From 177299977da63f69483f462da5d0ada7f0e521db Mon Sep 17 00:00:00 2001
From: John Wellbelove <jwellbelove@users.noreply.github.com>
Date: Sun, 11 Jun 2023 16:23:43 +0100
Subject: [PATCH] Refactored visitor and variant to allow const variants and
 fix passing references to visitors

---
 .gitignore                             |   1 +
 include/etl/private/variant_variadic.h | 153 +++++------
 include/etl/visitor.h                  | 335 +++++++++++++------------
 test/test_variant_variadic.cpp         |  66 +++--
 test/test_visitor.cpp                  | 292 +++++++++++----------
 5 files changed, 456 insertions(+), 391 deletions(-)

diff --git a/.gitignore b/.gitignore
index f96cf41c..4eda7252 100644
--- a/.gitignore
+++ b/.gitignore
@@ -376,3 +376,4 @@ test/syntax_check/c++20/bclang
 test/syntax_check/c++11/bgcc
 test/syntax_check/c++17/bgcc
 test/syntax_check/c++20/bgcc
+test/vs2022/Debug MSVC C++17
diff --git a/include/etl/private/variant_variadic.h b/include/etl/private/variant_variadic.h
index 498cdf63..3ff5b876 100644
--- a/include/etl/private/variant_variadic.h
+++ b/include/etl/private/variant_variadic.h
@@ -825,7 +825,9 @@ namespace etl
     //***************************************************************************
     /// Accept an etl::visitor.
     //***************************************************************************
-    void accept(etl::visitor<TTypes...>& v)
+    template <typename TVisitor>
+    etl::enable_if_t<etl::is_visitor<TVisitor>::value, void> 
+      accept(TVisitor& v)
     {
 #if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
       do_visitor(v, etl::make_index_sequence<sizeof...(TTypes)>{});
@@ -837,7 +839,9 @@ namespace etl
     //***************************************************************************
     /// Accept an etl::visitor.
     //***************************************************************************
-    void accept(etl::visitor<TTypes...>& v) const
+    template <typename TVisitor>
+    etl::enable_if_t<etl::is_visitor<TVisitor>::value, void> 
+      accept(TVisitor& v) const
     {
 #if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
       do_visitor(v, etl::make_index_sequence<sizeof...(TTypes)>{});
@@ -850,7 +854,7 @@ namespace etl
     /// Accept a generic functor.
     //***************************************************************************
     template <typename TVisitor>
-    etl::enable_if_t<!etl::is_base_of<etl::visitor<TTypes...>, TVisitor>::value, void>
+    etl::enable_if_t<!etl::is_visitor<TVisitor>::value, void>
       accept(TVisitor& v)
     {
 #if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
@@ -864,7 +868,7 @@ namespace etl
     /// Accept a generic functor.
     //***************************************************************************
     template <typename TVisitor>
-    etl::enable_if_t<!etl::is_base_of<etl::visitor<TTypes...>, TVisitor>::value, void>
+    etl::enable_if_t<!etl::is_visitor<TVisitor>::value, void>
       accept(TVisitor& v) const
     {
 #if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
@@ -1018,41 +1022,41 @@ namespace etl
     {
       switch (index())
       {
-        case 0: { auto arg = etl::get<0>(*this); visitor.visit(arg); break; }
-        case 1: { auto arg = etl::get<1>(*this); visitor.visit(arg); break; }
-        case 2: { auto arg = etl::get<2>(*this); visitor.visit(arg); break; }
-        case 3: { auto arg = etl::get<3>(*this); visitor.visit(arg); break; }
-        case 4: { auto arg = etl::get<4>(*this); visitor.visit(arg); break; }
-        case 5: { auto arg = etl::get<5>(*this); visitor.visit(arg); break; }
-        case 6: { auto arg = etl::get<6>(*this); visitor.visit(arg); break; }
-        case 7: { auto arg = etl::get<7>(*this); visitor.visit(arg); break; }
+        case 0: { visitor.visit(etl::get<0>(*this)); break; }
+        case 1: { visitor.visit(etl::get<1>(*this)); break; }
+        case 2: { visitor.visit(etl::get<2>(*this)); break; }
+        case 3: { visitor.visit(etl::get<3>(*this)); break; }
+        case 4: { visitor.visit(etl::get<4>(*this)); break; }
+        case 5: { visitor.visit(etl::get<5>(*this)); break; }
+        case 6: { visitor.visit(etl::get<6>(*this)); break; }
+        case 7: { visitor.visit(etl::get<7>(*this)); break; }
 #if !defined(ETL_VARIANT_CPP11_MAX_8_TYPES)
-        case 8: { auto arg = etl::get<8>(*this); visitor.visit(arg); break; }
-        case 9: { auto arg = etl::get<9>(*this); visitor.visit(arg); break; }
-        case 10: { auto arg = etl::get<10>(*this); visitor.visit(arg); break; }
-        case 11: { auto arg = etl::get<11>(*this); visitor.visit(arg); break; }
-        case 12: { auto arg = etl::get<12>(*this); visitor.visit(arg); break; }
-        case 13: { auto arg = etl::get<13>(*this); visitor.visit(arg); break; }
