nullopt);
+// If compliled with -D_LIBCPP_HAS_NO_LOCALIZATION
+// basic_format_context(Out out,
+//                      basic_format_args<basic_format_context> args);
+
+#include <format>
+#include <cassert>
+#include <type_traits>
+
+#include "make_string.h"
+#include "platform_support.h" // locale name macros
+#include "test_macros.h"
+
+template <class OutIt, class CharT>
+void test() {
+  static_assert(
+      !std::is_copy_constructible_v<std::basic_format_context<OutIt, CharT>>);
+  static_assert(
+      !std::is_copy_assignable_v<std::basic_format_context<OutIt, CharT>>);
+  // The move operations are implicitly deleted due to the
+  // deleted copy operations.
+  static_assert(
+      !std::is_move_constructible_v<std::basic_format_context<OutIt, CharT>>);
+  static_assert(
+      !std::is_move_assignable_v<std::basic_format_context<OutIt, CharT>>);
+
+  std::basic_string<CharT> string = MAKE_STRING(CharT, "string");
+  std::basic_format_args args =
+      std::make_format_args<std::basic_format_context<OutIt, CharT>, CharT>(
+          true, CharT('a'), 42, string);
+
+  {
+    std::basic_string<CharT> str;
+    OutIt out_it{str};
+    std::basic_format_context context{out_it, args};
+    assert(args.__size() == 4);
+    for (size_t i = 0, e = args.__size(); i != e; ++i)
+      assert(context.arg(i) == args.get(i));
+
+    context.out() = CharT('a');
+    assert(str.size() == 1);
+    assert(str.front() == CharT('a'));
+
+#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+    assert(context.locale() == std::locale());
+#endif
+  }
+
+#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+  std::locale en_US{LOCALE_en_US_UTF_8};
+  std::locale fr_FR{LOCALE_fr_FR_UTF_8};
+  {
+    std::basic_string<CharT> str;
+    OutIt out_it{str};
+    std::basic_format_context context{out_it, args, en_US};
+    assert(args.__size() == 4);
+    for (size_t i = 0, e = args.__size(); i != e; ++i)
+      assert(context.arg(i) == args.get(i));
+
+    context.out() = CharT('a');
+    assert(str.size() == 1);
+    assert(str.front() == CharT('a'));
+
+    assert(context.locale() != fr_FR);
+    assert(context.locale() == en_US);
+  }
+
+  {
+    std::basic_string<CharT> str;
+    OutIt out_it{str};
+    std::basic_format_context context{out_it, args, fr_FR};
+    assert(args.__size() == 4);
+    for (size_t i = 0, e = args.__size(); i != e; ++i)
+      assert(context.arg(i) == args.get(i));
+
+    context.out() = CharT('a');
+    assert(str.size() == 1);
+    assert(str.front() == CharT('a'));
+
+    assert(context.locale() == fr_FR);
+    assert(context.locale() != en_US);
+  }
+#endif
+}
+
+void test() {
+  test<std::back_insert_iterator<std::basic_string<char>>, char>();
+  test<std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>();
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  test<std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>();
+#endif
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+  test<std::back_insert_iterator<std::basic_string<char16_t>>, char16_t>();
+  test<std::back_insert_iterator<std::basic_string<char32_t>>, char32_t>();
+#endif
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.formatter/format.context/locale.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: libcpp-has-no-localization
+
+// REQUIRES: locale.en_US.UTF-8
+// REQUIRES: locale.fr_FR.UTF-8
+
+// <format>
+
+// std::locale locale();
+
+#include <format>
+#include <cassert>
+
+#include "make_string.h"
+#include "platform_support.h" // locale name macros
+#include "test_macros.h"
+
+template <class OutIt, class CharT>
+void test() {
+  std::locale en_US{LOCALE_en_US_UTF_8};
+  std::locale fr_FR{LOCALE_fr_FR_UTF_8};
+  std::basic_string<CharT> string = MAKE_STRING(CharT, "string");
+  std::basic_format_args args =
+      std::make_format_args<std::basic_format_context<OutIt, CharT>, CharT>(
+          true, CharT('a'), 42, string);
+
+  {
+    std::basic_string<CharT> str;
+    OutIt out_it{str};
+    std::basic_format_context context{out_it, args, en_US};
+    assert(args.__size() == 4);
+    for (size_t i = 0, e = args.__size(); i != e; ++i)
+      assert(context.arg(i) == args.get(i));
+
+    context.out() = CharT('a');
+    assert(str.size() == 1);
+    assert(str.front() == CharT('a'));
+
+    assert(context.locale() != fr_FR);
+    assert(context.locale() == en_US);
+  }
+
+  {
+    std::basic_string<CharT> str;
+    OutIt out_it{str};
+    std::basic_format_context context{out_it, args, fr_FR};
+    assert(args.__size() == 4);
+    for (size_t i = 0, e = args.__size(); i != e; ++i)
+      assert(context.arg(i) == args.get(i));
+
+    context.out() = CharT('a');
+    assert(str.size() == 1);
+    assert(str.front() == CharT('a'));
+
+    assert(context.locale() == fr_FR);
+    assert(context.locale() != en_US);
+  }
+}
+
+void test() {
+  test<std::back_insert_iterator<std::basic_string<char>>, char>();
+  test<std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>();
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  test<std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>();
+#endif
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+  test<std::back_insert_iterator<std::basic_string<char16_t>>, char16_t>();
+  test<std::back_insert_iterator<std::basic_string<char32_t>>, char32_t>();
+#endif
+}
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.formatter/format.context/out.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// iterator out();
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class OutIt, class CharT>
