std::expected and related types are not implemented.

[KOLANICH]

clang++ -std=gnu++2b ./test.cpp

std::expected<int, char>

error: no template named 'expected' in namespace 'std'

[philnik777]

Works just fine for me: https://godbolt.org/z/YPs13zYT3. What system are you on? Which compiler and library version are you using? What is the actual code you are trying to run?

[KOLANICH]

The distro is Debian sid. Clang is installed from apt.llvm.org.

The test program is

#include <expected>

int main(){
    using T = std::expected<int, char>;
    return 0;
}

packages:

libstdc++-12-dev/unstable,testing,now 12.2.0-14 amd64 [installed,automatic]
g++/unstable,testing,now 4:12.2.0-3 amd64 [installed,automatic]
clang/unknown,now 1:17.0-58~exp1~20230129224010.18 amd64 [installed]

Compilation with clang++:

clang++ -v -std=gnu++2b ./test_std_expected.cpp

(if I add -stdlib=libc++ and/or change to -std=c++2b, that doesn't help).

Debian clang version 17.0.0 (++20230515100629+528a9e46bd2f-1~exp1~20230515220757.1301)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/12
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/12
Candidate multilib: .;@m64
Selected multilib: .;@m64
 "/usr/lib/llvm-17/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -emit-obj -mrelax-all -dumpdir a- -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name test_std_expected.cpp -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -v -fcoverage-compilation-dir=. -resource-dir /usr/lib/llvm-17/lib/clang/17 -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12 -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/x86_64-linux-gnu/c++/12 -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/backward -internal-isystem /usr/lib/llvm-17/lib/clang/17/include -internal-isystem /usr/local/include -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -std=gnu++2b -fdeprecated-macro -fdebug-compilation-dir=. -ferror-limit 19 -fgnuc-version=4.2.1 -fcxx-exceptions -fexceptions -fcolor-diagnostics -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /tmp/test_std_expected-1e165d.o -x c++ ./test_std_expected.cpp
clang -cc1 version 17.0.0 based upon LLVM 17.0.0 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include"
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12
 /usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/x86_64-linux-gnu/c++/12
 /usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/backward
 /usr/lib/llvm-17/lib/clang/17/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
./test_std_expected.cpp:4:15: error: use of undeclared identifier 'std'
    using T = std::expected<int, char>;
              ^
1 error generated.

Compilation with g++:

g++ -v -std=gnu++2b ./test_std_expected.cpp

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 12.2.0-14' --with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-12 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-12-bTRWOB/gcc-12-12.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-bTRWOB/gcc-12-12.2.0/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (Debian 12.2.0-14) 
COLLECT_GCC_OPTIONS='-v' '-std=gnu++23' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a-'
 /usr/lib/gcc/x86_64-linux-gnu/12/cc1plus -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE ./test_std_expected.cpp -quiet -dumpdir a- -dumpbase test_std_expected.cpp -dumpbase-ext .cpp -mtune=generic -march=x86-64 -std=gnu++23 -version -fasynchronous-unwind-tables -o /tmp/ccvdD3qC.s
GNU C++23 (Debian 12.2.0-14) version 12.2.0 (x86_64-linux-gnu)
        compiled by GNU C version 12.2.0, GMP version 6.2.1, MPFR version 4.1.1-p1, MPC version 1.3.1, isl version isl-0.25-GMP

warning: MPFR header version 4.1.1-p1 differs from library version 4.2.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/12"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/12
 /usr/include/x86_64-linux-gnu/c++/12
 /usr/include/c++/12/backward
 /usr/lib/gcc/x86_64-linux-gnu/12/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C++23 (Debian 12.2.0-14) version 12.2.0 (x86_64-linux-gnu)
        compiled by GNU C version 12.2.0, GMP version 6.2.1, MPFR version 4.1.1-p1, MPC version 1.3.1, isl version isl-0.25-GMP

warning: MPFR header version 4.1.1-p1 differs from library version 4.2.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 62b090dbbefa50644117a3c13d47369a
COLLECT_GCC_OPTIONS='-v' '-std=gnu++23' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a-'
 as -v --64 -o /tmp/ccUDAK3w.o /tmp/ccvdD3qC.s
GNU ассемблер, версия 2.40 (x86_64-linux-gnu); используется BFD версии (GNU Binutils for Debian) 2.40
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-std=gnu++23' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a.'
 /usr/lib/gcc/x86_64-linux-gnu/12/collect2 -plugin /usr/lib/gcc/x86_64-linux-gnu/12/liblto_plugin.so -plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper -plugin-opt=-fresolution=/tmp/cc1tweqa.res -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie /usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/Scrt1.o /usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/12/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/12 -L/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/12/../../.. /tmp/ccUDAK3w.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/12/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-std=gnu++23' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a.'

(compiled succesfully)

[KOLANICH]

The problem seems to be with

#if __cplusplus > 202002L && __cpp_concepts >= 202002L

If we

#if __cpp_concepts < 202002L
#error "No concepts"
#endif

Then the condition triggers.

clang++ -std=gnu++2b -D__cpp_concepts=202002L ./test_std_expected.cpp

compiles.

[philnik777]

Yeah OK. That's a deliberate choice on the Clang side of things to not advertise concepts as completed and libstdc++ to require concepts. I think you either have to wait until clang claims full support for concepts or switch to libc++.

[KOLANICH]

or switch to libc++

As I have written,

(if I add -stdlib=libc++ and/or change to -std=c++2b, that doesn't help).

Currently I just use polyfills (https://github.com/TartanLlama/expected, https://github.com/martinmoene/expected-lite, https://github.com/RishabhRD/expected, the first one has CPack packaging implemented, so I use it).

[philnik777]

or switch to libc++

As I have written,

(if I add -stdlib=libc++ and/or change to -std=c++2b, that doesn't help).

Currently I just use polyfills (https://github.com/TartanLlama/expected, https://github.com/martinmoene/expected-lite, https://github.com/RishabhRD/expected, the first one has CPack packaging implemented, so I use it).

Does clang maybe use an old version of libc++? It should be implemented and available with libc++.

[KOLANICH]

Now I have installed both libc++-17-dev and libc++abi-17-dev (it turns out they were not installed and strangely I have to install both manually). Now it compiles with -std=c++2b.