diff --git a/applications/test/HashSet/Test-hashSet.C b/applications/test/HashSet/Test-hashSet.C
index c412b26523..b4bdd07cab 100644
--- a/applications/test/HashSet/Test-hashSet.C
+++ b/applications/test/HashSet/Test-hashSet.C
@@ -73,6 +73,22 @@ void printMinMax(const HashSet<Key, Hash>& set)
 }
 
 
+template<class Key, class Hash>
+void printHashSet(const HashSet<Key, Hash>& table)
+{
+    Info<< table.size() << '(' << nl;
+
+    for (const auto& key : table.sortedToc())
+    {
+        const auto iter = table.find(key);
+
+        Info<< "    " << key << " : " << Foam::name(&(iter.key())) << nl;
+    }
+
+    Info<< ')' << nl;
+}
+
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 // Main program:
 
@@ -84,33 +100,33 @@ int main(int argc, char *argv[])
         << typeid(HashSet<label>::hasher).name() << nl << nl;
 
     hashedWordList words
-    {
+    ({
         "abc",
         "def",
         "ghi"
-    };
+    });
     words = { "def", "ghi", "xy", "all", "end", "all" };
 
     wordHashSet setA
-    {
+    ({
         "xx",
         "yy",
         "zz"
-    };
+    });
 
     setA = { "kjhk", "kjhk2", "abced" };
 
     HashTable<label> tableA
-    {
+    ({
         { "value1", 1 },
         { "value2", 2 },
         { "value3", 3 }
-    };
+    });
 
     HashTable<nil> tableB;
-    tableB.insert("value4", nil());
-    tableB.insert("value5", nil());
-    tableB.insert("value6", nil());
+    tableB.emplace("value4");
+    tableB.emplace("value5");
+    tableB.emplace("value6");
 
     Info<< "tableA keys: "; tableA.writeKeys(Info) << endl;
 
@@ -123,11 +139,11 @@ int main(int argc, char *argv[])
     }
 
     Map<label> mapA
-    {
+    ({
         { 1, 1 },
         { 2, 2 },
         { 3, 3 }
-    };
+    });
     mapA.set(4, 4);
 
     Info<< "hashedWordList: " << words << nl
@@ -185,9 +201,9 @@ int main(int argc, char *argv[])
     }
 
     labelHashSet setB
-    {
+    ({
         1, 11, 42
-    };
+    });
 
     Info<<"Set with min/max:" << minMax(setB)
         << " min:" << min(setB) << " max:" << max(setB) << nl;
@@ -309,6 +325,26 @@ int main(int argc, char *argv[])
     Info<< "setA1: " << setA1 << nl
         << "setB1: " << setB1 << nl;
 
+
+    // Merging
+    {
+        wordHashSet set0({ "abc", "kjhk", "kjhk2" });
+        wordHashSet set1({ "abc", "def", "ghi", "jkl" });
+
+        Info<< nl
+            << "Set0" << nl;
+        printHashSet(set0);
+        Info<< "Set1" << nl;
+        printHashSet(set1);
+
+        set1.merge(set0);
+
+        Info<< "merged 0" << nl;
+        printHashSet(set0);
+        Info<< "merged 1" << nl;
+        printHashSet(set1);
+    }
+
     return 0;
 }
 
diff --git a/applications/test/HashTable2/Test-HashTable2.C b/applications/test/HashTable2/Test-HashTable2.C
index 010251f807..19406361fc 100644
--- a/applications/test/HashTable2/Test-HashTable2.C
+++ b/applications/test/HashTable2/Test-HashTable2.C
@@ -79,26 +79,56 @@ public:
 };
 
 
+template<class T, class Key, class Hash>
+void printTable(const HashPtrTable<T, Key, Hash>& table)
+{
+    Info<< table.size() << '(' << nl;
+
+    for (const auto& key : table.sortedToc())
+    {
+        const auto iter = table.find(key);
+
+        Info
+            << "    " << iter.key() << " (" << Foam::name(&(iter.key()))
+            << ") : ";
+
+        if (iter.val())
+        {
+            Info<< *(iter.val());
+        }
+        else
+        {
+            Info<< "nullptr";
+        }
+
+        Info<< " (" << Foam::name(iter.val()) << ")" << nl;;
+    }
+
+    Info<< ')' << nl;
+}
+
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 // Main program:
 
 int main(int argc, char *argv[])
 {
-    HashTable<label, Foam::string> table1
-    {
+    HashTable<Label, Foam::string> table0
+    ({
+        {"abc", 123},
         {"kjhk", 10},
         {"kjhk2", 12}
-    };
+    });
 
