compile error with gcc and -std=c++20: partial specialization of struct fmt::v8::formatter

[michael-brade]

Hi, this little test works with -std=c++17 but breaks when using -std=c++20. I am on Debian unstable using gcc 11.2.0 and fmt 8.1.1. The include order matters, including chrono.h after using fmt::format breaks with C++20.

Here is the test code:

#include <fmt/core.h>
//#include <fmt/chrono.h>    // works when including chrono.h here

std::string m = "hi";
std::string f = fmt::format("{}", m);

#include <fmt/chrono.h>

int main() {
   return 0;
}

And this is the output:

$ gcc -std=c++20 -v -o test.cpp.o -c test.cpp 
Using built-in specs.
COLLECT_GCC=gcc
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 11.2.0-18' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-11 --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-bootstrap --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-11-RK3FrC/gcc-11-11.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-RK3FrC/gcc-11-11.2.0/debian/tmp-gcn/usr --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (Debian 11.2.0-18) 
COLLECT_GCC_OPTIONS='-std=c++20' '-v' '-o' 'test.cpp.o' '-c' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/11/cc1plus -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE test.cpp -quiet -dumpbase test.cpp.cpp -dumpbase-ext .cpp -mtune=generic -march=x86-64 -std=c++20 -version -fasynchronous-unwind-tables -o /tmp/cc9K8HEh.s
GNU C++20 (Debian 11.2.0-18) version 11.2.0 (x86_64-linux-gnu)
        compiled by GNU C version 11.2.0, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/11"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/11/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/11
 /usr/include/x86_64-linux-gnu/c++/11
 /usr/include/c++/11/backward
 /usr/lib/gcc/x86_64-linux-gnu/11/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C++20 (Debian 11.2.0-18) version 11.2.0 (x86_64-linux-gnu)
        compiled by GNU C version 11.2.0, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 021593b70cdc7376787f3c2dbadd7218
In file included from /usr/include/fmt/chrono.h:19,
                 from test.cpp:7:
/usr/include/fmt/format.h:2597:1: error: partial specialization of ‘struct fmt::v8::formatter<std::__cxx11::basic_string<_CharT>, Char>’ after instantiation of ‘struct fmt::v8::formatter<std::__cxx11::basic_string<char>, char, void>’ [-fpermissive]
 2597 | FMT_FORMAT_AS(std::basic_string<Char>, basic_string_view<Char>);
      | ^~~~~~~~~~~~~

[vitaut]

Looks like a duplicate of #2769.

[michael-brade]

ah, sorry, indeed it is, I didn't expect it to be fixed already. How about an 8.1.2 release? 😋

[vitaut]

Yeah, this should be a part of the next release.