[libc++] does not define __cpp_lib_format

[h-2]

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?

[frederick-vs-ja]

Currently both __cpp_lib_format and __cpp_lib_chrono are blocked by missing chrono types. See #73953 (comment).

[h-2]

Ah, that is unfortunate... is there a timeline for adding this?

[mordante]

__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.)

[pawsaw]

@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

[frederick-vs-ja]

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

It's unlikely for libc++ to complete C++20 chrono types in LLVM19, so perhaps you need to wait a bit longer while.

[mordante]

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.

[pawsaw]

Thank you.

[mordante]

We've decided to define the macro regardless whether the missing chrono parts are done before LLVM 19. See #98275.