The proper way to avoid this seems to be using a function. Why can't you do that? Otherwise the only trick I can think of is something like s += strlen(s) where compound assignment guarantees that s is only evaluated once. But since s isn't necessarily an lvalue in this case, or even a read/write location that should be updated, that probably won't work.

The problem with the function is that you need a cast within the function (as it gets a const char * but returns a char *). I'd like to have zero casts in a program. I think I prefer having to manually keep the names unique (by adding more trailing underscores) and avoid the function.

With functions, it'd have to be

#define QVoidptrof(p)  typeof(1?(p):(void*){})
#define QCharptrof(s)  typeof                                         \
(                                                                     \
	_Generic((QVoidptrof(s)){},                                   \
		const void *: (const char *){},                       \
		void *:       (char *){}                              \
	)                                                             \
)

#define strnul(s)  ((void)0, (QCharptrof(s)){strnul(s)})
char *
(strnul)(const char *s)
{
    (char *) s + strlen(s);
}

alx‭ You don't need a cast, you can just pick char my_func() or const char my_func_const() with _Generic based on what was passed to the macro, much like QVoid/QChar will have to be implemented in C23.