-        case 14: { auto arg = etl::get<14>(*this); visitor.visit(arg); break; }
-        case 15: { auto arg = etl::get<15>(*this); visitor.visit(arg); break; }
+        case 8: { visitor.visit(etl::get<8>(*this)); break; }
+        case 9: { visitor.visit(etl::get<9>(*this)); break; }
+        case 10: { visitor.visit(etl::get<10>(*this)); break; }
+        case 11: { visitor.visit(etl::get<11>(*this)); break; }
+        case 12: { visitor.visit(etl::get<12>(*this)); break; }
+        case 13: { visitor.visit(etl::get<13>(*this)); break; }
+        case 14: { visitor.visit(etl::get<14>(*this)); break; }
+        case 15: { visitor.visit(etl::get<15>(*this)); break; }
 #if !defined(ETL_VARIANT_CPP11_MAX_16_TYPES)
-        case 16: { auto arg = etl::get<16>(*this); visitor.visit(arg); break; }
-        case 17: { auto arg = etl::get<17>(*this); visitor.visit(arg); break; }
-        case 18: { auto arg = etl::get<18>(*this); visitor.visit(arg); break; }
-        case 19: { auto arg = etl::get<19>(*this); visitor.visit(arg); break; }
-        case 20: { auto arg = etl::get<20>(*this); visitor.visit(arg); break; }
-        case 21: { auto arg = etl::get<21>(*this); visitor.visit(arg); break; }
-        case 22: { auto arg = etl::get<22>(*this); visitor.visit(arg); break; }
-        case 23: { auto arg = etl::get<23>(*this); visitor.visit(arg); break; }
+        case 16: { visitor.visit(etl::get<16>(*this)); break; }
+        case 17: { visitor.visit(etl::get<17>(*this)); break; }
+        case 18: { visitor.visit(etl::get<18>(*this)); break; }
+        case 19: { visitor.visit(etl::get<19>(*this)); break; }
+        case 20: { visitor.visit(etl::get<20>(*this)); break; }
+        case 21: { visitor.visit(etl::get<21>(*this)); break; }
+        case 22: { visitor.visit(etl::get<22>(*this)); break; }
+        case 23: { visitor.visit(etl::get<23>(*this)); break; }
 #if !defined(ETL_VARIANT_CPP11_MAX_24_TYPES)
-        case 24: { auto arg = etl::get<24>(*this); visitor.visit(arg); break; }
-        case 25: { auto arg = etl::get<25>(*this); visitor.visit(arg); break; }
-        case 26: { auto arg = etl::get<26>(*this); visitor.visit(arg); break; }
-        case 27: { auto arg = etl::get<27>(*this); visitor.visit(arg); break; }
-        case 28: { auto arg = etl::get<28>(*this); visitor.visit(arg); break; }
-        case 29: { auto arg = etl::get<29>(*this); visitor.visit(arg); break; }
-        case 30: { auto arg = etl::get<30>(*this); visitor.visit(arg); break; }
-        case 31: { auto arg = etl::get<31>(*this); visitor.visit(arg); break; }
+        case 24: { visitor.visit(etl::get<24>(*this)); break; }
+        case 25: { visitor.visit(etl::get<25>(*this)); break; }
+        case 26: { visitor.visit(etl::get<26>(*this)); break; }
+        case 27: { visitor.visit(etl::get<27>(*this)); break; }
+        case 28: { visitor.visit(etl::get<28>(*this)); break; }
+        case 29: { visitor.visit(etl::get<29>(*this)); break; }
+        case 30: { visitor.visit(etl::get<30>(*this)); break; }
+        case 31: { visitor.visit(etl::get<31>(*this)); break; }
 #endif
 #endif
 #endif
@@ -1068,41 +1072,41 @@ namespace etl
     {
       switch (index())
       {
-        case 0: { auto arg = etl::get<0>(*this); visitor.visit(arg); break; }
-        case 1: { auto arg = etl::get<1>(*this); visitor.visit(arg); break; }
-        case 2: { auto arg = etl::get<2>(*this); visitor.visit(arg); break; }
-        case 3: { auto arg = etl::get<3>(*this); visitor.visit(arg); break; }
-        case 4: { auto arg = etl::get<4>(*this); visitor.visit(arg); break; }
-        case 5: { auto arg = etl::get<5>(*this); visitor.visit(arg); break; }
-        case 6: { auto arg = etl::get<6>(*this); visitor.visit(arg); break; }
-        case 7: { auto arg = etl::get<7>(*this); visitor.visit(arg); break; }
+        case 0: { visitor.visit(etl::get<0>(*this)); break; }
+        case 1: { visitor.visit(etl::get<1>(*this)); break; }
+        case 2: { visitor.visit(etl::get<2>(*this)); break; }
+        case 3: { visitor.visit(etl::get<3>(*this)); break; }
+        case 4: { visitor.visit(etl::get<4>(*this)); break; }
+        case 5: { visitor.visit(etl::get<5>(*this)); break; }
+        case 6: { visitor.visit(etl::get<6>(*this)); break; }
+        case 7: { visitor.visit(etl::get<7>(*this)); break; }
 #if !defined(ETL_VARIANT_CPP11_MAX_8_TYPES)
-        case 8: { auto arg = etl::get<8>(*this); visitor.visit(arg); break; }
-        case 9: { auto arg = etl::get<9>(*this); visitor.visit(arg); break; }
-        case 10: { auto arg = etl::get<10>(*this); visitor.visit(arg); break; }
-        case 11: { auto arg = etl::get<11>(*this); visitor.visit(arg); break; }
-        case 12: { auto arg = etl::get<12>(*this); visitor.visit(arg); break; }
