Clang compilation issues with C++20 and C++23 features for std::chrono and std::ranges

[Sailanarmo]

Greetings,

I am unsure if this is a bug or a huge misunderstand on how I am using emscripten so if it is the latter please forgive my ignorance. I am attempting to compile a project that I wrote in Qt and trying my hand at using Wasm to render the UI. I have followed the instructions on the wiki where I:

• Cloned the emsdk git clone -b 5.0.2 <repo> --single-branch
• ran ./emsdk install latest
• ran ./emsdk activate latest
• Added emsdk to my .bashrc file by adding source "<location of emsdk_env.sh>

My project fully compiles with GCC 14.2.0 on both Rocky Linux and Ubuntu 24.04. However, trying to compile with emscripten I find features that should be in Clang are not properly found. These are std::chrono::current_zone and std::views::enumerate. I am unsure if this is a me issue or an issue with the compiler provided? I have pulled out two of the offenders and these are the compiler issues I am seeing.

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 5.0.2 (dc80f64)

Failing command line in full:

error: no member named 'enumerate' in namespace 'std::ranges::views'
   55 |     for(const auto& [index, entry] : std::views::enumerate(std::filesystem::directory_iterator{datasetPath}) )

error: no member named 'current_zone' in namespace 'std::chrono'
   13 |   auto localTime = std::chrono::current_zone()->to_local(now);

Full link command and output with -v appended:

emcc -std=c++23 test.cpp -v
 /home/sailanarmo/emsdk/upstream/bin/clang -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -std=c++23 -v -c test.cpp -o /tmp/emscripten_temp_846fy_g9/test.o
clang version 23.0.0git (https:/github.com/llvm/llvm-project 58f4da463e5b3cd3531cace17cc3f2d8d860964e)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/sailanarmo/emsdk/upstream/bin
 (in-process)
 "/home/sailanarmo/emsdk/upstream/bin/clang-23" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name test.cpp -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -fvisibility=hidden -debugger-tuning=gdb -fdebug-compilation-dir=/home/sailanarmo/AtlasImager -v -fcoverage-compilation-dir=/home/sailanarmo/AtlasImager -resource-dir /home/sailanarmo/emsdk/upstream/lib/clang/23 -D EMSCRIPTEN -isysroot /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /home/sailanarmo/emsdk/upstream/lib/clang/23/include -internal-isystem /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include -std=c++23 -fdeprecated-macro -ferror-limit 19 -fmessage-length=197 -fgnuc-version=4.2.1 -fno-implicit-modules -fskip-odr-check-in-gmf -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_846fy_g9/test.o -x c++ test.cpp
clang -cc1 version 23.0.0git based upon LLVM 23.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
 /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
 /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/compat
 /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
 /home/sailanarmo/emsdk/upstream/lib/clang/23/include
 /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
test.cpp:13:33: error: no member named 'current_zone' in namespace 'std::chrono'
   13 |   auto localTime = std::chrono::current_zone()->to_local(now);
      |                                 ^~~~~~~~~~~~
1 error generated.
emcc: error: '/home/sailanarmo/emsdk/upstream/bin/clang -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -std=c++23 -v -c test.cpp -o /tmp/emscripten_temp_846fy_g9/test.o' failed (returned 1)

