Type-checking container_of macro

It is an error to invoke the container_of() macro with a
`ptr` parameter that is not of type pointer-to-`member`.
But the macro hides that error by ignoring the 'ptr' type.

Make the macro safer by requiring `ptr` to have the
correct type.

Also, rename it to er_container_of(), to avoid potential
name collisions with other container_of() macros.

eventrouter/internal/defs.h

@@ -4,12 +4,16 @@
 #define ER_UNUSED(statement) (void)(statement);
 
 /// Returns the address of the structure containing the member.
-#ifndef container_of
-#define container_of(ptr, type, member)                    \
+
+/// The "?:" conditional operator requires its second and third operands to
+/// be of compatible type, so by writing "1 ? (ptr) : &((type *)0)->member"
+/// instead of simply writing "(ptr)", we ensure that `ptr` actually does
+/// point to a type compatible with `member` (compilation halts if it doesn't).
+
+#define er_container_of(ptr, type, member)                 \
     ({                                                     \
-        const typeof(((type *)0)->member) *__mptr = (ptr); \
+        void *__mptr = (1 ? (ptr) : &((type *)0)->member); \
         (type *)((char *)__mptr - offsetof(type, member)); \
     })
-#endif
 
 #endif /* EVENTROUTER_DEFS_H */

eventrouter/internal/event.h

@@ -89,7 +89,7 @@ extern "C"
     /// `a_event_p`. The developer must ensure that `a_type` is correct. This
     /// macro returns the surrounding struct by value instead of by reference.
 #define FROM_ER_EVENT(a_event_p, a_type) \
-    (*container_of(a_event_p, a_type, ER_EVENT_MEMBER))
+    (*er_container_of(a_event_p, a_type, ER_EVENT_MEMBER))
 
     /// Initialize the event fields of a struct which mixes-in `ErEvent_t`
     /// behavior. This differs from `ErEventInit_t` in that it can be used in

eventrouter/internal/eventrouter_baremetal.c

@@ -226,7 +226,7 @@ ErEvent_t *ErGetEventToDeliver(void)
     ErList_t *node = &s_context.m_events.m_deliver_now;
     if (node->m_next != NULL)
     {
-        ret          = container_of(node->m_next, ErEvent_t, m_next);
+        ret          = er_container_of(node->m_next, ErEvent_t, m_next);
         node->m_next = node->m_next->m_next;
         ret->m_next.m_next = NULL;
     }