Can't build as C++20 module since 10.1.0

[GregTheMadMonk]

Issue in the title. 10.0.0 builds successfully with -DFMT_MODULE=ON, 10.1.0 doesn't.

Compiler versions and build logs:

$ gcc --version
gcc (GCC) 13.2.1 20230801
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cmake .. -DFMT_MODULE=ON && make
-- CMake version: 3.27.1
-- The CXX compiler identification is GNU 13.2.1
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Testing is incompatible with build mode 'module'.
-- Version: 10.1.0
-- Build type: Release
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- Configuring done (1.9s)
-- Generating done (0.0s)
-- Build files have been written to: /home/greg/tmp/fmt/build
[ 12%] Building CXX object CMakeFiles/fmt.dir/src/fmt.cc.o
/home/greg/tmp/fmt/src/fmt.cc:75:20: error: ‘export’ may only occur once in an export declaration
   75 | #define FMT_EXPORT export
      |                    ^~~~~~
/home/greg/tmp/fmt/include/fmt/core.h:1665:1: note: in expansion of macro ‘FMT_EXPORT’
 1665 | FMT_EXPORT
      | ^~~~~~~~~~
In file included from /home/greg/tmp/fmt/include/fmt/chrono.h:21,
                 from /home/greg/tmp/fmt/src/fmt.cc:91:
/home/greg/tmp/fmt/include/fmt/format.h:1484:57: error: conflicting exporting declaration ‘fmt::v10::detail::uint128_fallback fmt::v10::detail::umul128(uint64_t, uint64_t)’
 1484 | inline uint128_fallback umul128(uint64_t x, uint64_t y) noexcept {
      |                                                         ^~~~~~~~
/home/greg/tmp/fmt/include/fmt/format.h:371:27: note: previous declaration ‘fmt::v10::detail::uint128_fallback fmt::v10::detail::umul128(uint64_t, uint64_t)’ here
  371 |   friend uint128_fallback umul128(uint64_t x, uint64_t y) noexcept;
      |                           ^~~~~~~
/home/greg/tmp/fmt/include/fmt/format.h:4319:71: error: conflicting exporting declaration ‘template<class Char> void fmt::v10::detail::vformat_to(buffer<T>&, fmt::v10::basic_string_view<Char>, typename vformat_args<Char>::type, locale_ref)’
 4319 |              typename vformat_args<Char>::type args, locale_ref loc) {
      |                                                                    ^

In file included from /home/greg/tmp/fmt/include/fmt/args.h:15,
                 from /home/greg/tmp/fmt/src/fmt.cc:90:
/home/greg/tmp/fmt/include/fmt/core.h:2676:6: note: previous declaration ‘template<class Char> void fmt::v10::detail::vformat_to(buffer<T>&, fmt::v10::basic_string_view<Char>, typename vformat_args<Char>::type, locale_ref)’ here
 2676 | void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
      |      ^~~~~~~~~~
/home/greg/tmp/fmt/src/fmt.cc:73:8: warning: not writing module ‘fmt’ due to errors
   73 | export module fmt;
      |        ^~~~~~
make[2]: *** [CMakeFiles/fmt.dir/build.make:76: CMakeFiles/fmt.dir/src/fmt.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:135: CMakeFiles/fmt.dir/all] Error 2
make: *** [Makefile:166: all] Error 2

$ clang --version
clang version 15.0.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

$ CXX=clang++ CC=clang cmake .. -DFMT_MODULE=ON && make
-- CMake version: 3.27.1
-- The CXX compiler identification is Clang 15.0.7
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Testing is incompatible with build mode 'module'.
-- Version: 10.1.0
-- Build type: Release
-- Performing Test HAS_NULLPTR_WARNING
-- Performing Test HAS_NULLPTR_WARNING - Success
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- Configuring done (2.7s)
-- Generating done (0.0s)
-- Build files have been written to: /home/greg/tmp/fmt/build
[ 11%] Generating fmt.pcm
In file included from /home/greg/tmp/fmt/src/fmt.cc:90:
In file included from /home/greg/tmp/fmt/include/fmt/args.h:15:
/home/greg/tmp/fmt/include/fmt/core.h:1665:1: error: export declaration appears within another export declaration
FMT_EXPORT
^
/home/greg/tmp/fmt/src/fmt.cc:75:20: note: expanded from macro 'FMT_EXPORT'
#define FMT_EXPORT export
                   ^
/home/greg/tmp/fmt/include/fmt/core.h:1600:1: note: export block begins here
FMT_BEGIN_EXPORT
^
/home/greg/tmp/fmt/src/fmt.cc:76:26: note: expanded from macro 'FMT_BEGIN_EXPORT'
#define FMT_BEGIN_EXPORT export {
                         ^
In file included from /home/greg/tmp/fmt/src/fmt.cc:91:
In file included from /home/greg/tmp/fmt/include/fmt/chrono.h:21:
/home/greg/tmp/fmt/include/fmt/format.h:4318:6: error: cannot export redeclaration 'vformat_to' here since the previous declaration has module linkage
void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
     ^
/home/greg/tmp/fmt/include/fmt/core.h:2676:6: note: previous declaration is here
void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
     ^
In file included from /home/greg/tmp/fmt/src/fmt.cc:109:
In file included from /home/greg/tmp/fmt/src/format.cc:8:
/home/greg/tmp/fmt/include/fmt/format-inl.h:1424:11: error: explicit instantiation of undefined function template 'vformat_to'
  detail::vformat_to(buffer, fmt, args);
          ^
/home/greg/tmp/fmt/include/fmt/core.h:2676:6: note: explicit instantiation refers here
void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
     ^
3 errors generated.
make[2]: *** [CMakeFiles/fmt.dir/build.make:74: fmt.pcm] Error 1
make[1]: *** [CMakeFiles/Makefile2:134: CMakeFiles/fmt.dir/all] Error 2
make: *** [Makefile:166: all] Error 2

[vitaut]

At least some of the module-related issues should be fixed by #3597.

[GregTheMadMonk]

Checked: it builds with Clang now. umul128 error with GCC is still present though.

[GregTheMadMonk]

Seems to compile fine now. Thank you!