+void test(
+    std::basic_format_args<std::basic_format_context<OutIt, CharT>> args) {
+  {
+    std::basic_string<CharT> str;
+    OutIt out_it{str};
+    std::basic_format_context context{out_it, args};
+    context.out() = CharT('a');
+    context.out() = CharT('b');
+    context.out() = CharT('c');
+
+    assert(str.size() == 3);
+    assert(str[0] == CharT('a'));
+    assert(str[1] == CharT('b'));
+    assert(str[2] == CharT('c'));
+  }
+}
+
+void test() {
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<char>>, char>>()));
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>()));
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>>()));
+#endif
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<char16_t>>,
+          char16_t>>()));
+  test(std::basic_format_args(
+      std::make_format_args<std::basic_format_context<
+          std::back_insert_iterator<std::basic_string<char32_t>>,
+          char32_t>>()));
+#endif
+}
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.formatter/format.context/types.compile.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// <format>
+
+// Class typedefs:
+// template<class Out, class charT>
+// class basic_format_context {
+// public:
+//   using iterator = Out
+//   using char_type = charT;
+//   template<class T> using formatter_type = formatter<T, charT>;
+// }
+//
+// Namespace std typedefs:
+// using format_context = basic_format_context<unspecified, char>;
+// using wformat_context = basic_format_context<unspecified, wchar_t>;
+
+#include <format>
+#include <type_traits>
+
+#include "test_macros.h"
+
+template <class OutIt, class CharT>
+constexpr void test() {
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<OutIt, CharT>::iterator,
+                     OutIt>);
+  static_assert(
+      std::is_same_v<
+          typename std::basic_format_context<OutIt, CharT>::char_type, CharT>);
+  static_assert(std::is_same_v<typename std::basic_format_context<
+                                   OutIt, CharT>::template formatter_type<bool>,
+                               std::formatter<bool, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<CharT>,
+                     std::formatter<CharT, CharT>>);
+  static_assert(std::is_same_v<typename std::basic_format_context<
+                                   OutIt, CharT>::template formatter_type<int>,
+                               std::formatter<int, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<unsigned>,
+                     std::formatter<unsigned, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<long long>,
+                     std::formatter<long long, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<OutIt, CharT>::
+                         template formatter_type<unsigned long long>,
+                     std::formatter<unsigned long long, CharT>>);
+#ifndef _LIBCPP_HAS_NO_INT128
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<__int128_t>,
+                     std::formatter<__int128_t, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<__uint128_t>,
+                     std::formatter<__uint128_t, CharT>>);
+#endif
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<float>,
+                     std::formatter<float, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<double>,
+                     std::formatter<double, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<long double>,
+                     std::formatter<long double, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<const CharT*>,
+                     std::formatter<const CharT*, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<OutIt, CharT>::
+                         template formatter_type<std::basic_string_view<CharT>>,
+                     std::formatter<std::basic_string_view<CharT>, CharT>>);
+  static_assert(
+      std::is_same_v<typename std::basic_format_context<
+                         OutIt, CharT>::template formatter_type<const void*>,
+                     std::formatter<const void*, CharT>>);
+}
+
+constexpr void test() {
+  test<std::back_insert_iterator<std::basic_string<char>>, char>();
+  test<std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>();
+  test<std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>();
+  test<std::back_insert_iterator<std::basic_string<char16_t>>, char16_t>();
+  test<std::back_insert_iterator<std::basic_string<char32_t>>, char32_t>();
+}
+
+static_assert(std::is_same_v<
+              std::format_context,
+              std::basic_format_context<
+                  std::back_insert_iterator<std::basic_string<char>>, char>>);
+static_assert(
+    std::is_same_v<
+        std::wformat_context,
+        std::basic_format_context<
+            std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>);
+
+// Required for MSVC internal test runner compatibility.
+int main(int, char**) { return 0; }

libcxx/test/std/utilities/format/format.formatter/format.parse.ctx/check_arg_id.verify.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// constexpr void check_arg_id(size_t id);
+
+#include <format>
+
+#include "test_macros.h"
+
+constexpr bool test() {
+  // [format.parse.ctx]/11
+  // Remarks: Call expressions where id >= num_args_ are not
+  // core constant expressions ([expr.const]).
+  std::format_parse_context context("", 0);
+  context.check_arg_id(1);
+
+  return true;
+}
+
+int main(int, char**) {
+  // expected-error@+1 {{static_assert expression is not an integral constant expression}}
+  static_assert(test());
+
+  return 0;
+}