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h-2 opened this issue Jan 11, 2024 · 8 comments · Fixed by #98275
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[libc++] does not define __cpp_lib_format #77773

h-2 opened this issue Jan 11, 2024 · 8 comments · Fixed by #98275
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@mordante
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libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi.

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@h-2
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h-2 commented Jan 11, 2024

dff62f5

states that

Some std::formatter specializations are not yet available since the class used in the specialization has not been implemented in libc++. This prevents the feature-test macro to be set.

Which specializations are that? I think it is unfortunate that the header is available for a while, but conforming code that relies on the macro cannot use it. Furthermore, __cpp_lib_format_ranges and __cpp_lib_format_uchar are set, which is weird, because they are features that build on __cpp_lib_format so implementations might even think that checking one of the former is sufficient, even though in this situation it is not?
@github-actions github-actions bot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Jan 11, 2024
@frederick-vs-ja
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Currently both __cpp_lib_format and __cpp_lib_chrono are blocked by missing chrono types. See #73953 (comment).

@h-2
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h-2 commented Jan 11, 2024 •
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Ah, that is unfortunate... is there a timeline for adding this?

@mordante
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__cpp_lib_format requires the chrono formatters to be available. 3 papers set the macro to the same initial value. __cpp_lib_format_ranges and __cpp_lib_format_uchar are defined separately. I don't have a timeline but this is high on the top of my priority list for LLVM-19. (LLVM-18 will branch in later this month.)

@ldionne ldionne mentioned this issue Apr 30, 2024
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@pawsaw
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pawsaw commented Jul 4, 2024 •
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@mordante Could you give us a status update? I'm working with Homebrew clang version 18.1.6 and the corresponding libc++. Does this version suppose to have this __cpp_lib_format macro defined? According to my experiments, it does not. However, [1] says this feature is implemented?

Edit: Ok, I misunderstood your comment. So, this will be fixed with LLVM-19 and not backported to LLVM-18, correct?

[1] https://en.cppreference.com/w/cpp/compiler_support/20

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mordante commented Jul 4, 2024

cppreference is not the authoritative source for the status, https://libcxx.llvm.org/Status/Cxx20.html is the authoritative source. (Still I try to keep the status up-to-date on the parts I work on.)

I disagree with @frederick-vs-ja. I think it might be possible to get all chrono parts required for the initial version of __cpp_lib_format done before LLVM-19 is branched. But not all chrono parts for the chrono FTM. The latter would require parsing support, which will not be done before LLVM-19.

So it might be possible to get it in before LLVM-19, but it will be tight. It will not be backported to LLVM-18.

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pawsaw commented Jul 4, 2024

Thank you.

@mordante mordante mentioned this issue Jul 4, 2024
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mordante added a commit to mordante/llvm-project that referenced this issue Jul 10, 2024
@mordante
[libc++][format] define __cpp_lib_format.
691a4d0 
In order to define the format __cpp_lib_format to its initial value
(201907) these papers need to be completed:
- P0645R10 Text Formatting
- P1652R1 Printf corner cases in std::format
- 1361R2 Integration of chrono with text formatting
The first two have been implemented for a while the latter is almost done.

The next value (202106) requires:
- P2216R3 std::format improvements
which has been implemented

The next value (202110) requires:
- P2418R2 Add support for std::generator-like types to std::format
- P2372R3 Fixing locale handling in chrono formatters
The first one has been implemented for a while the latter is almost
done. The latter paper is a DR against 1361R2 and both are implemented
at the same time.

We've had user feedback that the missing of the __cpp_lib_format makes
their lives harder (llvm#77773).
The missing papers 1361R2 and P2372R3 are very close to completion and
might completed before LLVM-19, but it will be a close call. This has
been discussed in the monthly libc++ meeting and we decided to set the
__cpp_lib_format so it will be set in LLVM-19.

