<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/138558>138558</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [C++20 modules]
        </td>
    </tr>

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

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

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

<pre>
    (This is an outgrowth of my work discussed in #127561 and related to https://github.com/dealii/dealii/issues/18071. @ChuanqiXu9: FYI)

The collection of module partition units below produces a bunch of errors of the following kind:
```
/usr/bin/clang++-20 -std=c++20 -stdlib=libc++ -MD -MT CMakeFiles/test.dir/b.cc.o -MF CMakeFiles/test.dir/b.cc.o.d @CMakeFiles/test.dir/b.cc.o.modmap -o CMakeFiles/test.dir/b.cc.o -c /home/bangerth/tmp/bug/b.cc
In module 'test:A' imported from /home/bangerth/tmp/bug/b.cc:6:
/usr/include/c++/v1/__random/mersenne_twister_engine.h:223:15: error: cannot befriend target of using declaration
  223 |   friend bool operator==(
      |               ^
/home/bangerth/tmp/bug/b.cc:12:16: note: in instantiation of template class 'std::mersenne_twister_engine<unsigned long, 32, 624, 397, 31, 2567483615, 11, 4294967295, 7, 2636928640, 15, 4022730752, 18, 1812433253>' requested here
   12 |   std::mt19937 x1, x2;
      | ^
/usr/include/c++/v1/__random/mersenne_twister_engine.h:764:1: note: target of using declaration
  764 | operator==(const mersenne_twister_engine<_UInt, _Wp, _Np, _Mp, _Rp, _Ap, _Up, _Dp, _Sp, _Bp, _Tp, _Cp, _Lp, _Fp>& __x,
 | ^
/home/bangerth/tmp/bug/std.ccm:23:14: note: using declaration
   23 | using std::operator==;
      |              ^
In module 'test:A' imported from /home/bangerth/tmp/bug/b.cc:6:
/usr/include/c++/v1/__random/mersenne_twister_engine.h:241:15: error: cannot befriend target of using declaration
  241 |   friend bool operator!=(
      | ^
/usr/include/c++/v1/__random/mersenne_twister_engine.h:807:1: note: target of using declaration
  807 | operator!=(const mersenne_twister_engine<_UInt, _Wp, _Np, _Mp, _Rp, _Ap, _Up, _Dp, _Sp, _Bp, _Tp, _Cp, _Lp, _Fp>& __x,
 | ^
/home/bangerth/tmp/bug/std.ccm:24:14: note: using declaration
   24 | using std::operator!=;
      |              ^
[...]
```
I can't see any good reason for this -- it seems like a bug to me.

Here are the relevant files:
```
// a.ccm
module;
#include <random>
export module test:A;
export {}
```
```
// std.ccm
module;

#include <mutex>
#include <random>

export module test:std;

export {
 namespace std {
    using std::mt19937;
    using std::mutex;
    using std::operator==;
    using std::operator!=;
    // Would ordinarily contain much more stuff here...
  } // namespace std


#if defined(_LIBCPP_VERSION) // For CLang's support library

 _LIBCPP_BEGIN_NAMESPACE_STD

  using std::operator==;
  using std::operator!=;

  _LIBCPP_END_NAMESPACE_STD

#endif
} // export
```
```
// b.cc
module;

module test:B;

import :std;
import :A;

bool test()
{
  static std::mutex rand_mutex; // Necessary, for unclear reasons

  std::mt19937 x1, x2;
  return (x1 == x2) && (x1 != x2);
}
```
```
# CMakeLists.txt
cmake_minimum_required(VERSION 3.28 FATAL_ERROR)

project(test LANGUAGES CXX)
add_library(test)
target_compile_features(test PUBLIC cxx_std_20)
target_sources(test PUBLIC
        FILE_SET CXX_MODULES
        FILES
         std.ccm
          a.ccm
          b.cc)
```

