> MSVC rejects this standard-conforming TU when __declspec(dllimport) is added: https://godbolt.org/z/GYrfTqaGn  I am pretty sure this is out of compliance with C99.

Windows/MSVC defines DLLs as separate programs, with their own lifetime and entry point (e.g. you can reload a DLL multiple times and it will be reinitialised each time). So there's no conflict with the standard here, and certainly nothing that affects the real discussion.

If you'd like to continue this sideline, feel free to take it elsewhere - I'm done with it. Let's keep the focus on making sure the added feature is useful for users.