-        case 13: { auto arg = etl::get<13>(*this); visitor.visit(arg); break; }
-        case 14: { auto arg = etl::get<14>(*this); visitor.visit(arg); break; }
-        case 15: { auto arg = etl::get<15>(*this); visitor.visit(arg); break; }
+        case 8: { visitor.visit(etl::get<8>(*this)); break; }
+        case 9: { visitor.visit(etl::get<9>(*this)); break; }
+        case 10: { visitor.visit(etl::get<10>(*this)); break; }
+        case 11: { visitor.visit(etl::get<11>(*this)); break; }
+        case 12: { visitor.visit(etl::get<12>(*this)); break; }
+        case 13: { visitor.visit(etl::get<13>(*this)); break; }
+        case 14: { visitor.visit(etl::get<14>(*this)); break; }
+        case 15: { visitor.visit(etl::get<15>(*this)); break; }
 #if !defined(ETL_VARIANT_CPP11_MAX_16_TYPES)
-        case 16: { auto arg = etl::get<16>(*this); visitor.visit(arg); break; }
-        case 17: { auto arg = etl::get<17>(*this); visitor.visit(arg); break; }
-        case 18: { auto arg = etl::get<18>(*this); visitor.visit(arg); break; }
-        case 19: { auto arg = etl::get<19>(*this); visitor.visit(arg); break; }
-        case 20: { auto arg = etl::get<20>(*this); visitor.visit(arg); break; }
-        case 21: { auto arg = etl::get<21>(*this); visitor.visit(arg); break; }
-        case 22: { auto arg = etl::get<22>(*this); visitor.visit(arg); break; }
-        case 23: { auto arg = etl::get<23>(*this); visitor.visit(arg); break; }
+        case 16: { visitor.visit(etl::get<16>(*this)); break; }
+        case 17: { visitor.visit(etl::get<17>(*this)); break; }
+        case 18: { visitor.visit(etl::get<18>(*this)); break; }
+        case 19: { visitor.visit(etl::get<19>(*this)); break; }
+        case 20: { visitor.visit(etl::get<20>(*this)); break; }
+        case 21: { visitor.visit(etl::get<21>(*this)); break; }
+        case 22: { visitor.visit(etl::get<22>(*this)); break; }
+        case 23: { visitor.visit(etl::get<23>(*this)); break; }
 #if !defined(ETL_VARIANT_CPP11_MAX_24_TYPES)
-        case 24: { auto arg = etl::get<24>(*this); visitor.visit(arg); break; }
-        case 25: { auto arg = etl::get<25>(*this); visitor.visit(arg); break; }
-        case 26: { auto arg = etl::get<26>(*this); visitor.visit(arg); break; }
-        case 27: { auto arg = etl::get<27>(*this); visitor.visit(arg); break; }
-        case 28: { auto arg = etl::get<28>(*this); visitor.visit(arg); break; }
-        case 29: { auto arg = etl::get<29>(*this); visitor.visit(arg); break; }
-        case 30: { auto arg = etl::get<30>(*this); visitor.visit(arg); break; }
-        case 31: { auto arg = etl::get<31>(*this); visitor.visit(arg); break; }
+        case 24: { visitor.visit(etl::get<24>(*this)); break; }
+        case 25: { visitor.visit(etl::get<25>(*this)); break; }
+        case 26: { visitor.visit(etl::get<26>(*this)); break; }
+        case 27: { visitor.visit(etl::get<27>(*this)); break; }
+        case 28: { visitor.visit(etl::get<28>(*this)); break; }
+        case 29: { visitor.visit(etl::get<29>(*this)); break; }
+        case 30: { visitor.visit(etl::get<30>(*this)); break; }
+        case 31: { visitor.visit(etl::get<31>(*this)); break; }
 #endif
 #endif
 #endif
@@ -1119,7 +1123,10 @@ namespace etl
     {
       if (Index == index())
       {
-        auto v = etl::get<Index>(*this);
+        // Workaround for MSVC (2023/05/13)
+        // It doesn't compile 'visitor.visit(etl::get<Index>(*this))' correctly for C++17 & C++20.
+        // Changed all of the instances for consistancy.   
+        decltype(auto) v = etl::get<Index>(*this);
         visitor.visit(v);
         return true;
       }
@@ -1139,8 +1146,8 @@ namespace etl
       {
         // Workaround for MSVC (2023/05/13)
         // It doesn't compile 'visitor.visit(etl::get<Index>(*this))' correctly for C++17 & C++20.
-        // Changed all of the instances for consistancy.
-        auto v = etl::get<Index>(*this);
+        // Changed all of the instances for consistancy.      
+        decltype(auto) v = etl::get<Index>(*this);
         visitor.visit(v);
         return true;
       }
@@ -1306,7 +1313,7 @@ namespace etl
     {
       if (Index == index())
       {
-        auto v = etl::get<Index>(*this);
+        decltype(auto) v = etl::get<Index>(*this);
         visitor(v);
         return true;
       }
@@ -1324,7 +1331,7 @@ namespace etl
     {
       if (Index == index())
       {
-        auto v = etl::get<Index>(*this);
+        decltype(auto) v = etl::get<Index>(*this);
         visitor(v);
         return true;
       }
diff --git a/include/etl/visitor.h b/include/etl/visitor.h
index 6544edd0..2975c678 100644
--- a/include/etl/visitor.h
+++ b/include/etl/visitor.h
@@ -32,6 +32,7 @@ SOFTWARE.
 #define ETL_VISITOR_INCLUDED
 
