From 2085295358bf05b17c80a3b3418d6e5bf2940913 Mon Sep 17 00:00:00 2001
From: John Wellbelove <john.wellbelove@asterconsulting.co.uk>
Date: Fri, 6 Jun 2025 12:17:49 +0100
Subject: [PATCH] #1119 Forced compiler error in nth_type.h breaks containers
 for C++03

---
 include/etl/nth_type.h | 41 +++++++++++++++++++++++++++++++++++------
 test/etl_profile.h     |  1 +
 test/test_nth_type.cpp |  6 ++++++
 3 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/include/etl/nth_type.h b/include/etl/nth_type.h
index 7617dc68..b1e62af5 100644
--- a/include/etl/nth_type.h
+++ b/include/etl/nth_type.h
@@ -32,13 +32,9 @@ SOFTWARE.
 #include "platform.h"
 #include "static_assert.h"
 
-#if ETL_NOT_USING_CPP11
-  #if !defined(ETL_IN_UNIT_TEST)
-    #error NOT SUPPORTED FOR C++03 OR BELOW
-  #endif
-#else
 namespace etl
 {
+#if ETL_USING_CPP11 && !defined(ETL_NTH_TYPE_FORCE_CPP03_IMPLEMENTATION)
   namespace private_nth_type
   {
     //***********************************
@@ -67,6 +63,39 @@ namespace etl
   //***********************************
   template <size_t N, typename... TTypes>
   using nth_type_t = typename nth_type<N, TTypes...>::type;
+#else
+  // Primary template: Not defined, will cause a compile error if used with N > 7 or not enough types.
+  template <int N, typename T0 = void, typename T1 = void, typename T2 = void, typename T3 = void,
+                   typename T4 = void, typename T5 = void, typename T6 = void, typename T7 = void>
+  struct nth_type;
+
+  // Specialisations for N = 0..7
+  template <typename T0, typename T1, typename T2>
+  struct nth_type<0, T0, T1, T2, void, void, void, void, void> { typedef T0 type; };
+
+  template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
+  struct nth_type<0, T0, T1, T2, T3, T4, T5, T6, T7> { typedef T0 type; };
+  
+  template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
+  struct nth_type<1, T0, T1, T2, T3, T4, T5, T6, T7> { typedef T1 type; };
+
+  template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
+  struct nth_type<2, T0, T1, T2, T3, T4, T5, T6, T7> { typedef T2 type; };
+
+  template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
+  struct nth_type<3, T0, T1, T2, T3, T4, T5, T6, T7> { typedef T3 type; };
+
+  template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
+  struct nth_type<4, T0, T1, T2, T3, T4, T5, T6, T7> { typedef T4 type; };
+
+  template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
+  struct nth_type<5, T0, T1, T2, T3, T4, T5, T6, T7> { typedef T5 type; };
+
+  template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
+  struct nth_type<6, T0, T1, T2, T3, T4, T5, T6, T7> { typedef T6 type; };
+
+  template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
+  struct nth_type<7, T0, T1, T2, T3, T4, T5, T6, T7> { typedef T7 type; };
+#endif
 }
 #endif
-#endif
diff --git a/test/etl_profile.h b/test/etl_profile.h
index e5cf5fd4..4edf41d1 100644
--- a/test/etl_profile.h
+++ b/test/etl_profile.h
@@ -103,6 +103,7 @@ SOFTWARE.
   #define ETL_FLAT_SET_FORCE_CPP03_IMPLEMENTATION
   #define ETL_FLAT_MULTISET_FORCE_CPP03_IMPLEMENTATION
   #define ETL_VARIANT_POOL_FORCE_CPP03_IMPLEMENTATION
+  #define ETL_NTH_TYPE_FORCE_CPP03_IMPLEMENTATION
 #endif
 
 #if defined(ETL_FORCE_TEST_CPP11)
diff --git a/test/test_nth_type.cpp b/test/test_nth_type.cpp
index a5339292..192608c2 100644
--- a/test/test_nth_type.cpp
+++ b/test/test_nth_type.cpp
@@ -38,9 +38,15 @@ namespace
     //*************************************************************************
     TEST(test_nth_type)
     {
+#if defined(ETL_NTH_TYPE_FORCE_CPP03_IMPLEMENTATION)
+      CHECK((std::is_same<etl::nth_type<0, int, long, double>::type, int>::value));
+      CHECK((std::is_same<etl::nth_type<1, int, long, double>::type, long>::value));
+      CHECK((std::is_same<etl::nth_type<2, int, long, double>::type, double>::value));
+#else
       CHECK((std::is_same<etl::nth_type_t<0, int, long, double>, int>::value));
       CHECK((std::is_same<etl::nth_type_t<1, int, long, double>, long>::value));
       CHECK((std::is_same<etl::nth_type_t<2, int, long, double>, double>::value));
+#endif
     }
   }
 }