-    Info<< "table1: " << table1 << nl
-        << "toc: " << flatOutput(table1.toc()) << nl;
+    Info<< "table0: " << table0 << nl
+        << "toc: " << flatOutput(table0.toc()) << nl;
 
     HashTable<label, label, Hash<label>> table2
-    {
+    ({
         {3, 10},
         {5, 12},
         {7, 16}
-    };
+    });
 
     Info<< "table2: " << table2 << nl
         << "toc: " << flatOutput(table2.toc()) << nl;
@@ -127,7 +157,7 @@ int main(int argc, char *argv[])
         table1.insert("ghi", 15);
         table1.insert("jkl", 20);
 
-        Info<< nl << "Table toc: " << flatOutput(table1.toc()) << nl;
+        Info<< nl << "Table toc: " << flatOutput(table1.sortedToc()) << nl;
 
         for (const word k : { "abc" })
         {
@@ -153,16 +183,30 @@ int main(int argc, char *argv[])
                     ;
             }
         }
+
+        Info<< nl
+            << "Table0: " << flatOutput(table0.sortedToc()) << nl
+            << "Table1: " << flatOutput(table1.sortedToc()) << nl;
+
+        table1.merge(table0);
+
+        Info<< "merged 0: " << flatOutput(table0.sortedToc()) << nl
+            << "merged 1: " << flatOutput(table1.sortedToc()) << nl;
     }
 
     {
+        HashPtrTable<Label> ptable0(0);
+        ptable0.emplace("abc", 123),
+        ptable0.emplace("kjhk", 10);
+        ptable0.emplace("kjhk2", 12);
+
         HashPtrTable<Label> ptable1(0);
         ptable1.insert("abc", autoPtr<Label>::New(5));
-        ptable1.insert("def", autoPtr<Label>::New(10));
-        ptable1.insert("ghi", autoPtr<Label>::New(15));
-        ptable1.insert("jkl", autoPtr<Label>::New(20));
+        ptable1.emplace("def", 10);
+        ptable1.emplace("ghi", 15);
+        ptable1.emplace("jkl", 20);
 
-        Info<< nl << "PtrTable toc: " << flatOutput(ptable1.toc()) << nl;
+        Info<< nl << "PtrTable toc: " << flatOutput(ptable1.sortedToc()) << nl;
 
         for (const word k : { "abc" })
         {
@@ -242,6 +286,20 @@ int main(int argc, char *argv[])
             }
         }
 
+        Info<< nl
+            << "Table0" << nl;
+        printTable(ptable0);
+
+        Info<< "Table1" << nl;
+        printTable(ptable1);
+
+        ptable1.merge(ptable0);
+
+        Info<< "merged 0" << nl;
+        printTable(ptable0);
+        Info<< "merged 1" << nl;
+        printTable(ptable1);
+
         Info<< nl << "Ending scope" << nl;
     }
 
@@ -277,6 +335,28 @@ int main(int argc, char *argv[])
             Info<< "got with " << (*iter).size() << nl;
         }
 
+        Info<< nl
+            << "Test (DIY) insert_or_assign" << nl;
+
+        label nKeys = 0;
+
+        for (const auto& key : { "abc", "foo", "mno", "xyz" })
+        {
+            Info<< key;
+            if (ltable1.contains(key))
+            {
+                Info<< " : " << ltable1[key];
+            }
+            else
+            {
+                Info<< " : n/a";
+            }
+
+            /// ltable1.insert_or_assign(key, identity(++nKeys));
+            ltable1(key) = identity(++nKeys);
+
+            Info<< "  --> " << ltable1[key] << nl;
+        }
     }
 
     Info<< "\nEnd\n" << endl;
diff --git a/applications/test/IOobjectList/Test-IOobjectList.C b/applications/test/IOobjectList/Test-IOobjectList.C
index 2e1e261a2f..5a74e1adfd 100644
--- a/applications/test/IOobjectList/Test-IOobjectList.C
+++ b/applications/test/IOobjectList/Test-IOobjectList.C
@@ -334,7 +334,7 @@ int main(int argc, char *argv[])
             Info<< "==target==" << nl; reportDetail(objects);
             Info<< "==source==" << nl; reportDetail(other);
 
