MacOS std::chrono::clock_cast not found

I am trying to use the new C++20 chrono library, more specifically the new clock_cast.

If I compile on Linux with clang++ 16.0.6 it works as expected.
However, if I try to compile on MacOS I encounter the following error:

error: no member named 'clock_cast' in namespace 'std::chrono'

Clang is version 17.0.5 on the Mac, so I would expect the chrono library to be implemented, do I need to add another include?
I couldn't find anything online about this issue.

Clang version:

Homebrew clang version 17.0.5
Target: arm64-apple-darwin23.1.0
Thread model: posix
InstalledDir: /opt/homebrew/opt/llvm/bin

Also tried with the default xcode clang version, same error.

Minimum reproducible example:

#include <chrono>

int main() {
    auto time = std::chrono::high_resolution_clock::now();
    auto system_time = std::chrono::clock_cast<std::chrono::system_clock>(time);
}

Compilation options:

/opt/homebrew/opt/llvm/bin/clang++ -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE -D_LARGEFILE_SOURCE -isystem /opt/homebrew/Cellar/openssl@3/3.1.4/include -std=gnu++20 -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX14.0.sdk -fPIC -o test.cpp.o -c test.cpp

Solution

This has not yet been implemented.

Here is the libc++ status page for C++20:

https://libcxx.llvm.org/Status/Cxx20.html

The pertinent row is:

P0355R7 LWG Extending chrono to Calendars and Time Zones Jacksonville Partial [9]

The footnote is:

[9] P0355: The implementation status is:

Calendars mostly done in Clang 7

Input parsers not done

Stream output Obsolete due to P1361R2 “Integration of chrono with text formatting”

Time zone and leap seconds In Progress

TAI clock not done

GPS clock not done

UTC clock not done

Note that this:

auto time = std::chrono::high_resolution_clock::now();
auto system_time = std::chrono::clock_cast<std::chrono::system_clock>(time);

will never portably work as there is no defined relationship between high_resolution_clock and system_clock. On gcc these are the same clock and so clock_cast just performs the identity conversion. On libc++ high_resolution_clock is the same as steady_clock, and steady_clock has no defined relationship to system_clock.

clock_cast will perform conversions among: system_clock, utc_clock, tai_clock, gps_clock, file_clock, and any user-defined clock which defines one of these pairs of static member functions:

to_sys
from_sys

to_utc
from_utc

See this answer (section "Update for C++20") for an example of a user-defined clock which participates in the clock_cast infrastructure.