apt.llvm.org clang-15 is not able to compile with C++17 feature std::execution::par

Question

for cross-reference: i asked also at llvm-bug-tracker: https://github.com/llvm/llvm-project/issues/57898

Steps to reproduce:

Ubuntu with clang from apt.llvm.org and with potential pstl backends installed via libtbb2-dev and clang-15-openmp is not able to compile with C++17 feature std::execution::par.

Dockerfile:

FROM ubuntu:22.04

RUN apt-get update && apt-get -y install \
    software-properties-common gpg wget \
    make libssl-dev

RUN apt-get install -y build-essential libthrust-dev libtbb2-dev libtbb2

# https://apt.llvm.org/
RUN wget https://apt.llvm.org/llvm.sh && \
    chmod +x llvm.sh && \
    ./llvm.sh 15 all
ENV PATH=$PATH:/usr/lib/llvm-15/bin

RUN echo "\
\n#include <execution> \n \
\n#include <algorithm> \n \
\n#include <vector> \n \
int main(void) { \n \
    std::vector<float> images = {1,2,3,4,5}; \n \
    std::vector<float> results; \n \
    results.resize(images.size()); \n \
    std::transform(std::execution::par, images.begin(), images.end(), results.begin(), [](const auto &input){ return input; }); \n \
}" > hello.cpp

RUN clang -std=c++17 -stdlib=libc++ hello.cpp -o hello && ./hello

produces following error:

hello.cpp:11:26: error: no member named 'execution' in namespace 'std'; did you mean 'exception'?
     std::transform(std::execution::par, images.begin(), images.end(), results.begin(), [](const auto &input){ return input; }); 
                    ~~~~~^~~~~~~~~
                         exception
/usr/lib/llvm-15/bin/../include/c++/v1/exception:100:29: note: 'exception' declared here
class _LIBCPP_EXCEPTION_ABI exception
                            ^
hello.cpp:11:37: error: no member named 'par' in 'std::exception'
     std::transform(std::execution::par, images.begin(), images.end(), results.begin(), [](const auto &input){ return input; }); 
                    ~~~~~~~~~~~~~~~~^
2 errors generated.
The command '/bin/sh -c clang -std=c++17 -stdlib=libc++ hello.cpp -o hello && ./hello' returned a non-zero code: 1

Nevertheless, godbold shows that clang-15 should be possible to compile so: https://godbolt.org/z/3TxoWEMxb

Am i am missing something ubuntu specific here, or do i really have to compile the llvm-stack myself because of potentially missing pstl flag of apt.llvm.org shipped packages?

Answer 1

The problem is not related to compilers, but to implementations of the C++ standard library.

According to the compiler suppor table as well as libc++ documentation, libc++ is still missing the support for C++17 parallel algorithms and execution policies.

On Godbolt, Clang uses libstdc++ by default. That's why it compiles, even with Clang. With using libc++ instead, the compilation will fail. Live demo: https://godbolt.org/z/dMPP7sssa.