-            objects.merge(std::move(other));
+            objects.merge(other);
             Info<< nl << "After merge" << nl;
 
             Info<< "==target==" << nl; reportDetail(objects);
diff --git a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C
index f9b7c10fb4..10d7fad18c 100644
--- a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C
+++ b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2021 OpenCFD Ltd.
+    Copyright (C) 2017-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -150,6 +150,30 @@ void Foam::HashPtrTable<T, Key, Hash>::clear()
 }
 
 
+template<class T, class Key, class Hash>
+void Foam::HashPtrTable<T, Key, Hash>::merge
+(
+    HashPtrTable<T, Key, Hash>& source
+)
+{
+    // Use parent merge (of raw pointer entries)
+    // and by-pass any pointer deletions etc.
+    parent_type::merge(static_cast<parent_type&>(source));
+}
+
+
+template<class T, class Key, class Hash>
+void Foam::HashPtrTable<T, Key, Hash>::merge
+(
+    HashPtrTable<T, Key, Hash>&& source
+)
+{
+    // Use parent merge (of raw pointer entries)
+    // and by-pass any pointer deletions etc.
+    parent_type::merge(static_cast<parent_type&>(source));
+}
+
+
 // * * * * * * * * * * * * * * * Member Operators  * * * * * * * * * * * * * //
 
 template<class T, class Key, class Hash>
diff --git a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H
index 765c34b89e..39d2fbff68 100644
--- a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H
+++ b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2022 OpenCFD Ltd.
+    Copyright (C) 2017-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -173,6 +173,16 @@ public:
         //- Clear all entries from table and delete any allocated pointers
         void clear();
 
+        //- Attempts to extract entries from source parameter and insert them
+        //- into \c this, does not overwrite existing entries.
+        //- The source will contains any items that could not be merged.
+        void merge(HashPtrTable<T, Key, Hash>& source);
+
+        //- Attempts to extract entries from source parameter and insert them
+        //- into \c this, does not overwrite existing entries.
+        //- The source will contains any items that could not be merged.
+        void merge(HashPtrTable<T, Key, Hash>&& source);
+
 
     // Reading/writing
 
diff --git a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableI.H b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableI.H
index f207bbceac..2d3f8a4921 100644
--- a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableI.H
+++ b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableI.H
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2022 OpenCFD Ltd.
+    Copyright (C) 2018-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
diff --git a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
index 02d2390f66..7457fc6b30 100644
--- a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
+++ b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -232,6 +232,20 @@ inline Foam::label Foam::HashSet<Key, Hash>::unset
 }
 
 
+template<class Key, class Hash>
+void Foam::HashSet<Key, Hash>::merge(HashSet<Key, Hash>& source)
+{
+    parent_type::merge(source);
+}
+
+
+template<class Key, class Hash>
+void Foam::HashSet<Key, Hash>::merge(HashSet<Key, Hash>&& source)
+{
+    parent_type::merge(source);
+}
+
+
 // * * * * * * * * * * * * * * * Member Operators  * * * * * * * * * * * * * //
 
 template<class Key, class Hash>
diff --git a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
index 141fad2696..2288d57ead 100644
--- a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
+++ b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
@@ -197,7 +197,7 @@ public:
         //- Same as insert (no value to overwrite)
         bool set(const Key& key)
         {
-            return insert(key);
+            return this->parent_type::emplace(key);
         }
 
         //- Unset the specified key - same as erase
@@ -207,6 +207,16 @@ public:
             return this->parent_type::erase(key);
         }
 
+        //- Attempts to extract entries from source parameter and insert them
+        //- into \c this, does not overwrite existing entries.
+        //- The source will contains any items that could not be merged.
+        void merge(HashSet<Key, Hash>& source);
+
+        //- Attempts to extract entries from source parameter and insert them
+        //- into \c this, does not overwrite existing entries.
+        //- The source will contains any items that could not be merged.
+        void merge(HashSet<Key, Hash>&& source);
+
 
     // Convenience
 
@@ -370,7 +380,7 @@ public:
         //- Add entries to this HashSet
         this_type& operator|=(const this_type& rhs);
 