 #include "platform.h"
+#include "type_traits.h"
 
 //*****************************************************************************
 ///\defgroup visitor visitor
@@ -149,6 +150,14 @@ namespace etl
 
 #endif
 
+  //*****************************************************************
+  /// The tag to identify an etl::visitor.
+  ///\ingroup visitor
+  //*****************************************************************
+  struct visitor_tag
+  {
+  };
+
 #if ETL_USING_CPP11 && !defined(ETL_VISITOR_FORCE_CPP03_IMPLEMENTATION)
 
   //*****************************************************************
@@ -169,13 +178,13 @@ namespace etl
   ///\ingroup visitor
   //*****************************************************************
   template <typename T1>
-  class visitor<T1>
+  class visitor<T1> : public virtual visitor_tag
   {
   public:
 
     virtual ~visitor() = default;
 
-    virtual void visit(T1&) = 0;
+    virtual void visit(T1) = 0;
   };
 
 #else
@@ -189,28 +198,28 @@ namespace etl
             typename T5  = void, typename T6  = void, typename T7  = void, typename T8  = void,
             typename T9  = void, typename T10 = void, typename T11 = void, typename T12 = void,
             typename T13 = void, typename T14 = void, typename T15 = void, typename T16 = void>
-  class visitor
+  class visitor : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
-    virtual void visit(T9&) = 0;
-    virtual void visit(T10&) = 0;
-    virtual void visit(T11&) = 0;
-    virtual void visit(T12&) = 0;
-    virtual void visit(T13&) = 0;
-    virtual void visit(T14&) = 0;
-    virtual void visit(T15&) = 0;
-    virtual void visit(T16&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
+    virtual void visit(T9) = 0;
+    virtual void visit(T10) = 0;
+    virtual void visit(T11) = 0;
+    virtual void visit(T12) = 0;
+    virtual void visit(T13) = 0;
+    virtual void visit(T14) = 0;
+    virtual void visit(T15) = 0;
+    virtual void visit(T16) = 0;
   };
 
   //*****************************************************************
@@ -222,27 +231,27 @@ namespace etl
             typename T5,  typename T6,  typename T7,  typename T8,
             typename T9,  typename T10, typename T11, typename T12,
             typename T13, typename T14, typename T15>
-  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15>
+  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
-    virtual void visit(T9&) = 0;
-    virtual void visit(T10&) = 0;
-    virtual void visit(T11&) = 0;
-    virtual void visit(T12&) = 0;
-    virtual void visit(T13&) = 0;
-    virtual void visit(T14&) = 0;
-    virtual void visit(T15&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
+    virtual void visit(T9) = 0;
+    virtual void visit(T10) = 0;
+    virtual void visit(T11) = 0;
+    virtual void visit(T12) = 0;
+    virtual void visit(T13) = 0;
+    virtual void visit(T14) = 0;
+    virtual void visit(T15) = 0;
   };
 
   //*****************************************************************
@@ -254,26 +263,26 @@ namespace etl
             typename T5,  typename T6,  typename T7,  typename T8,
             typename T9,  typename T10, typename T11, typename T12,
             typename T13, typename T14>
-  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14>
+  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
-    virtual void visit(T9&) = 0;
-    virtual void visit(T10&) = 0;
-    virtual void visit(T11&) = 0;
-    virtual void visit(T12&) = 0;
-    virtual void visit(T13&) = 0;
-    virtual void visit(T14&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
+    virtual void visit(T9) = 0;
+    virtual void visit(T10) = 0;
+    virtual void visit(T11) = 0;
+    virtual void visit(T12) = 0;
+    virtual void visit(T13) = 0;
+    virtual void visit(T14) = 0;
   };
 
   //*****************************************************************
@@ -285,25 +294,25 @@ namespace etl
             typename T5, typename T6,  typename T7,  typename T8,
             typename T9, typename T10, typename T11, typename T12,
             typename T13>
-  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13>
+  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
-    virtual void visit(T9&) = 0;
-    virtual void visit(T10&) = 0;
-    virtual void visit(T11&) = 0;
-    virtual void visit(T12&) = 0;
-    virtual void visit(T13&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
+    virtual void visit(T9) = 0;
+    virtual void visit(T10) = 0;
+    virtual void visit(T11) = 0;
+    virtual void visit(T12) = 0;
+    virtual void visit(T13) = 0;
   };
 
   //*****************************************************************
@@ -314,24 +323,24 @@ namespace etl
   template <typename T1, typename T2,  typename T3,  typename T4,
             typename T5, typename T6,  typename T7,  typename T8,
             typename T9, typename T10, typename T11, typename T12>
-  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12>
+  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
-    virtual void visit(T9&) = 0;
-    virtual void visit(T10&) = 0;
-    virtual void visit(T11&) = 0;
-    virtual void visit(T12&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
+    virtual void visit(T9) = 0;
+    virtual void visit(T10) = 0;
+    virtual void visit(T11) = 0;
+    virtual void visit(T12) = 0;
   };
 
