<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/108831>108831</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang-19, rc4] [cxx-modules] error: declaration attached to named module can't be attached to other modules
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          Silverlan
      </td>
    </tr>
</table>

<pre>
    Repository with reduced code for reproduction: https://github.com/Silverlan/test_cxx_modules

I have a simple cxx-module library consisting of two module fragments, `a` and `b`.
`a.cppm`:
```cpp
module;

export module test_module.a;

export namespace test_module {
        class A {
        public:
#ifdef DEFINE_IN_CLASS
                class B* GetB() { return nullptr; }
#else
                class B* GetB();
#endif
        };
};
```

`b.cppm`:
```cpp
module;

export module test_module.b;

export namespace test_module {
        class B {
        public:

        };
};
```

The method in class `A` uses class `B` by forward declaration. The method is defined in a separate cpp-file, `a.cpp` (unless `DEFINE_IN_CLASS` is defined).
`a.cpp`:
```cpp
module;

module test_module.a;

import test_module.b;

#ifndef DEFINE_IN_CLASS
class test_module::B* test_module::A::GetB() { return nullptr; }
#endif
```

There are four test cases:
1) clang-18, `DEFINE_IN_CLASS` defined: Build is successful.
2) clang-18, `DEFINE_IN_CLASS` *not* defined: Build is successful.
3) clang-19, `DEFINE_IN_CLASS` defined: Build is successful.
3) clang-19, `DEFINE_IN_CLASS` *not* defined: Build fails with the following error:
```
FAILED: CMakeFiles/test_lib.dir/Debug/src/a.cpp.o 
/home/runner/work/test_cxx_modules/test_cxx_modules/LLVM-19.1.0-rc4-Linux-X64/bin/clang++ -DCMAKE_INTDIR=\"Debug\"  -g -std=gnu++20 -MD -MT CMakeFiles/test_lib.dir/Debug/src/a.cpp.o -MF CMakeFiles/test_lib.dir/Debug/src/a.cpp.o.d @CMakeFiles/test_lib.dir/Debug/src/a.cpp.o.modmap -o CMakeFiles/test_lib.dir/Debug/src/a.cpp.o -c /home/runner/work/test_cxx_modules/test_cxx_modules/src/a.cpp
In file included from /home/runner/work/test_cxx_modules/test_cxx_modules/src/a.cpp:9:
/home/runner/work/test_cxx_modules/test_cxx_modules/src/b.cppm:6:8: error: declaration 'B' attached to named module 'test_module.b' can't be attached to other modules
    6 |         class B {
      | ^
/home/runner/work/test_cxx_modules/test_cxx_modules/src/a.cppm:11:9: note: also found
   11 |                 class B* GetB();
      | ^
1 error generated.
ninja: build stopped: subcommand failed.
Error: Process completed with exit code 1.
```

Steps to reproduce:
1) Fork https://github.com/Silverlan/test_cxx_modules
2) Run the action/workflow `Clang Build Error`
3) You can see each test case in the workflow log:

