On Tue, 18 Feb 2025 at 09:11, Thomas Khyn via Gcc-help
<gcc-help@xxxxxxxxxxx> wrote:
>
> Hi,
>
> I am trying to build {fmt} as a C++20 module with GCC-15 (15.0.1 20250214).
>
> I get an error that neither me nor the {fmt} maintainer can make sense
> of. We are unsure if it's a code issue or a GCC issue (other compilers
> work fine, however they do not implement P1815
> <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1815r2>).
>
> This is about the following code fragment (os.cc L72
> <https://github.com/fmtlib/fmt/blob/master/src/os.cc#L72>, curated from
> #ifdefs below), which ends up being included in fmt.cc:
>
> namespace {
> // Return type of read and write functions.
> using rwresult = ssize_t;
>
> inline std::size_t convert_rwcount(std::size_t count) { return count; }
> } // namespace
>
> The compiler reports an error for the line using rwresult = ssize_t; :
>
> /usr/bin/c++ -I/mnt/d/dev/libs/tools/cpp/fmt/include -O3 -DNDEBUG
> -std=gnu++20 -fvisibility=hidden -fvisibility-inlines-hidden
> -fdiagnostics-color=always -fmodules-ts -MD -MT
> CMakeFiles/fmt.dir/src/fmt.cc.o -MF CMakeFiles/fmt.dir/src/fmt.cc.o.d
> -fmodules-ts -fmodule-mapper=CMakeFiles/fmt.dir/src/fmt.cc.o.modmap -MD
> -fdeps-format=p1689r5 -x c++ -o CMakeFiles/fmt.dir/src/fmt.cc.o -c
> /mnt/d/dev/libs/tools/cpp/fmt/src/fmt.cc
> In file included from /mnt/d/dev/libs/tools/cpp/fmt/src/fmt.cc:148:
> /mnt/d/dev/libs/tools/cpp/fmt/src/os.cc:74:7: error: ‘using
> {anonymous}::rwresult = ssize_t’ exposes TU-local entity ‘{anonymous}’
> 74 | using rwresult = ssize_t;
> | ^~~~~~~~
> /mnt/d/dev/libs/tools/cpp/fmt/src/os.cc:62:1: note: ‘{anonymous}’
> declared with internal linkage
> 62 | namespace {
> | ^~~~~~~~~
>
> My shallow understanding of the standard
> <https://timsong-cpp.github.io/cppwp/n4659/basic.link#8> and P1815
> <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1815r2> is
> that it looks like gcc is correct to disallow this code (anonymous
> namespace is exposed as TU-local because rwresult has linkage because
> ssize_t is a fundamental type).
>
> However, when working around this error (either by moving the `using`
> statement within the functions bodies or by creating a macro for
> `rwresult`
> <https://github.com/fmtlib/fmt/commit/9381bba8d967428dd4829b2fcc0c32199ef41ed3#diff-261f435b8f5004993798511d1dc333e917901cbb2e83e73592d5bd2f3082c7bc>),
> gcc is happy with `convert_rwcount`, which should also have internal
> linkage and therefore expose the anonymous namespace as TU-local and
> therefore report the same error.
>
> Would anyone know what am I missing? If GCC is correct about
> convert_rwcount then I would like to understand if there is anything we
> can do to rwresult to make it happy.
This is apparently https://gcc.gnu.org/PR118799
>
> Thanks in advance,
>
> Thomas
>
>
> PS: full steps to reproduce the error, with GCC 15.0.1 20250214
> (installed in /opt/gcc-dev) and cmake 3.31.5:
>
> git clone -b gcc-15-fixes-test https://github.com/tkhyn/fmt
> mkdir fmt/build && cd fmt/build
> export CXX=/opt/gcc-dev/bin/g++
> export CC=/opt/gcc-dev/bin/gcc
> cmake .. -DFMT_MODULE=ON
> cmake --build . --target fmt
>
>
> To make the error disappear, make these changes
> <https://github.com/fmtlib/fmt/commit/9381bba8d967428dd4829b2fcc0c32199ef41ed3#diff-261f435b8f5004993798511d1dc333e917901cbb2e83e73592d5bd2f3082c7bc>
> to os.cc.
