Newer gccs produce a warning in

struct __attribute__((visibility("hidden"))) foo {
};
struct bar {
  foo x; // bar has default visibility, but foo is hidden
};
struct zed : public foo { // zed has default visibility, but foo is hidden
};

I think it is a reasonable warning because

* If the type has a hidden member or base, that member or base was created locally (in the same DSO), so the type could also be hidden.
* If there is something strange going on, the warning is really easy to avoid, just make the visibility of bar and zed explicit.