-        //- Only retain entries found in both HashSets
+        //- Only retain entries contained in both HashSets
         inline this_type& operator&=(const this_type& rhs);
 
         //- Only retain unique entries (xor)
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.C b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.C
index 94d91e5d80..e0cface7c1 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.C
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2022 OpenCFD Ltd.
+    Copyright (C) 2017-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -359,7 +359,8 @@ bool Foam::HashTable<T, Key, Hash>::setEntry
 {
     if (!capacity_)
     {
-        resize(2);
+        // Same as default sizing
+        resize(128);
     }
 
     const label index = hashKeyIndex(key);
@@ -384,13 +385,9 @@ bool Foam::HashTable<T, Key, Hash>::setEntry
             new node_type(table_[index], key, std::forward<Args>(args)...);
 
         ++size_;
-        if (double(size_)/capacity_ > 0.8 && capacity_ < maxTableSize)
+        if (0.8*capacity_ < size_)  // Resize after 80% fill factor
         {
-            #ifdef FULLDEBUG
-            DebugInFunction << "Doubling table size\n";
-            #endif
-
-            resize(2*capacity_);
+            if (capacity_ < maxTableSize) resize(2*capacity_);
         }
     }
     else if (overwrite)
@@ -425,10 +422,7 @@ bool Foam::HashTable<T, Key, Hash>::setEntry
     }
     else
     {
-        // Do not overwrite existing entry (STL 'insert' convention)
-        #ifdef FULLDEBUG
-        DebugInFunction << "Not inserting " << key << ": already in table\n";
-        #endif
+        // Not overwriting existing entry
         return false;
     }
 
@@ -436,6 +430,52 @@ bool Foam::HashTable<T, Key, Hash>::setEntry
 }
 
 
+template<class T, class Key, class Hash>
+void Foam::HashTable<T, Key, Hash>::insert_node(node_type* entry)
+{
+    if (!entry) return;
+
+    if (!capacity_)
+    {
+        // Same as default sizing
+        resize(128);
+    }
+
+    const label index = hashKeyIndex(entry->key());
+
+    node_type* curr = nullptr;
+    //node_type* prev = nullptr;
+
+    for (node_type* ep = table_[index]; ep; ep = ep->next_)
+    {
+        if (entry->key() == ep->key())
+        {
+            curr = ep;
+            break;
+        }
+        //prev = ep;
+    }
+
+    if (!curr)
+    {
+        // Not found, insert it at the head
+        table_[index] = entry;
+
+        ++size_;
+        if (0.8*capacity_ < size_)  // Resize after 80% fill factor
+        {
+            if (capacity_ < maxTableSize) resize(2*capacity_);
+        }
+    }
+    else
+    {
+        FatalErrorInFunction
+            << "Not inserting " << entry->key() << ": already in table\n"
+            << exit(FatalError);
+    }
+}
+
+
 template<class T, class Key, class Hash>
 bool Foam::HashTable<T, Key, Hash>::erase(const iterator& iter)
 {
@@ -444,9 +484,7 @@ bool Foam::HashTable<T, Key, Hash>::erase(const iterator& iter)
     // The parameter should be (iterator&), but then the compiler doesn't find
     // it correctly and tries to call as (iterator) instead.
 
-    iterator& it = const_cast<iterator&>(iter);
-
-    return iterator_erase(it.entry_, it.index_);
+    return iterator_erase(const_cast<iterator&>(iter));
 }
 
 
@@ -454,7 +492,7 @@ template<class T, class Key, class Hash>
 bool Foam::HashTable<T, Key, Hash>::erase(const Key& key)
 {
     iterator iter(find(key));
-    return erase(iter);
+    return iterator_erase(iter);
 }
 
 
@@ -605,10 +643,6 @@ void Foam::HashTable<T, Key, Hash>::resize(const label sz)
 
     if (newCapacity == oldCapacity)
     {
-        #ifdef FULLDEBUG
-        DebugInFunction << "New table size == old table size\n";
-        #endif
-
         return;
     }
     else if (!newCapacity)
@@ -617,7 +651,8 @@ void Foam::HashTable<T, Key, Hash>::resize(const label sz)
         if (size_)
         {
             WarningInFunction
-                << "HashTable contains " << size_ << " cannot resize(0)" << nl;
+                << "HashTable contains " << size_
+                << " elements, cannot resize(0)" << nl;
         }
         else
         {
@@ -646,8 +681,7 @@ void Foam::HashTable<T, Key, Hash>::resize(const label sz)
 
     // Move to new table[] but with new chaining.
 