   //*****************************************************************
@@ -342,23 +351,23 @@ namespace etl
   template <typename T1, typename T2,  typename T3, typename T4,
             typename T5, typename T6,  typename T7, typename T8,
             typename T9, typename T10, typename T11>
-  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11>
+  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
-    virtual void visit(T9&) = 0;
-    virtual void visit(T10&) = 0;
-    virtual void visit(T11&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
+    virtual void visit(T9) = 0;
+    virtual void visit(T10) = 0;
+    virtual void visit(T11) = 0;
   };
 
   //*****************************************************************
@@ -369,22 +378,22 @@ namespace etl
   template <typename T1, typename T2, typename T3, typename T4,
             typename T5, typename T6, typename T7, typename T8,
             typename T9, typename T10>
-  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10>
+  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
-    virtual void visit(T9&) = 0;
-    virtual void visit(T10&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
+    virtual void visit(T9) = 0;
+    virtual void visit(T10) = 0;
   };
 
   //*****************************************************************
@@ -395,21 +404,21 @@ namespace etl
   template <typename T1, typename T2, typename T3, typename T4,
             typename T5, typename T6, typename T7, typename T8,
             typename T9>
-  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9>
+  class visitor<T1, T2, T3, T4, T5, T6, T7, T8, T9> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
-    virtual void visit(T9&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
+    virtual void visit(T9) = 0;
   };
 
   //*****************************************************************
@@ -419,20 +428,20 @@ namespace etl
   //*****************************************************************
   template <typename T1, typename T2, typename T3, typename T4,
             typename T5, typename T6, typename T7, typename T8>
-  class visitor<T1, T2, T3, T4, T5, T6, T7, T8>
+  class visitor<T1, T2, T3, T4, T5, T6, T7, T8> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
-    virtual void visit(T8&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
+    virtual void visit(T8) = 0;
   };
 
   //*****************************************************************
@@ -442,19 +451,19 @@ namespace etl
   //*****************************************************************
   template <typename T1, typename T2, typename T3, typename T4,
             typename T5, typename T6, typename T7>
-  class visitor<T1, T2, T3, T4, T5, T6, T7>
+  class visitor<T1, T2, T3, T4, T5, T6, T7> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
-    virtual void visit(T7&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
+    virtual void visit(T7) = 0;
   };
 
   //*****************************************************************
@@ -464,18 +473,18 @@ namespace etl
   //*****************************************************************
   template <typename T1, typename T2, typename T3, typename T4,
             typename T5, typename T6>
-  class visitor<T1, T2, T3, T4, T5, T6>
+  class visitor<T1, T2, T3, T4, T5, T6> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
-    virtual void visit(T6&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
+    virtual void visit(T6) = 0;
   };
 
   //*****************************************************************
@@ -485,17 +494,17 @@ namespace etl
   //*****************************************************************
   template <typename T1, typename T2, typename T3, typename T4,
             typename T5>
-  class visitor<T1, T2, T3, T4, T5>
+  class visitor<T1, T2, T3, T4, T5> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
-    virtual void visit(T5&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
+    virtual void visit(T5) = 0;
   };
 
   //*****************************************************************
@@ -504,16 +513,16 @@ namespace etl
   ///\ingroup visitor
   //*****************************************************************
   template <typename T1, typename T2, typename T3, typename T4>
-  class visitor<T1, T2, T3, T4>
+  class visitor<T1, T2, T3, T4> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
-    virtual void visit(T4&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
+    virtual void visit(T4) = 0;
   };
 
   //*****************************************************************
@@ -522,15 +531,15 @@ namespace etl
   ///\ingroup visitor
   //*****************************************************************
   template <typename T1, typename T2, typename T3>
-  class visitor<T1, T2, T3>
+  class visitor<T1, T2, T3> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
-    virtual void visit(T3&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
+    virtual void visit(T3) = 0;
   };
 
   //*****************************************************************
@@ -539,14 +548,14 @@ namespace etl
   ///\ingroup visitor
   //*****************************************************************
   template <typename T1, typename T2>
-  class visitor<T1, T2>
+  class visitor<T1, T2> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
-    virtual void visit(T2&) = 0;
+    virtual void visit(T1) = 0;
+    virtual void visit(T2) = 0;
   };
 
   //*****************************************************************
@@ -555,15 +564,33 @@ namespace etl
   ///\ingroup visitor
   //*****************************************************************
   template <typename T1>
-  class visitor<T1>
+  class visitor<T1> : public visitor_tag
   {
   public:
 
     virtual ~visitor() {}
 
-    virtual void visit(T1&) = 0;
+    virtual void visit(T1) = 0;
   };
 
