thread: Zombification is NORETURN when called from threads

core/include/thread.h

@@ -356,6 +356,9 @@ THREAD_MAYBE_INLINE void thread_yield_higher(void);
  *          but its scheduler entry and stack will be kept.
  *          A zombie state thread is supposed to be cleaned up
  *          by @ref thread_kill_zombie().
+ *
+ *          This function would be NORETURN, but its no-op behavior inside an
+ *          ISR precludes such an annotation.
  */
 void thread_zombify(void);
 

core/sched.c

@@ -320,6 +320,8 @@ NORETURN void sched_task_exit(void)
     sched_num_threads--;
 
     thread_zombify();
+    /* We're in a thread, so thread_zombify is really NORETURN. */
+    UNREACHABLE();
 }
 
 #ifdef MODULE_SCHED_CB