* Thomas Weißschuh:

quoted

If you call the data member sa_data just like glibc, it will only fail
in C++, not C.  GCC considers the two definitions sufficiently
equivalent (even though glibc adds a may_alias attribute to meet POSIX
requirements), and duplicate definitions are permitted in C.

clang is not so lenient and will error out.

It seems it accepts it if you switch to C23 mode.

Thanks,
Florian