+#endif
+
+  //*****************************************************************
+  /// Is the type an etl::visitor?
+  ///\ingroup visitor
+  //*****************************************************************
+  template <typename T>
+  struct is_visitor : public etl::bool_constant<etl::is_base_of<etl::visitor_tag, T>::value>
+  {
+  };
+
+#if ETL_USING_CPP17
+  //*****************************************************************
+  /// Is the type an etl::visitor?
+  ///\ingroup visitor
+  //*****************************************************************
+  template <typename T>
+  constexpr bool is_visitor_v = etl::is_visitor<T>::value;
 #endif
 }
 
diff --git a/test/test_variant_variadic.cpp b/test/test_variant_variadic.cpp
index 3e4a08d3..fd6157b9 100644
--- a/test/test_variant_variadic.cpp
+++ b/test/test_variant_variadic.cpp
@@ -810,7 +810,7 @@ namespace
     //*************************************************************************
     TEST(test_variant_accept_visitor)
     {    
-      struct Visitor : public etl::visitor<char, int, std::string>
+      struct Visitor : public etl::visitor<char&, int&, std::string&>
       {
         Visitor()
           : result_c(0)
@@ -822,16 +822,19 @@ namespace
         void visit(char& c)
         {
           result_c = c;
+          ++c;
         }
 
         void visit(int& i)
         {
           result_i = i;
+          ++i;
         }
 
         void visit(std::string& s)
         {
           result_s = s;
+          s = "4";
         }
 
         char result_c;
@@ -846,20 +849,23 @@ namespace
       variant_etl = char(1);
       variant_etl.accept(visitor);
       CHECK_EQUAL(1, visitor.result_c);
+      CHECK_EQUAL(2, etl::get<char>(variant_etl));
       
       variant_etl = int(2);
       variant_etl.accept(visitor);
       CHECK_EQUAL(2, visitor.result_i);
+      CHECK_EQUAL(3, etl::get<int>(variant_etl));
 
       variant_etl = std::string("3");
       variant_etl.accept(visitor);
       CHECK_EQUAL("3", visitor.result_s);
+      CHECK_EQUAL("4", etl::get<std::string>(variant_etl));
     }
 
     //*************************************************************************
     TEST(test_variant_accept_visitor_deprecated)
     {
-      struct Visitor : public etl::visitor<char, int, std::string>
+      struct Visitor : public etl::visitor<char&, int&, std::string&>
       {
         Visitor()
           : result_c(0)
@@ -871,16 +877,19 @@ namespace
         void visit(char& c)
         {
           result_c = c;
+          ++c;
         }
 
         void visit(int& i)
         {
           result_i = i;
+          ++i;
         }
 
         void visit(std::string& s)
         {
           result_s = s;
+          s = "4";
         }
 
         char result_c;
@@ -895,21 +904,23 @@ namespace
       variant_etl = char(1);
       variant_etl.accept_visitor(visitor);
       CHECK_EQUAL(1, visitor.result_c);
+      CHECK_EQUAL(2, etl::get<char>(variant_etl));
 
       variant_etl = int(2);
       variant_etl.accept_visitor(visitor);
       CHECK_EQUAL(2, visitor.result_i);
+      CHECK_EQUAL(3, etl::get<int>(variant_etl));
 
       variant_etl = std::string("3");
-      //variant_etl.accept_visitor(visitor);
-      variant_etl.accept(visitor);
+      variant_etl.accept_visitor(visitor);
       CHECK_EQUAL("3", visitor.result_s);
+      CHECK_EQUAL("4", etl::get<std::string>(variant_etl));
     }
 
     //*************************************************************************
     TEST(test_const_variant_accept_visitor)
     {
-      struct Visitor : public etl::visitor<char, int, std::string>
+      struct Visitor : public etl::visitor<const char&, const int&, const std::string&>
       {
         Visitor()
           : result_c(0)
@@ -918,17 +929,17 @@ namespace
         {
         }
 
-        void visit(char& c)
+        void visit(const char& c)
         {
           result_c = c;
         }
 
-        void visit(int& i)
+        void visit(const int& i)
         {
           result_i = i;
         }
 
-        void visit(std::string& s)
+        void visit(const std::string& s)
         {
           result_s = s;
         }
@@ -961,7 +972,7 @@ namespace
     //*************************************************************************
     TEST(test_const_variant_accept_visitor_deprecated)
     {
-      struct Visitor : public etl::visitor<char, int, std::string>
+      struct Visitor : public etl::visitor<char, int, const std::string&>
       {
         Visitor()
           : result_c(0)
@@ -970,17 +981,17 @@ namespace
         {
         }
 
-        void visit(char& c)
+        void visit(char c)
         {
           result_c = c;
         }
 
-        void visit(int& i)
+        void visit(int i)
         {
           result_i = i;
         }
 
-        void visit(std::string& s)
+        void visit(const std::string& s)
         {
           result_s = s;
         }
@@ -1024,19 +1035,22 @@ namespace
         {
         }
 
-        void operator()(char c)
+        void operator()(char& c)
         {
           result_c = c;
+          ++c;
         }
 
-        void operator()(int i)
+        void operator()(int& i)
         {
           result_i = i;
+          ++i;
         }
 
-        void operator()(const std::string& s)
+        void operator()(std::string& s)
         {
           result_s = s;
+          s = "4";
         }
 
         char result_c;
@@ -1051,14 +1065,17 @@ namespace
       variant_etl = char(1);
       variant_etl.accept(visitor);
       CHECK_EQUAL(1, visitor.result_c);
-      
+      CHECK_EQUAL(2, etl::get<char>(variant_etl));
+
       variant_etl = int(2);
       variant_etl.accept(visitor);
       CHECK_EQUAL(2, visitor.result_i);
+      CHECK_EQUAL(3, etl::get<int>(variant_etl));
 
       variant_etl = std::string("3");
       variant_etl.accept(visitor);
       CHECK_EQUAL("3", visitor.result_s);
+      CHECK_EQUAL("4", etl::get<std::string>(variant_etl));
     }
 
