[Bug c++/112621] New: g++ error: "references internal linkage entity" when compiling a module interface

["amaikinono at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112621

            Bug ID: 112621
           Summary: g++ error: "references internal linkage entity" when
                    compiling a module interface
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amaikinono at gmail dot com
  Target Milestone: ---

Created attachment 56641
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56641&action=edit
Preprocessed files.

Here's a minimal example:

```
// class.cxx
export module mod;

const int default_val { 0 };

export class Example
{
  // This line causes the problem.
  int value { default_val };
};
```

Compile it:

```
$ g++ -c -std=c++20 -fmodules-ts class.cxx
class.cxx:5:14: error: ‘class Example’ references internal linkage entity
‘const int default_val’
    5 | export class Example
      |              ^~~~~~~
class.cxx:1:8: error: failed to write compiled module: Bad file data
    1 | export module mod;
      |        ^~~~~~
class.cxx:1:8: note: compiled module file is ‘gcm.cache/mod.gcm’
```

If we don't use `default_val` in the class:

```
export module mod;

const int default_val { 0 };

export class Example
{
  int value { 0 };
};
```

or if the file is not a module interface:

```
const int default_val { 0 };

class Example
{
  int value { default_val };
};
```

Then it compiles.

I've also reported this at
<https://stackoverflow.com/questions/77461399/g-error-references-internal-linkage-entity-when-compiling-a-module-interfac>.

Outputs of `gcc -v`:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=ada,c,c++,d,fortran,go,lto,objc,obj-c++ --enable-bootstrap
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-build-config=bootstrap-lto --with-linker-hash-style=gnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.1 20230801 (GCC)