RC: suppress compiler warnings with GCC 12 and 13

We're pretty sure that these warnings are false positives.
Both are related to destructing MultiCompleteType objects.

GCC 12 (thread_unsafe_refcount):
rc.hpp:322:18: pointer used after ‘void operator delete(void*, std::size_t)’
[-Wuse-after-free]

GCC 13 (thread_safe_refcount, only arm64):
inlined from ‘...thread_safe_refcount::operator--()’ at .../rc.hpp:404:39
atomic_base.h:645:34: ‘long unsigned int __atomic_sub_fetch_8(...)’ writing
8 bytes into a region of size 0 overflows the destination
[-Werror=stringop-overflow=]

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>

openvpn/common/rc.hpp

@@ -304,6 +304,26 @@ class RCWeakPtr
     typename T::Controller::Ptr controller;
 };
 
+/* We're pretty sure these are false positives. They only occur with very
+   specific compiler versions and/or architectures.
+
+   For some reason some gcc versions think that the reference counter goes
+   away too soon when destructing certain MultiCompleteType objects. The
+   warnings/errors do not tell us why they think that.
+
+   So for now we display the warnings, but do not fail -Werror builds over
+   them. So that we can fail them for any other new warnings.
+ */
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic push
+#if __GNUC__ == 12
+#pragma GCC diagnostic warning "-Wuse-after-free"
+#endif
+#if __GNUC__ == 13
+#pragma GCC diagnostic warning "-Wstringop-overflow"
+#endif
+#endif
+
 class thread_unsafe_refcount
 {
   public:
@@ -479,6 +499,10 @@ class thread_safe_refcount
     std::atomic<olong> rc;
 };
 
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
+
 // Reference count base class for objects tracked by RCPtr.
 // Disallows copying and assignment.
 template <typename RCImpl> // RCImpl = thread_safe_refcount or thread_unsafe_refcount