-    label nMove = size_;  // Allow early completion
-    for (label i=0; nMove && i < oldCapacity; ++i)
+    for (label i = 0, nPending = size_; nPending && i < oldCapacity; ++i)
     {
         for (node_type* ep = oldTable[i]; ep; /*nil*/)
         {
@@ -661,8 +695,8 @@ void Foam::HashTable<T, Key, Hash>::resize(const label sz)
                 table_[newIdx] = ep;
             }
 
-            ep = next;  // continue in the linked-list
-            --nMove;    // note any early completion
+            ep = next;      // continue in the linked-list
+            --nPending;     // note any early completion
         }
         oldTable[i] = nullptr;
     }
@@ -809,6 +843,46 @@ Foam::label Foam::HashTable<T, Key, Hash>::filterEntries
 }
 
 
+template<class T, class Key, class Hash>
+void Foam::HashTable<T, Key, Hash>::merge(HashTable<T, Key, Hash>& source)
+{
+    // Self-merge implicitly a no-op
+
+    if (node_type::stores_value())
+    {
+        // key/val pair
+        for (iterator iter = source.begin(); iter != source.end(); ++iter)
+        {
+            if (!contains(iter.key()))
+            {
+                node_type* entry = source.iterator_extract(iter);
+                this->insert_node(entry);
+            }
+        }
+    }
+    else
+    {
+        // key only, does not store a value.
+        // Since the key is const, juggling the node does not work
+
+        for (iterator iter = source.begin(); iter != source.end(); ++iter)
+        {
+            if (emplace(iter.key()))
+            {
+                source.erase(iter);
+            }
+        }
+    }
+}
+
+
+template<class T, class Key, class Hash>
+void Foam::HashTable<T, Key, Hash>::merge(HashTable<T, Key, Hash>&& source)
+{
+    merge(source);
+}
+
+
 // * * * * * * * * * * * * * * * Member Operators  * * * * * * * * * * * * * //
 
 template<class T, class Key, class Hash>
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
index dcec3f00fa..7209f093a6 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
@@ -206,6 +206,17 @@ private:
         template<class... Args>
         bool setEntry(const bool overwrite, const Key& key, Args&&... args);
 
+        //- Insert node (low-level). The node must not previously exist!
+        void insert_node(node_type* entry);
+
+        //- Low-level entry erasure using iterator internals.
+        //  This invalidates the iterator until the next ++ operation.
+        //  \return True if the corresponding entry existed and was removed
+        bool iterator_erase(iterator& iter);
+
+        //- Unlink node from iterator (low-level).
+        node_type* iterator_extract(iterator& iter);
+
         //- Read hash table
         Istream& readTable(Istream& is);
 
@@ -532,6 +543,16 @@ public:
         //- Transfer contents into this table.
         void transfer(HashTable<T, Key, Hash>& rhs);
 
+        //- Attempts to extract entries from source parameter and insert them
+        //- into \c this, does not overwrite existing entries.
+        //- The source will contains any items that could not be merged.
+        void merge(HashTable<T, Key, Hash>& source);
+
+        //- Attempts to extract entries from source parameter and insert them
+        //- into \c this, does not overwrite existing entries.
+        //- The source will contains any items that could not be merged.
+        void merge(HashTable<T, Key, Hash>&& source);
+
 
     // Member Operators
 
@@ -714,11 +735,6 @@ protected:
             }
         };
 
-        //- Low-level entry erasure using iterator internals.
-        //  This invalidates the iterator until the next ++ operation.
-        //  \return True if the corresponding entry existed and was removed
-        bool iterator_erase(node_type*& entry, label& index);
-
 public:
 
         //- Forward iterator with non-const access
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H
index 4fa30d87f0..4cf990347e 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H
@@ -198,6 +198,32 @@ inline bool Foam::HashTable<T, Key, Hash>::set
 }
 
