> Yes, those are the cases where it isn't possible to avoid forward declarations, so it's fine to use them? But my point was that it _is_ possible to avoid them by type-punning via void*. Now, one can argue that it's unreasonably bad way to avoid them. But will a reviewer following this guide make the same assumption?

> exit runs global destructors, so if one thread calls exit while some other thread is still doing stuff, it might destroy an object that's in use, leading to explosions.

Or you can structure your code in a way that all your threads join prior to main exiting. Never call exit() or _exit() or even pthread_exit(). I think it's much cleaner. Not to mention that not every program is multi-threaded.

> It is very non-obvious which constructors will get automatically generated, because it depends on the types of the members. `Foo(Foo&& move) = default;` makes it immediately obvious.

Agreed but can you imagine what a simple

struct Point { int x, y; };

become if we were to make everything explicit?