     //*************************************************************************
@@ -1150,7 +1167,7 @@ namespace
       variant_etl = char(1);
       const test_variant_etl_3 const_variant_etl1(variant_etl);
       const_variant_etl1.accept(visitor);
-      CHECK_EQUAL(1, visitor.result_c);
+      CHECK_EQUAL(1, int(visitor.result_c));
 
       variant_etl = int(2);
       const test_variant_etl_3 const_variant_etl2(variant_etl);
@@ -1206,13 +1223,11 @@ namespace
 
       variant_etl = int(2);
       const test_variant_etl_3 const_variant_etl2(variant_etl);
-      //const_variant_etl2.accept_functor(visitor);
       const_variant_etl2.accept(visitor);
       CHECK_EQUAL(2, visitor.result_i);
 
       variant_etl = std::string("3");
       const test_variant_etl_3 const_variant_etl3(variant_etl);
-      //const_variant_etl3.accept_functor(visitor);
       const_variant_etl3.accept(visitor);
       CHECK_EQUAL("3", visitor.result_s);
     }
@@ -1225,23 +1240,26 @@ namespace
       int  result_i;
       std::string result_s;
 
-      auto visitor = etl::make_overload([&result_c](char) { result_c = 1; },
-                                        [&result_i](int) { result_i = 2; },
-                                        [&result_s](const std::string&) { result_s = "3"; });
+      auto visitor = etl::make_overload([&result_c](char& c) { result_c = 1; ++c; },
+                                        [&result_i](int& i) { result_i = 2; ++i; },
+                                        [&result_s](std::string& s) { result_s = "3"; s = "4"; });
 
       test_variant_etl_3 variant_etl;
 
       variant_etl = char(1);
       variant_etl.accept(visitor);
       CHECK_EQUAL(1, result_c);
+      CHECK_EQUAL(2, etl::get<char>(variant_etl));
 
       variant_etl = int(2);
       variant_etl.accept(visitor);
       CHECK_EQUAL(2, result_i);
+      CHECK_EQUAL(3, etl::get<int>(variant_etl));
 
       variant_etl = std::string("3");
       variant_etl.accept(visitor);
       CHECK_EQUAL("3", result_s);
+      CHECK_EQUAL("4", etl::get<std::string>(variant_etl));
     }
 
     //*************************************************************************
@@ -1262,12 +1280,10 @@ namespace
       CHECK_EQUAL(1, result_c);
 
       variant_etl = int(2);
-      //variant_etl.accept_functor(visitor);
       variant_etl.accept(visitor);
       CHECK_EQUAL(2, result_i);
 
       variant_etl = std::string("3");
-      //variant_etl.accept_functor(visitor);
       variant_etl.accept(visitor);
       CHECK_EQUAL("3", result_s);
     }
@@ -1321,13 +1337,11 @@ namespace
 
       variant_etl = int(2);
       const test_variant_etl_3 const_variant_etl2(variant_etl);
-      //const_variant_etl2.accept_functor(visitor);
       const_variant_etl2.accept(visitor);
       CHECK_EQUAL(2, result_i);
 
       variant_etl = std::string("3");
       const test_variant_etl_3 const_variant_etl3(variant_etl);
-      //const_variant_etl3.accept_functor(visitor);
       const_variant_etl3.accept(visitor);
       CHECK_EQUAL("3", result_s);
     }
diff --git a/test/test_visitor.cpp b/test/test_visitor.cpp
index dc1cac19..4ea72356 100644
--- a/test/test_visitor.cpp
+++ b/test/test_visitor.cpp
@@ -43,14 +43,14 @@ class Triangle;
 // Circle will be passed by reference.
 // Triangle will be passed by const reference.
 //*****************************************************************************
-typedef etl::visitor<Square, Circle, const Triangle> DrawVisitorType;
+typedef etl::visitor<Square&, Circle&, const Triangle&> DrawVisitorType;
 
 //*****************************************************************************
 // What classes do you want the visitors to handle?
 // Square will be passed by reference.
 // Triangle will be passed by const reference.
 //*****************************************************************************
-typedef etl::visitor<Square, const Triangle> LogVisitorType;
+typedef etl::visitor<Square&, const Triangle&> LogVisitorType;
 