Based on the discussion and the implementation status the
__cpp_lib_format is set to 202110
@mordante mordante mentioned this issue Jul 10, 2024
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We've decided to define the macro regardless whether the missing chrono parts are done before LLVM 19. See #98275.

swift-ci pushed a commit to swiftlang/llvm-project that referenced this issue Jul 10, 2024
@mordante
[libc++][format] define __cpp_lib_format. (llvm#98275)
deeb936 
In order to define the format __cpp_lib_format to its initial value
(201907) these papers need to be completed:
- P0645R10 Text Formatting
- P1652R1 Printf corner cases in std::format
- 1361R2 Integration of chrono with text formatting The first two have
been implemented for a while the latter is almost done.

The next value (202106) requires:
- P2216R3 std::format improvements which has been implemented

The next value (202110) requires:
- P2418R2 Add support for std::generator-like types to std::format
- P2372R3 Fixing locale handling in chrono formatters The first one has
been implemented for a while the latter is almost done. The latter paper
is a DR against 1361R2 and both are implemented at the same time.

We've had user feedback that the missing of the __cpp_lib_format makes
their lives harder (llvm#77773).
The missing papers 1361R2 and P2372R3 are very close to completion and
might completed before LLVM-19, but it will be a close call. This has
been discussed in the monthly libc++ meeting and we decided to set the
__cpp_lib_format so it will be set in LLVM-19.

Based on the discussion and the implementation status the
__cpp_lib_format is set to 202110.

Fixes llvm#77773
aaryanshukla pushed a commit to aaryanshukla/llvm-project that referenced this issue Jul 14, 2024
@mordante @aaryanshukla
[libc++][format] define __cpp_lib_format. (llvm#98275)
0fcaaa4 
In order to define the format __cpp_lib_format to its initial value
(201907) these papers need to be completed:
- P0645R10 Text Formatting
- P1652R1 Printf corner cases in std::format
- 1361R2 Integration of chrono with text formatting The first two have
been implemented for a while the latter is almost done.

The next value (202106) requires:
- P2216R3 std::format improvements which has been implemented

The next value (202110) requires:
- P2418R2 Add support for std::generator-like types to std::format
- P2372R3 Fixing locale handling in chrono formatters The first one has
been implemented for a while the latter is almost done. The latter paper
is a DR against 1361R2 and both are implemented at the same time.

We've had user feedback that the missing of the __cpp_lib_format makes
their lives harder (llvm#77773).
The missing papers 1361R2 and P2372R3 are very close to completion and
might completed before LLVM-19, but it will be a close call. This has
been discussed in the monthly libc++ meeting and we decided to set the
__cpp_lib_format so it will be set in LLVM-19.

Based on the discussion and the implementation status the
__cpp_lib_format is set to 202110.

Fixes llvm#77773
blueboxd pushed a commit to blueboxd/libcxx that referenced this issue Apr 19, 2025
@mordante @copybara-github
[libc++][format] define __cpp_lib_format. (#98275)
af24a58 
In order to define the format __cpp_lib_format to its initial value
(201907) these papers need to be completed:
- P0645R10 Text Formatting
- P1652R1 Printf corner cases in std::format
- 1361R2 Integration of chrono with text formatting The first two have
been implemented for a while the latter is almost done.

The next value (202106) requires:
- P2216R3 std::format improvements which has been implemented

The next value (202110) requires:
- P2418R2 Add support for std::generator-like types to std::format
- P2372R3 Fixing locale handling in chrono formatters The first one has
been implemented for a while the latter is almost done. The latter paper
is a DR against 1361R2 and both are implemented at the same time.

We've had user feedback that the missing of the __cpp_lib_format makes
their lives harder (llvm/llvm-project#77773).
The missing papers 1361R2 and P2372R3 are very close to completion and
might completed before LLVM-19, but it will be a close call. This has
been discussed in the monthly libc++ meeting and we decided to set the
__cpp_lib_format so it will be set in LLVM-19.

Based on the discussion and the implementation status the
__cpp_lib_format is set to 202110.

Fixes #77773

NOKEYCHECK=True
GitOrigin-RevId: deeb936863f26e124cc870319148b0576266f11c
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[libc++][format] define __cpp_lib_format. mordante/llvm-project
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