Ensure each container type can be explicitly instantiated

include/boost/unordered/concurrent_flat_map.hpp

@@ -190,6 +190,7 @@ namespace boost {
       }
 
 
+      template <bool avoid_explicit_instantiation = true>
       concurrent_flat_map(
         unordered_flat_map<Key, T, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))

include/boost/unordered/concurrent_flat_set.hpp

@@ -187,6 +187,7 @@ namespace boost {
       }
 
 
+      template <bool avoid_explicit_instantiation = true>
       concurrent_flat_set(
         unordered_flat_set<Key, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))

include/boost/unordered/concurrent_node_map.hpp

@@ -197,6 +197,7 @@ namespace boost {
       {
       }
 
+      template <bool avoid_explicit_instantiation = true>
       concurrent_node_map(
         unordered_node_map<Key, T, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))

include/boost/unordered/concurrent_node_set.hpp

@@ -194,6 +194,7 @@ namespace boost {
       {
       }
 
+      template <bool avoid_explicit_instantiation = true>
       concurrent_node_set(
         unordered_node_set<Key, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))

include/boost/unordered/unordered_flat_map.hpp

@@ -182,6 +182,7 @@ namespace boost {
       {
       }
 
+      template <bool avoid_explicit_instantiation = true>
       unordered_flat_map(
         concurrent_flat_map<Key, T, Hash, KeyEqual, Allocator>&& other)
           : table_(std::move(other.table_))

include/boost/unordered/unordered_flat_set.hpp

@@ -178,6 +178,7 @@ namespace boost {
       {
       }
 
+      template <bool avoid_explicit_instantiation = true>
       unordered_flat_set(
         concurrent_flat_set<Key, Hash, KeyEqual, Allocator>&& other)
           : table_(std::move(other.table_))

include/boost/unordered/unordered_node_map.hpp

@@ -189,6 +189,7 @@ namespace boost {
       {
       }
 
+      template <bool avoid_explicit_instantiation = true>
       unordered_node_map(
         concurrent_node_map<Key, T, Hash, KeyEqual, Allocator>&& other)
           : table_(std::move(other.table_))

include/boost/unordered/unordered_node_set.hpp

@@ -187,6 +187,7 @@ namespace boost {
       {
       }
 
+      template <bool avoid_explicit_instantiation = true>
       unordered_node_set(
         concurrent_node_set<Key, Hash, KeyEqual, Allocator>&& other)
           : table_(std::move(other.table_))