 //*****************************************************************************
 // Base shape.
@@ -369,7 +369,7 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -383,8 +383,8 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -398,9 +398,9 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -414,10 +414,10 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -432,11 +432,11 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -451,12 +451,12 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -471,13 +471,13 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -492,14 +492,14 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -515,15 +515,15 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
-        void visit(Shape<9, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
+        void visit(Shape<9, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -539,16 +539,16 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
-        void visit(Shape<9, ShapeVisitor>&) {}
-        void visit(Shape<10, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
+        void visit(Shape<9, ShapeVisitor>) {}
+        void visit(Shape<10, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -565,17 +565,17 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
-        void visit(Shape<9, ShapeVisitor>&) {}
-        void visit(Shape<10, ShapeVisitor>&) {}
-        void visit(Shape<11, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
+        void visit(Shape<9, ShapeVisitor>) {}
+        void visit(Shape<10, ShapeVisitor>) {}
+        void visit(Shape<11, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -591,18 +591,18 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
-        void visit(Shape<9, ShapeVisitor>&) {}
-        void visit(Shape<10, ShapeVisitor>&) {}
-        void visit(Shape<11, ShapeVisitor>&) {}
-        void visit(Shape<12, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
+        void visit(Shape<9, ShapeVisitor>) {}
+        void visit(Shape<10, ShapeVisitor>) {}
+        void visit(Shape<11, ShapeVisitor>) {}
+        void visit(Shape<12, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -619,19 +619,19 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
-        void visit(Shape<9, ShapeVisitor>&) {}
-        void visit(Shape<10, ShapeVisitor>&) {}
-        void visit(Shape<11, ShapeVisitor>&) {}
-        void visit(Shape<12, ShapeVisitor>&) {}
-        void visit(Shape<13, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
+        void visit(Shape<9, ShapeVisitor>) {}
+        void visit(Shape<10, ShapeVisitor>) {}
+        void visit(Shape<11, ShapeVisitor>) {}
+        void visit(Shape<12, ShapeVisitor>) {}
+        void visit(Shape<13, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -648,20 +648,20 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
-        void visit(Shape<9, ShapeVisitor>&) {}
-        void visit(Shape<10, ShapeVisitor>&) {}
-        void visit(Shape<11, ShapeVisitor>&) {}
-        void visit(Shape<12, ShapeVisitor>&) {}
-        void visit(Shape<13, ShapeVisitor>&) {}
-        void visit(Shape<14, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
+        void visit(Shape<9, ShapeVisitor>) {}
+        void visit(Shape<10, ShapeVisitor>) {}
+        void visit(Shape<11, ShapeVisitor>) {}
+        void visit(Shape<12, ShapeVisitor>) {}
+        void visit(Shape<13, ShapeVisitor>) {}
+        void visit(Shape<14, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -678,21 +678,21 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
-        void visit(Shape<9, ShapeVisitor>&) {}
-        void visit(Shape<10, ShapeVisitor>&) {}
-        void visit(Shape<11, ShapeVisitor>&) {}
-        void visit(Shape<12, ShapeVisitor>&) {}
-        void visit(Shape<13, ShapeVisitor>&) {}
-        void visit(Shape<14, ShapeVisitor>&) {}
-        void visit(Shape<15, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
+        void visit(Shape<9, ShapeVisitor>) {}
+        void visit(Shape<10, ShapeVisitor>) {}
+        void visit(Shape<11, ShapeVisitor>) {}
+        void visit(Shape<12, ShapeVisitor>) {}
+        void visit(Shape<13, ShapeVisitor>) {}
+        void visit(Shape<14, ShapeVisitor>) {}
+        void visit(Shape<15, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
@@ -709,27 +709,43 @@ namespace
       {
       public:
 
-        void visit(Shape<1, ShapeVisitor>&) {}
-        void visit(Shape<2, ShapeVisitor>&) {}
-        void visit(Shape<3, ShapeVisitor>&) {}
-        void visit(Shape<4, ShapeVisitor>&) {}
-        void visit(Shape<5, ShapeVisitor>&) {}
-        void visit(Shape<6, ShapeVisitor>&) {}
-        void visit(Shape<7, ShapeVisitor>&) {}
-        void visit(Shape<8, ShapeVisitor>&) {}
-        void visit(Shape<9, ShapeVisitor>&) {}
-        void visit(Shape<10, ShapeVisitor>&) {}
-        void visit(Shape<11, ShapeVisitor>&) {}
-        void visit(Shape<12, ShapeVisitor>&) {}
-        void visit(Shape<13, ShapeVisitor>&) {}
-        void visit(Shape<14, ShapeVisitor>&) {}
-        void visit(Shape<15, ShapeVisitor>&) {}
-        void visit(Shape<16, ShapeVisitor>&) {}
+        void visit(Shape<1, ShapeVisitor>) {}
+        void visit(Shape<2, ShapeVisitor>) {}
+        void visit(Shape<3, ShapeVisitor>) {}
+        void visit(Shape<4, ShapeVisitor>) {}
+        void visit(Shape<5, ShapeVisitor>) {}
+        void visit(Shape<6, ShapeVisitor>) {}
+        void visit(Shape<7, ShapeVisitor>) {}
+        void visit(Shape<8, ShapeVisitor>) {}
+        void visit(Shape<9, ShapeVisitor>) {}
+        void visit(Shape<10, ShapeVisitor>) {}
+        void visit(Shape<11, ShapeVisitor>) {}
+        void visit(Shape<12, ShapeVisitor>) {}
+        void visit(Shape<13, ShapeVisitor>) {}
+        void visit(Shape<14, ShapeVisitor>) {}
+        void visit(Shape<15, ShapeVisitor>) {}
+        void visit(Shape<16, ShapeVisitor>) {}
       };
 
       // This test just needs to compile without errors.
       CHECK(true);
     }
+
+    //*************************************************************************
+    struct NotVisitor {};
+    
+    TEST(test_is_visitor)
+    {
+#if ETL_USING_CPP17
+      CHECK_TRUE(etl::is_visitor_v<DrawVisitor>);
+      CHECK_TRUE(etl::is_visitor_v<LogVisitor>);
+      CHECK_FALSE(etl::is_visitor_v<NotVisitor>);
+#else
+      CHECK_TRUE(etl::is_visitor<DrawVisitor>::value);
+      CHECK_TRUE(etl::is_visitor<LogVisitor>::value);
+      CHECK_FALSE(etl::is_visitor<NotVisitor>::value);
+#endif
+    }
   }
 }