refactored proxy_erase to reduce template instantiations and strengthen misuse avoidance

include/boost/unordered/detail/foa/table.hpp

@@ -123,6 +123,7 @@ public:
 
 private:
   template<typename,typename,bool> friend class table_iterator;
+  template<typename> friend class table_erase_return_type;
   template<typename,typename,typename,typename> friend class table;
 
   table_iterator(Group* pg,std::size_t n,const table_element_type* p_):
@@ -198,6 +199,45 @@ private:
   table_element_type *p=nullptr;
 };
 
+/* Returned by table::erase([const_]iterator) to avoid iterator increment
+ * if discarded.
+ */
+
+template<typename Iterator>
+class table_erase_return_type; 
+
+template<typename TypePolicy,typename Group,bool Const>
+class table_erase_return_type<table_iterator<TypePolicy,Group,Const>>
+{
+  using iterator=table_iterator<TypePolicy,Group,Const>;
+  using const_iterator=table_iterator<TypePolicy,Group,true>;
+
+public:
+  /* can't delete it because VS in pre-C++17 mode needs to see it for RVO */
+  table_erase_return_type(const table_erase_return_type&);
+
+  operator iterator()const noexcept
+  {
+    auto it=pos;
+    it.increment(); /* valid even if *it was erased */
+    return iterator(const_iterator_cast_tag{},it);
+  }
+
+  template<
+    bool dependent_value=false,
+    typename std::enable_if<!Const||dependent_value>::type* =nullptr
+  >
+  operator const_iterator()const noexcept{return this->operator iterator();}
+
+private:
+  template<typename,typename,typename,typename> friend class table;
+
+  table_erase_return_type(const_iterator pos_):pos{pos_}{}
+  table_erase_return_type& operator=(const table_erase_return_type&)=delete;
+
+  const_iterator pos;
+};
+
 /* foa::table interface departs in a number of ways from that of C++ unordered
  * associative containers because it's not for end-user consumption
  * (boost::unordered_(flat|node)_(map|set) wrappers complete it as
@@ -271,6 +311,7 @@ public:
     has_mutable_iterator,
     table_iterator<type_policy,group_type,false>,
     const_iterator>::type;
+  using erase_return_type=table_erase_return_type<iterator>;
 
   table(
     std::size_t n=default_bucket_count,const Hash& h_=Hash(),
@@ -348,40 +389,19 @@ public:
   >::type
   insert(element_type&& x){return emplace_impl(std::move(x));}
 
-  struct proxy_erase {
-    const_iterator pos;
-    operator iterator()const noexcept
-    {
-      auto it=pos;
-      it.increment();
-      return iterator(const_iterator_cast_tag{},it);
-    }
-
   template<
     bool dependent_value=false,
     typename std::enable_if<
       has_mutable_iterator||dependent_value>::type* =nullptr
   >
-    operator const_iterator()const noexcept
+  erase_return_type erase(iterator pos)noexcept
-    {
+  {return erase(const_iterator(pos));}
-      auto it=pos;
-      it.increment();
-      return it;
-    }
-  };
-
-  template<
-    bool dependent_value=false,
-    typename std::enable_if<
-      has_mutable_iterator||dependent_value>::type* =nullptr
-  >
-  proxy_erase erase(iterator pos)noexcept{return erase(const_iterator(pos));}
 
   BOOST_FORCEINLINE
-  proxy_erase erase(const_iterator pos)noexcept
+  erase_return_type erase(const_iterator pos)noexcept
   {
     super::erase(pos.pc,pos.p);
-    return proxy_erase{pos};
+    return {pos};
   }
 
   template<typename Key>

include/boost/unordered/unordered_flat_map.hpp

@@ -381,12 +381,13 @@ namespace boost {
           .first;
       }
 
-      BOOST_FORCEINLINE typename table_type::proxy_erase erase(iterator pos)
+      BOOST_FORCEINLINE typename table_type::erase_return_type erase(
+        iterator pos)
       {
         return table_.erase(pos);
       }
 
-      BOOST_FORCEINLINE typename table_type::proxy_erase erase(
+      BOOST_FORCEINLINE typename table_type::erase_return_type erase(
         const_iterator pos)
       {
         return table_.erase(pos);

include/boost/unordered/unordered_flat_set.hpp

@@ -282,7 +282,7 @@ namespace boost {
         return table_.emplace(std::forward<Args>(args)...).first;
       }
 
-      BOOST_FORCEINLINE typename table_type::proxy_erase erase(
+      BOOST_FORCEINLINE typename table_type::erase_return_type erase(
         const_iterator pos)
       {
         return table_.erase(pos);

include/boost/unordered/unordered_node_map.hpp

@@ -458,12 +458,13 @@ namespace boost {
           .first;
       }
 
-      BOOST_FORCEINLINE typename table_type::proxy_erase erase(iterator pos)
+      BOOST_FORCEINLINE typename table_type::erase_return_type erase(
+        iterator pos)
       {
         return table_.erase(pos);
       }
 
-      BOOST_FORCEINLINE typename table_type::proxy_erase erase(
+      BOOST_FORCEINLINE typename table_type::erase_return_type erase(
         const_iterator pos)
       {
         return table_.erase(pos);

include/boost/unordered/unordered_node_set.hpp

@@ -352,7 +352,7 @@ namespace boost {
         return table_.emplace(std::forward<Args>(args)...).first;
       }
 
-      BOOST_FORCEINLINE typename table_type::proxy_erase erase(
+      BOOST_FORCEINLINE typename table_type::erase_return_type erase(
         const_iterator pos)
       {
         return table_.erase(pos);