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diff --git a/indra/llcommon/llinstancetrackersubclass.h b/indra/llcommon/llinstancetrackersubclass.h
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+++ b/indra/llcommon/llinstancetrackersubclass.h
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+/**
+ * @file   llinstancetrackersubclass.h
+ * @author Nat Goodspeed
+ * @date   2022-12-09
+ * @brief  Intermediate class to get subclass-specific types from
+ *         LLInstanceTracker instance-retrieval methods.
+ *
+ * $LicenseInfo:firstyear=2022&license=viewerlgpl$
+ * Copyright (c) 2022, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_LLINSTANCETRACKERSUBCLASS_H)
+#define LL_LLINSTANCETRACKERSUBCLASS_H
+
+#include <memory>                   // std::shared_ptr, std::weak_ptr
+
+/**
+ * Derive your subclass S of a subclass T of LLInstanceTracker<T> from
+ * LLInstanceTrackerSubclass<S, T> to perform appropriate downcasting and
+ * filtering for LLInstanceTracker access methods.
+ *
+ * LLInstanceTracker<T> uses CRTP, so that getWeak(), getInstance(), snapshot
+ * and instance_snapshot return pointers and references to T. The trouble is
+ * that subclasses T0 and T1 derived from T also get pointers and references
+ * to their base class T, requiring explicit downcasting. Moreover,
+ * T0::getInstance() shouldn't find an instance of any T subclass other than
+ * T0. Nor should T0::snapshot.
+ *
+ * @code
+ * class Tracked: public LLInstanceTracker<Tracked, std::string>
+ * {
+ * private:
+ *     using super = LLInstanceTracker<Tracked, std::string>;
+ * public:
+ *     Tracked(const std::string& name): super(name) {}
+ *     // All references to Tracked::ptr_t, Tracked::getInstance() etc.
+ *     // appropriately use Tracked.
+ *     // ...
+ * };
+ *
+ * // But now we derive SubTracked from Tracked. We need SubTracked::ptr_t,
+ * // SubTracked::getInstance() etc. to use SubTracked, not Tracked.
+ * // This LLInstanceTrackerSubclass specialization is itself derived from
+ * // Tracked.
+ * class SubTracked: public LLInstanceTrackerSubclass<SubTracked, Tracked>
+ * {
+ * private:
+ *     using super = LLInstanceTrackerSubclass<SubTracked, Tracked>;
+ * public:
+ *     // LLInstanceTrackerSubclass's constructor forwards to Tracked's.
+ *     SubTracked(const std::string& name): super(name) {}
+ *     // SubTracked::getInstance() returns std::shared_ptr<SubTracked>, etc.
+ *     // ...
+ * @endcode
+ */
+template <typename SUBCLASS, typename T>
+class LLInstanceTrackerSubclass: public T
+{
+public:
+    using ptr_t  = std::shared_ptr<SUBCLASS>;
+    using weak_t = std::weak_ptr<SUBCLASS>;
+
+    // forward any constructor call to the corresponding T ctor
+    template <typename... ARGS>
+    LLInstanceTrackerSubclass(ARGS&&... args):
+        T(std::forward<ARGS>(args)...)
+    {}
+
+    weak_t getWeak()
+    {
+        // call base-class getWeak(), try to lock, downcast to SUBCLASS
+        return std::dynamic_pointer_cast<SUBCLASS>(T::getWeak().lock());
+    }
+
+    template <typename KEY>
+    static ptr_t getInstance(const KEY& k)
+    {
+        return std::dynamic_pointer_cast<SUBCLASS>(T::getInstance(k));
+    }
+
+    using snapshot          = typename T::template snapshot_of<SUBCLASS>;
+    using instance_snapshot = typename T::template instance_snapshot_of<SUBCLASS>;
+    using key_snapshot      = typename T::template key_snapshot_of<SUBCLASS>;
+
+    static size_t instanceCount()
+    {
+        // T::instanceCount() lies because our snapshot, et al., won't
+        // necessarily return all the T instances -- only those that are also
+        // SUBCLASS instances. Count those.
+        size_t count = 0;
+        for (const auto& pair : snapshot())
+            ++count;
+        return count;
+    }
+};
+
+#endif /* ! defined(LL_LLINSTANCETRACKERSUBCLASS_H) */