Here are a couple of observations:
* The errors go away if I remove the `#include <random>` from `a.ccm`. Because `a.ccm` does not actually export anything, the presence of `<random>` does not strike me as wrong. In my original testcase, I cannot remove it because it is transitively `#include`d by something else I actually need in that file -- `a.ccm` wraps an external library of which one of the headers happens to `#include <random>`.
* The `:std` partition also exports the symbols in `_LIBCPP_BEGIN_NAMESPACE_STD`. This is needed to avoid other errors, and is something that @davidstone had already realized in https://github.com/davidstone/std_module/blob/d121f1701884953c9ca468b159b530556e2475a3/source/std_module.cpp, for example. If I remove these lines, the error goes away, but I get many more errors in other places in return (just not for this minimal testcase).
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzcWFtv4zoO_jXqC1HDlm_JQx-c22yAtjOYdvbMPhmyzSQ6Y0s-ktw2--sXkpw0zfa2wDwsTlFEtkRJ5MePFC2mNd8KxCuSzki6uGCD2Ul1VTGxRWV2F5Vs9leETu53XAPXwATIwWyVfDQ7kBvo9vAo1S9ouK4HrbEBLoDQOKJ5mkXARAMKW2awASNhZ0yvSVwQuiJ0teVmN1RBLTtCVw2ylvPTB671gJrQVTQJ8ygAkoTz3cDEX_znMCVxAat_rQmdkrAgYXG_Q6hl22JtuBROM9kMLULPlOGubxDcaKiwlY_QK9kMNWpgUA2idqagUlJp-2R2CBvZtvKRiy384qKxOocFycLxP7QmDFoRuqq4IHRVt0xsCZ0ROrukIVxq05B4UfuesaPlFYkXLa_Gbri8WcDlzT3Mb9gvXPHWWWtQm6DhbumgrgMJlzerD0SCxqHzvkwnm471cCk_3K8GQlc72aHtG5lgJbvedgzbUZSExVoccCY0tyuRuCgIzYF3vVTW6xslu08uFxfZiPMBWy7qdmjsvBEyQlcPEaGrslRMNI44HSqNQmBpHrk2qEoUWy4w2JG4oDQmcRGlli3OvfahZkJIAxVuFEfRgGFqi8b6fdDW3w3WLVPMcoaEBQClMZB8DgDjhErKFmSPihm74sL-04mTtX9e9vSPpEtv1adAiKj9sVCAkAZtywVwoQ0ThrMDvw12vQ0sqFumtcXfca4gcfEGJCSeD8KFewOttHSdQ0ztb0YT9zLNXRPZX5pmeTKJsyi1b5HrS-g0mWY5nbo-J0yzOJvSSZaETswNJCGleRzmqVs8mvjfiCZxTNOYxEvLEIV_DagtRXao0KMX0RG9Z1NMNJ3GOTy5_Z8oiWcvgD5C-3sIk2eJBf8U-4_okWeJ0-S_CFFLoQ287Yvyx1oYa1X5R--aW9_c-Oa7bwrf_PDNwjd3vpn55t43c99c-2bVO5wzKMsnQudW1Rd4vU9FbZqgrjsbQi6CklNEXscBxjDxo0cHnsFy5r5XwuT_PaUk0W9IKUn0Xkqh0Ssp5TczfRLm_yvTJ2H-kukHPf8mTE8-zfTkXaZ7WD5mOklnQRCQdHFWWqwtnyz5QSMCE3vYSmnrKKalgI1UYGw1dnkJ3Il0Glr-C10ts7VlVoeBr4r-gQqBKXQVjcIWH5gwsHEn_2sVDaErYA6OsPBB6M0gNB75BiSej_yKrQ34ZIPyELHHcHWzxjGSz0h-buMrGx88cbb1-e7dYPDJb_62Wm9p5pw1eyFg1QsLEKxD3bMarR6HToBzJ4_n0bN3z8e9dm-MvpUMP8mkEag_5NA2IFXDBVO83UMthWFcQDfUO-iksjYMm407WC3D7GySLw7zX5g6QuzB3ECDGy6wIXRSXq9n82_fyn8uv9-tv94SOj3MX0kF82tX7-Ya9NA7HFteKab2fjE4zJ4tv6xvy9viZnn3rZgvy7v7xSjxOWw-gYyTO-y3vF28thuhMYqGb-zjMxCeAR8zcyx2z2j5klmzY78_p-CUbc9dxVHMpXw3mU7Gz5iRdNoww-szToGld3mg18GCW6xRaws7nbvMMIi6RabGZKEP8HxYTyk0g7JfbpOnCLwL7KhzemaT6zjicPcjoyUfhHbsvzauuTY6ME8W7bpjv7DsuODd0JW2DuTKUW6kGsQBncCquC-uy-X371-_H7_yeiX_xNoiZoGD6-L2y4_iy_IO5j9_eiHWNOWBiV7K9_uzraxl1_MWyw0yMyj7_eNX-vZjdr2eQ_30VGrTlDR8MUvLQdXnwk6j6Wp9vSzvlvdWg_Lm6-LH9fLueWR8PElt9u2YYt2LI5c38BS4k-TNoJZD36I9mWWlUT24s-iQwmkB9ut3_HzdSmCPbA98A2tQ2MkHn_3tsm-kyywcS6os9JplYQAzrNmg8bQTGonano3AajOwtt2PEWTPKLPj_ovCbtYr1Chqp7Hd-Gy34zraKHtwdQhMw6OSYhuArf_2IBXfcsF8hNRMo116fSi0Rru4Lbm8mtwA12AUE5ob_oDt_qXFJAsbqPagZYdOVcBWI6yfTRHoLy_Mjvkj0h6wp9Y_Kta7CxB8MqisbiPRrJWPO17vQAo83B_skDWoNOxY36PQ9lh-1wXBiSsdZC57ZOHJBQZrtRwR124Pve8q2Wp35ZKF76Vc69HDFY411F_HsAfJG5Bmh2qkj0WZicaKPSPlECFJ2LAH3mhjjdyxBlirkDV7m2pa_m8P3nsXPMfZvuoqx3RKV1UrKysQ0WgT5WE0mSTTNK6nNUuySRWl0yqNwzTNkCZ5ymI73QXki3WCuu8PSRCfWNe3GMD6ZRBohJYL1AeaOptha9loY8Z2V4OBNdgyuLOFlztMx8jiYoSqb1mN7v05a_45aOMofSzPXH57QeBpcNFcxc00nrILvIryJEuyLI6Ti91VVGOeTGlEmzqLw2YSN0laTbLppk6raMqmF_yKhjQN0zCN8phGcRBOojqLWZVlNTaTyYYkIXaMt0HbPnSBVNsLd392FcWTNJ1ctKzCVrtbPkoFPoIbJZSSdHGhruyky2rYapKErUvVx2UMN627HpwfL7M84tpOHVR79Y7T7Spjc3lM3ic3e165hyv6nwAAAP__4Vg8nQ">