https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125524
--- Comment #3 from Sam James <sjames at gcc dot gnu.org> --- Clang's requirements seem different. At https://clang.llvm.org/docs/AttributeReference.html#counted-by-counted-by-or-null-sized-by-sized-by-or-null, they give an example where the invariant is broken but it's okay as long as it's fixed up later (i.e. no atomicity needed).
