[Bug c++/120498] error: either all initializer clauses should be designated or none of them should be when designated-initialising nested union members(?) (same code accepted as C)

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120498

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And "expected primary-expression before '.' token" because that syntax for
initializing a member of a subobject isn't valid at all in C++.

So you're not initializing 'sa.u' and that's why you get an error.