<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/131814>131814</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [modules] Compilation success depends on order of declarations in the modulemap
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang,
            clang:modules
      </td>
    </tr>

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

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

<pre>
    Consider the following set of headers:

```c++
// myvector.h
#ifndef MYVECTOR_H
#define MYVECTOR_H

#include <bit_ref.h>

template <class _Allocator>
struct MyVector {
  typedef bool value_type;
  typedef unsigned long __storage_type;
  friend class MyBitIterator<MyVector, false>;
};

#endif
```

```c++
// bit_ref.h
#ifndef BIT_REF_H
#define BIT_REF_H

#include <bit_ref_fwd.h>

#endif
```

```c++
// bit_ref_fwd.h
#ifndef BIT_REF_FWD_H
#define BIT_REF_FWD_H

template <class _Cp, bool _IsConst, typename _Cp::__storage_type = 0>
class MyBitIterator {};

#endif
```

And the following modulemap:

```
module mystd {
 module myvector {
    header "myvector.h"
    export mystd.bit_ref
 }

  module bit_ref_fwd {
    header "bit_ref_fwd.h"
  }
  module bit_ref {
    header "bit_ref.h"
    export mystd.bit_ref_fwd
 }
}
```

The following fails to compile:

```
echo '#include <myvector.h>' | clang++ -x c++ - -std=c++17 -fmodules -fcxx-modules -Xclang -fmodules-local-submodule-visibility -fsyntax-only -I $(dirname ${0}) -fmodules-cache-path="$(mktemp -d)"
```

The error is:

```
While building module 'mystd' imported from <stdin>:1:
In file included from <module-includes>:1:
./repro-modules/myvector.h:11:16: error: missing '#include "bit_ref_fwd.h"'; 'MyBitIterator' must be declared before it is used
   11 |   friend class MyBitIterator<MyVector, false>;
      | ^
./repro-modules/bit_ref_fwd.h:5:7: note: declaration here is not visible
    5 | class MyBitIterator {};
      |       ^
<stdin>:1:10: fatal error: could not build module 'mystd'
    1 | #include <myvector.h>
 |  ~~~~~~~~^
2 errors generated.
```

A few things can be observed:
1. If we move the definition of `bit_ref_fwd` and `bit_ref` *before* the definition of `myvector` in the modulemap, the issue goes away.
2. If we remove `export mystd.bit_ref` from the `myvector` module, the issue goes away.
3. If we add a direct include of `#include <bit_ref_fwd.h>` in `myvector.h`, the issue goes away.

Is this behavior intended? If the order of module definitions in a modulemap matters, there should be a warning that tells us when the compiler encounters something that has not been defined yet in the modulemap. If the order is irrelevant, then there's a bug in Clang.

This was discovered while investigating https://github.com/llvm/llvm-project/pull/127015.
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJykV0-P4j4S_TTmUgIlDpDmwCFNN9o-jFb6afSb3RNy4grxrmMj24Hmsp99VXZooIfp2T8IdYeyXfXq1asyCO_V3iCu2eKZLV4mYgiddWstlTUGJ7WV5_XGGq8kOggdQmu1tidl9uAxgG2hQyHReVZULIvvZZbeDePP9M4qxreMb6E_H7EJ1s26aCtUayS28O3vf75uvv_1j91fklliqwx-MqcDptGDRGDFplZh57Cddax4TesB-4MWIa42WngPu0pr24hgXdrkgxuaAN_Of0YYwEoCBxDOByQgtbUajkIPuCMTK-6XBxOpkqCt2cNu54N1Yn-_t3UKjYQU_9v5WYW3gC5B2FwCM76BVmiPBCseZOXL-BDzRCNVe0vl75i9snFL7PPb990fr9tPvN5Zf0Hrrj3JG2r_D0ijp0ewtj9efgHtY-VhVTcHIjAWa_fmSZyBDFQGI3qMG4qKFdV9iYAVL5ClnB7UJ6rhPypDZeSnTuitHDT24vBzE7CsSqvQn32QF8192I73UoSxnYBxftMunI-r-H6wLiRfs5FiWiLkMTBcXN_w_9D7fX3GAMnNZx9fnf8dOPJ_AzD9vefz-x2XrVDaQ7DQ2P6gND6kFJvOAuPlvXhvCCteGS-BlRtqRbNP4oTpOzSXR5j6IFnxMhryEqZtStvDtG3e36cfn_4WfVzXpzRV9NQPdTJMj8qrWmkVzjBt_dkE8T61Rp9h-gaMzxl_kspFbdKn8jkjHvjqxmMjmg6nBxE6VrwQo_FU_0-SP0wl46tE8wPq0DnrQD0YwCyrfnSK6jgoLa9CJeZikYgj1VPRUELrbE8s-iCViZOpypPPNwMtuRmpvm4d0x_t_u7QjPGtw4OzFx4Z394WqMppY75kRZVSoIdeeU84P5X2gVp5yYpn2nc_ZHkJ_eAD1AgSGy0cSqixtQ5BBVAeBo8yyTXPoz7-55kN8UUu2OL1Fwl_GqfVghVVSYkaG0jZI0gRlDXQIaH0tAZRURrHMIuLkr8cWldA41OE9XNF84wityIIfWW-sYOWMXQUywOljBESa181Xuz2DcC_xlfCwVMsD3s0BB7l7MFohRZPEDpl9h4aYaiOtvbojiiTrPIZvLVwQujtEeMYjneHigzaFtgyu508ywyEkTdWsjBeJU0wXj12cUmIdisT91xnPN01HVXKDwh7ix7ESZwpG34B5zDCY8vs4cBeZqmFyM2naCnMFzGKSwwhJQiQymETLq05wv_NnZ6Sugk86-KhX8akGeCpLB5q7MRR0cAxAY2ksmwJEJ20ju4G217Ec6XVU0Bx5RB6EQJ9aUwxHYLvogBrBAEn4QxNgdCJAAG1praFU4epEuPF4ABNYwdDfsDbHqNs0qFOpDaqEU2CgRLOGH4q5uweu_KgnEONR2HCCM4khIyXHgTUw56cbOhGmF2GsPJwEh6k8o09Is2cU5cG5hF9UHsRCFkXwiFO6fgFaa9CN9SzxvaMb7U-Xv5ND87-A5vA-PYwaM34Nudlli9mE7ku5KpYiQmu83LOi2z1lJWTbj3n81rm2OJTkz3VmXjKJa54xlerNuNiUU_Ummd8kRX5Uz5fLOfljGcrgcvVouSrhSxWSzbPsBdKzwjAzLr9JIpgnccjEy1q1D7-QuB8vE8545uPT0X1MfI4_Yhw65hJPew9m2da-eCvroMKOv7cuJxZvMAmljSNQT80DXoPEg9opAdqyouwbsal_6mWk8Hp9X9NcsyUZvWY7HHN_x0AAP__VPIY7g">