Name namespace-scope unscoped enums

[avikivity]

Unnamed unscoped enums at namespace scope are problematic
(llvm/llvm-project#207581). Giving each such
enum a name (derived from the constant it declares) is enough to avoid
the issue, so do that for the header-level integral constants instead of
leaving them anonymous.

[avikivity]

Update:

• rebased
• simplified: it's enough to name the unscoped enum to make it module-friendly

[avikivity]

@vitaut please review

[vitaut]

Thanks for the PR.

The linked issue says

Two conditions are both necessary (and, together, sufficient):

1. ...
2. A module re-exports that enumerator with a using-declaration. Re-exporting
   a sibling (a type, or the dependent alias itself) does not trigger it.

Could you elaborate where these enumerators are re-exported?

[avikivity]

The first module to export the enum is Seastar. It doesn't name the enumerator directly. Rather it uses fmt via headers, (Seastar cannnot assume all its users consume it as a module), and some of Seastar's exported symbols depend on this enum (max_packed_args, easy to see how it would become a dependency).

The second module is my fmt.cppm (I should probably switch to fmt.cc, but it isn't packaged by Fedora). Whether it is named there or not, it is referred to.

When the two modules are imported by ScyllaDB, the two declarations clash.

Naming those enums makes the problem go away. It was also fixed in clang, but I ask that you accept this patch as a band-aid for people who use ancient compilers like clang 22.1.8 (clang 22 isn't backporting any more, as well).

gcc is also vulnerable, I'll file a bug report there too.

[avikivity]

gcc is also vulnerable, I'll file a bug report there too.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126192

[vitaut]

But fmt's module doesn't re-export the max_packed_args enumerator with a using-declaration, so I still don't understand where the issue is coming from.

[avikivity]

Modules will stealthily export enough stuff to be able to build the exported inline functions.

IIRC my fmt.cppm did not export max_packed_args using a using statement, and I only added it later trying to workaround the failures. I will verify and report.