Add means to handle negative capabilities in thread safety annotations

src/sync.h

@@ -103,6 +103,12 @@ public:
     }
 
     using UniqueLock = std::unique_lock<PARENT>;
+#ifdef __clang__
+    //! For negative capabilities in the Clang Thread Safety Analysis.
+    //! A negative requirement uses the EXCLUSIVE_LOCKS_REQUIRED attribute, in conjunction
+    //! with the ! operator, to indicate that a mutex should not be held.
+    const AnnotatedMixin& operator!() const { return *this; }
+#endif // __clang__
 };
 
 /**

src/threadsafety.h

@@ -60,6 +60,13 @@
 // and should only be used when sync.h Mutex/LOCK/etc are not usable.
 class LOCKABLE StdMutex : public std::mutex
 {
+public:
+#ifdef __clang__
+    //! For negative capabilities in the Clang Thread Safety Analysis.
+    //! A negative requirement uses the EXCLUSIVE_LOCKS_REQUIRED attribute, in conjunction
+    //! with the ! operator, to indicate that a mutex should not be held.
+    const StdMutex& operator!() const { return *this; }
+#endif // __clang__
 };
 
 // StdLockGuard provides an annotated version of std::lock_guard for us,