 
+// FUTURE?
+// If 'key' already exists, assign \c std::forward<T>(obj) to the
+// entry. If it does not exist, uses insert()
+//
+// template<class T, class Key, class Hash>
+// inline void Foam::HashTable<T, Key, Hash>::insert_or_assign
+// (
+//     const Key& key,
+//     T&& obj
+// )
+// {
+//     iterator iter(this->find(key));
+//
+//     if (iter.good())
+//     {
+//         iter.val() = std::forward<T>(obj);
+//     }
+//     else
+//     {
+//         this->setEntry(true, key, std::forward<T>(obj));
+//         // iter = this->find(key);
+//     }
+//     // return iter.val();
+// }
+
+
 template<class T, class Key, class Hash>
 inline const T& Foam::HashTable<T, Key, Hash>::lookup
 (
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIter.C b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIter.C
index 33708cfa00..027696b373 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIter.C
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIter.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2017 OpenCFD Ltd.
+    Copyright (C) 2017-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -39,7 +39,7 @@ Foam::HashTable<T, Key, Hash>::Iterator<Const>::Iterator
     container_(tbl),
     index_(0)
 {
-    if (tbl->size())
+    if (container_ && container_->size())
     {
         const label index = container_->hashKeyIndex(key);
 
@@ -62,12 +62,11 @@ Foam::HashTable<T, Key, Hash>::Iterator<Const>::Iterator
 // Any changes here may need changes in the iterator increment() method
 //
 template<class T, class Key, class Hash>
-bool Foam::HashTable<T, Key, Hash>::iterator_erase
-(
-    node_type*& entry,
-    label& index
-)
+bool Foam::HashTable<T, Key, Hash>::iterator_erase(iterator& iter)
 {
+    node_type* entry = iter.entry_;
+    const label index = iter.index_;
+
     // Safeguard against the following:
     // - empty table
     // - nullptr entry
@@ -98,8 +97,8 @@ bool Foam::HashTable<T, Key, Hash>::iterator_erase
         // Had previous element in linked list - reposition to there
         prev->next_ = entry->next_;
         delete entry;
-        entry = prev;
 
+        iter.entry_ = prev;
         return true;
     }
 
@@ -108,7 +107,7 @@ bool Foam::HashTable<T, Key, Hash>::iterator_erase
     delete entry;
 
     // Assign any non-nullptr value so it doesn't look like end()
-    entry = reinterpret_cast<node_type*>(this);
+    iter.entry_ = reinterpret_cast<node_type*>(this);
 
     // Mark the present index to continue and bring it back to the present
     // location with the next index.
@@ -116,10 +115,72 @@ bool Foam::HashTable<T, Key, Hash>::iterator_erase
     // Save: (-index-1), which has no ambiguity for index 0.
     // Retrieve:  (-(index+1))
 
-    index = (-index - 1);
+    iter.index_ = (-index - 1);
 
     return true;
 }
 
 
+//
+// Any changes here may need changes in the iterator increment() method
+//
+template<class T, class Key, class Hash>
+typename Foam::HashTable<T, Key, Hash>::node_type*
+Foam::HashTable<T, Key, Hash>::iterator_extract(iterator& iter)
+{
+    node_type* entry = iter.entry_;
+    const label index = iter.index_;
+
+    // Safeguard against the following:
+    // - empty table
+    // - nullptr entry
+    // - end iterator (which is also a nullptr)
+    // - negative index from a previous erase. See comment below.
+    if (!size_ || !entry || index < 0)
+    {
+        return nullptr;
+    }
+
+    // Decrease count
+    --size_;
+
+    // The previous element in the singly linked list
+    node_type* prev = nullptr;
+
+    for (node_type* ep = table_[index]; ep; ep = ep->next_)
+    {
+        if (ep == entry)
+        {
+            break;
+        }
+        prev = ep;
+    }
+
+    if (prev)
+    {
+        // Had previous element in linked list - reposition iterator to there
+        prev->next_ = entry->next_;
+        iter.entry_ = prev;
+
+        entry->next_ = nullptr;
+        return entry;
+    }
+
+    // Was first element on linked list
+    table_[index] = entry->next_;
+    iter.entry_ = table_[index];
+    entry->next_ = nullptr;
+
+    // Mark the present index to continue and bring it back to the present
+    // location with the next index.
+    //
+    // Save: (-index-1), which has no ambiguity for index 0.
+    // Retrieve:  (-(index+1))
+
+    iter.index_ = (-index - 1);
+
+    return entry;
+}
+
+
 // ************************************************************************* //
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIterI.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIterI.H
index ab36301c5f..9139b0ad78 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIterI.H
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIterI.H
@@ -70,7 +70,8 @@ inline Foam::HashTable<T, Key, Hash>::Iterator<Const>::Iterator
 