Alternatively you can also reproduce it locally by cloning the repository and building it with clang-19 (rc4). `DEFINE_IN_CLASS` can be enabled/disabled with CMake:
```
git clone https://github.com/Silverlan/test_cxx_modules
cd test_cxx_modules
mkdir -p build
cd build
cmake -DDEFINE_IN_CLASS=OFF -DCMAKE_C_COMPILER=../LLVM-19.1.0-rc4-Linux-X64/bin/clang -DCMAKE_CXX_COMPILER=../LLVM-19.1.0-rc4-Linux-X64/bin/clang++ -G "Ninja Multi-Config" -DCMAKE_BUILD_TYPE=Release ..
cmake --build .
```
(Replace the paths to clang-19 compiler. I've also only tested it with ninja.)
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJysWF1zmzwW_jXyzRkYEBjsC1_YJryT2aTtpt2ddm8yQhxsvRESK4l8_PsdgT_bJJsm7XRiWRw9Ot_PwcxasVGICzJdkWkxYb3barP4KuQ9GsnUpNL10-IGO22F0-YJHoTbgsG651gD1zVCow0Y7Iyue-6EViRZwta5zpJkSWhJaLkRbttXIdctoeUBmtDSoXW3_PHxttV1L9GSqCDRcvx7CVt2j8DAiraTCPzxMRjFQIrKMPMEXCsrrBNqA7oB96BhJ9AYtmlROUvoGkgWMZJFwFTt1xXJonB3URaxkHddS7LIK7vfHP_zrht3RlCSrE7Vw8dOG7e_cDBkXIfseUnFWrQd42fCQPKD7JxLZi0sz_a6vpKCH5WjiWhqbKC4KC8_XdxefrpdXy2_fj3IH2BWhC7hL3QrQmeEzj0oGHS9UaB6KTtnSLICkhcHYJQW_y_O0TaaoKpFczyRF8eHp-u9P0994uPw5z1ffczzq1c9_y47v20RWnRbXYNQMF5DskEYeov2uLXyW9WTr6YHZmqokUtmmC-oEE5hLNTYCIUDIgOLnRdD4F0XNELiPuW9ez0mobNeSRxv-TltsugEkND5T4Xxjui8oSBEO4TltcgNea5eTPTRaycAXstkOeTqL7vL8eP3auEktV-Iq0Fgxne_3gx3AmcW7cFdsb-IS6Y2QTzbxeQZ7-9dnyxh1Qs5xNf2nKO1TS934aBvwyJ0qbTzPngTanKCOv-ghm_EekXDhglpR3ZxW-9WKfWD7-xojDa_ZuH4tVxeXl0UHmV9ze6wFJ5EdrwiRRXWwhBaFlj1G0JLazih5ZDZoYZ9sMutbpHQ0vRKoRd_0ObuOXJ6duvq6t_XQTwP4zAKDE-DK6H6x-B7lhJaVsKT3OAWQleEriAo1tfLf3infCsub0hSkOmaUDoqOCwBgg0E1tUkKTaqH8_RCILrAoLrb--wM7guf_9UWANJo98_1uq6ZR0E-j2KcvhoNE4Ad1OEAt8UQSgu-xpraIxu_-g1yXJ-Qs9_AnfHjMkyI8ly5pN7XwOnrACE5itCc2DOMb7FGpweqK7eUyOh-XmPpTlwP3flDio8O6fdFg2cTWEAABmQfA37f7-w5LjtRcj04k96gO09EMc7_4LSzvdzYNJq33RVfdAhjs_U_EndF-aXZ3SPRz_DBhV6Sq13zU0J9Tfzd1dDp7JOd93Yumxfcd22frL0_etw4mIfry9G-0YJXPsZ1mE9djh8FG4cnuPwFZb56rCzPjz78RrP6aXU5u5jo_bALDe9GnouG8f3MWKN1A--i69999r16NGsvY5Dz_-he59TYBEBGd8eqdCPJx71ACb15qRQYjJdNcJgox9v__PPH1H038950ZJpQejsFZN6iyYYM3c34JfMWhwWSc7SeTTDIErm8yDN8iiomiwOsrRJkziaJimf-hQ48fBSOjSKOXGP8gmedsYMOXbwOQgHUnMm5ZMf0LjUyvOSt80c34t8Dgz54Z8JN8Z5z4h-CDM89QPWC8zor60QULFK-kmsrIUdliPQ0ExfIsGNTyapFX4sF3gNzz9o72phIOhG-w7Cp99adocQFD8blhSfy_JAeuvb9efrL5dXF574wvDt5HlE-P79vRh7Av4LCKWffEXDdS-dCNZaNWLjmXd_y-pfl1fF7bcfXy5IUtygRJ_NYXhmajD2gheql85usJPDG8cWoWNuO5TxIR18OxASTQiXhOb-LddnnFbyaQiBn-13GTT0npDQ-aReJPU8mbMJLuKcZuk8i9Jksl3kVVoxnGFGs6TJ0oxXdZ7nSZ5OeRRlWTURCxrRNJrHWZzGaZSETTxjsyRP6zjHLKENSSNsmZChlPdtqM1mIqztcRFHs1kSTySrUNrhBwJKFT7A8JRQSqbFxCz8oaDqN5akkRTW2SOME04OvyycToa-DqYF-N3DG731O8-S3Ivk9hYim_RGLl6pCa_o7iPojP4buSO0HMzz_WRn__2C_i8AAP__9Kg5og">