New upstream version 6.0.20upstream

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diff --git a/src/mongo/db/query/util/deferred.h b/src/mongo/db/query/util/deferred.h
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+/**
+ *    Copyright (C) 2023-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include <functional>
+
+namespace mongo {
+
+/**
+ * A template class that provides a way to defer the initialization of an object until its value is
+ * actually required. This is also commonly referred to as lazy initialization.
+ *
+ * Dangers:
+ * - This implementation is currently not thread safe, and it shouldn't be used in multi-threaded
+ *   fashion.
+ * - Be careful about using this for lazy initialization of data members and capturing the 'this'
+ *   variable. Code like this will result in buggy/unsafe move constructors, which would have a
+ *   dangling reference to the moved-from type:
+ *
+ *   class MyType {
+ *       int x;
+ *       // !!! Dangling 'this' when moved !!!
+ *       Deferred<int> xSquared{[this]() { return this->x * this-> x; };
+ *   };
+ *   Instead, it is better to do something like this:
+ *   class MyType {
+ *       int xSquared() const {
+ *          return *_xSquared.get(_x);
+ *       }
+ *
+ *       int _x;
+ *       Deferred<int, int> _xSquared{[](int x) { return x * x; };
+ *   };
+ * - As a similar danger, the value is only computed once. if you initialize it with arguments like
+ *   the above 'xSquared()' implementation, then be cogniscent that the value will never change. If
+ *   '_x' changes, '_xSquared' will not.
+ *
+ * A Deferred class can be constructed with either an initial value (eager initialization) or a
+ * function that will generate the value when needed.
+ */
+template <typename T, typename... Args>
+class Deferred {
+public:
+    /**
+     * Instantiates a Deffered<T> with the given data - no callbacks or lazy initialization.
+     */
+    Deferred(T data) : _data(data) {}
+
+    /**
+     * Stores a function to compute a T later. Please note the warnings described in this class
+     * comment.
+     */
+    Deferred(std::function<T(Args&&...)> initializer) : _initializer(std::move(initializer)) {}
+
+    /**
+     * Returns a pointer to the managed object. Initializes the object if it hasn't done so already.
+     */
+    T& get(Args&&... args) const {
+        if (_initializer) {
+            _data = _initializer(std::forward<Args>(args)...);
+            _initializer = nullptr;
+        }
+        return _data;
+    }
+
+    /**
+     * Dereferences the pointer to the managed object. Note this is only a valid shortcut if there
+     * are no arguments to '_initializer'.
+     */
+    T* operator->() const {
+        return &get();
+    }
+
+    /**
+     * Returns a referenced to the managed object. Initializes the object if it hasn't done so
+     * already. Note this is only a valid shortcut if there are no arguments to '_initializer'.
+     */
+    const T& operator*() const {
+        return get();
+    }
+
+    bool isInitialized() const {
+        return _initializer ? false : true;
+    }
+
+private:
+    mutable T _data;
+    mutable std::function<T(Args&&...)> _initializer;
+};
+
+}  // namespace mongo