emcc -std=c++23 test.cpp -v
 /home/sailanarmo/emsdk/upstream/bin/clang -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -std=c++23 -v -c test.cpp -o /tmp/emscripten_temp_7vv33nqx/test.o
clang version 23.0.0git (https:/github.com/llvm/llvm-project 58f4da463e5b3cd3531cace17cc3f2d8d860964e)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/sailanarmo/emsdk/upstream/bin
 (in-process)
 "/home/sailanarmo/emsdk/upstream/bin/clang-23" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name test.cpp -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -fvisibility=hidden -debugger-tuning=gdb -fdebug-compilation-dir=/home/sailanarmo/AtlasImager -v -fcoverage-compilation-dir=/home/sailanarmo/AtlasImager -resource-dir /home/sailanarmo/emsdk/upstream/lib/clang/23 -D EMSCRIPTEN -isysroot /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /home/sailanarmo/emsdk/upstream/lib/clang/23/include -internal-isystem /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include -std=c++23 -fdeprecated-macro -ferror-limit 19 -fmessage-length=197 -fgnuc-version=4.2.1 -fno-implicit-modules -fskip-odr-check-in-gmf -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_7vv33nqx/test.o -x c++ test.cpp
clang -cc1 version 23.0.0git based upon LLVM 23.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
 /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
 /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/compat
 /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
 /home/sailanarmo/emsdk/upstream/lib/clang/23/include
 /home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
test.cpp:13:46: error: no member named 'enumerate' in namespace 'std::ranges::views'
   13 |   for(const auto& [index, val] : std::views::enumerate(myVec))
      |                                              ^~~~~~~~~
test.cpp:13:56: error: use of undeclared identifier 'myVec'
   13 |   for(const auto& [index, val] : std::views::enumerate(myVec))
      |                                                        ^~~~~
2 errors generated.
emcc: error: '/home/sailanarmo/emsdk/upstream/bin/clang -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/sailanarmo/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -std=c++23 -v -c test.cpp -o /tmp/emscripten_temp_7vv33nqx/test.o' failed (returned 1)

I can see that the -std=c++23 is being passed in and I am honestly unsure why this would be failing since the clang version is 23.0.0. According to Godbolt both of these should be in the main branch so I don't really understand what is failing here.

Code from test program:

#include <print>
#include <chrono>
#include <ranges>

auto main() -> int
{

  auto now = std::chrono::system_clock::now();
  auto localTime = std::chrono::current_zone()->to_local(now);
  std::print("Current time: {}\n", localTime);

  const auto myVec = std::vector{0,1,2,3,4};

  for(const auto& [index, val] : std::views::enumerate(myVec))
    std::println("Index: {} Value: {} ", index, val);

  return 0;
}

[sbc100]

Regarding the std::chrono::current_zone it looks like those timezone features are gated on _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION:

emscripten/system/lib/libcxx/include/chrono

Lines 1079 to 1089 in 7d909ca

	# if _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION
	# include <__chrono/gps_clock.h>
	# include <__chrono/leap_second.h>
	# include <__chrono/tai_clock.h>
	# include <__chrono/time_zone.h>
	# include <__chrono/time_zone_link.h>
	# include <__chrono/tzdb.h>
	# include <__chrono/tzdb_list.h>
	# include <__chrono/utc_clock.h>
	# include <__chrono/zoned_time.h>
	# endif

Emscripten currently defines _LIBCPP_HAS_FILESYSTEM and _LIBCPP_HAS_LOCALIZATION but it does not look like we defined _LIBCPP_HAS_TIME_ZONE_DATABASE:

emscripten/system/lib/libcxx/include/__config_site

Line 20 in 7d909ca

#define _LIBCPP_HAS_TIME_ZONE_DATABASE 0

I'm not sure what it would have to do to support that but I can take a look.

Regarding 'enumerate' in namespace 'std::ranges::views' it looks like maybe LLVM's libc++ simply doesn't support that yet: llvm/llvm-project#105251. At least assuming that is the same thing?

[sbc100]

It seems like std::chrono::current_zone also currently requires -fexperimental-library. At least with the current release version of libc++ that we use in emscripten (21.1.8).

[Sailanarmo]

I must be linking against the wrong version of clang in Godbolt?

Thank you for looking into that so quickly by the way.

[sbc100]

Can you tell godbolt to also use lib++ (the LLVM c++ stdlib)?

[sbc100]

To make current_time work we would need give libc++ access to the timezone database somehow. Currently we disable that:

emscripten/tools/system_libs.py

Lines 1755 to 1759 in 7d909ca

	# Emscripten does not have C++20's time zone support which requires access
	# to IANA Time Zone Database. TODO Implement this using JS timezone
	'time_zone.cpp',
	'tzdb.cpp',
	'tzdb_list.cpp',

[Sailanarmo]

Can you tell godbolt to also use lib++ (the LLVM c++ stdlib)?

Sorry I have been trying to figure out how to do this but I haven't had any success yet. I could fire up my rocky linux install really quickly and install the latest clang toolset and see if this is accurate or not. It sounds like the current_time would be complicated to implement right now.