 
 //
-// Any changes here may need changes in iterator_erase() method too
+// Any changes here may need changes in iterator_erase(), iterator_extract()
+// methods too
 //
 template<class T, class Key, class Hash>
 template<bool Const>
diff --git a/src/OpenFOAM/db/IOobjectList/IOobjectList.C b/src/OpenFOAM/db/IOobjectList/IOobjectList.C
index 05729db4fb..9cf21f9022 100644
--- a/src/OpenFOAM/db/IOobjectList/IOobjectList.C
+++ b/src/OpenFOAM/db/IOobjectList/IOobjectList.C
@@ -152,36 +152,6 @@ Foam::IOobjectList::IOobjectList
 }
 
 
-// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
-
-Foam::label Foam::IOobjectList::merge(IOobjectList&& other)
-{
-    // Remove by name to avoid uncertainties about invalid iterators
-
-    label count = 0;
-
-    wordList keys(other.toc());
-
-    for (const word& key : keys)
-    {
-        if (!found(key))
-        {
-            if (IOobject::debug)
-            {
-                InfoInFunction << "Merge " << key << nl;
-            }
-
-            if (add(other.remove(key)))
-            {
-                ++count;
-            }
-        }
-    }
-
-    return count;
-}
-
-
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 const Foam::IOobject* Foam::IOobjectList::cfindObject
diff --git a/src/OpenFOAM/db/IOobjectList/IOobjectList.H b/src/OpenFOAM/db/IOobjectList/IOobjectList.H
index 91d02f54a4..a108b17687 100644
--- a/src/OpenFOAM/db/IOobjectList/IOobjectList.H
+++ b/src/OpenFOAM/db/IOobjectList/IOobjectList.H
@@ -224,19 +224,11 @@ public:
 
     // Basic methods
 
-        //- Move insert IOobject into the list
-        inline bool add(autoPtr<IOobject>&& objectPtr);
-
         //- Move insert IOobject into the list
         inline bool add(autoPtr<IOobject>& objectPtr);
 
-        //- Add objects from other to this list without overwriting
-        //- existing keys.
-        //  After calling this, the other parameter will contains any items
-        //  that could not be merged.
-        //
-        //  \return number of items merged
-        label merge(IOobjectList&& other);
+        //- Move insert IOobject into the list
+        inline bool add(autoPtr<IOobject>&& objectPtr);
 
         //- Remove object from the list, by name or by iterator.
         //
@@ -362,7 +354,7 @@ public:
         //  word checkType = volScalarField::typeName;
         //
         //  Info<< checkType << "="
-        //      << (classes.found(checkType) ? classes[checkType].size() : 0)
+        //      << (classes.contains(checkType) ? classes[checkType].size() : 0)
         //      << nl;
         //  \endcode
         //  Using the two-parameter HashTable::lookup method lets us avoid
diff --git a/src/OpenFOAM/db/IOobjectList/IOobjectListI.H b/src/OpenFOAM/db/IOobjectList/IOobjectListI.H
index 31f3fe6f1d..e9f90e6361 100644
--- a/src/OpenFOAM/db/IOobjectList/IOobjectListI.H
+++ b/src/OpenFOAM/db/IOobjectList/IOobjectListI.H
@@ -109,7 +109,7 @@ inline Foam::IOobjectList::IOobjectList
 
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
-inline bool Foam::IOobjectList::add(autoPtr<IOobject>&& objectPtr)
+inline bool Foam::IOobjectList::add(autoPtr<IOobject>& objectPtr)
 {
     if (objectPtr)
     {
@@ -120,7 +120,7 @@ inline bool Foam::IOobjectList::add(autoPtr<IOobject>&& objectPtr)
 }
 
 
-inline bool Foam::IOobjectList::add(autoPtr<IOobject>& objectPtr)
+inline bool Foam::IOobjectList::add(autoPtr<IOobject>&& objectPtr)
 {
     if (objectPtr)
     {