From dffd86544ce2e8e76350fed7a93aacb0fd97aaa8 Mon Sep 17 00:00:00 2001
From: John Wellbelove <john.wellbelove@asterconsulting.co.uk>
Date: Sat, 12 Jul 2025 12:52:42 +0100
Subject: [PATCH] Added etl::type_lists_are_convertible

---
 include/etl/type_list.h | 29 +++++++++++++++++++++++++++++
 test/test_type_list.cpp | 29 ++++++++++++++++++++++++++++-
 2 files changed, 57 insertions(+), 1 deletion(-)

diff --git a/include/etl/type_list.h b/include/etl/type_list.h
index 4dbc66ec..2042ff79 100644
--- a/include/etl/type_list.h
+++ b/include/etl/type_list.h
@@ -287,6 +287,35 @@ namespace etl
 
   template <typename... TypeLists>
   using type_list_cat_t = typename type_list_cat<TypeLists...>::type;
+
+  //***************************************************************************
+  /// Checks that two type lists are convertible.
+  /// Static asserts if the type lists are not the same length.
+  //***************************************************************************
+  // Primary template
+  template <typename TFromList, typename TToList>
+  struct type_lists_are_convertible;
+
+  // Specialization: both lists empty, convertible
+  template <>
+  struct type_lists_are_convertible<etl::type_list<>, etl::type_list<>> 
+    : public etl::true_type
+  {
+  };
+
+  // Recursive case: check head types, then recurse
+  template <typename TFromHead, typename... TFromTail, typename TToHead, typename... TToTail>
+  struct type_lists_are_convertible<etl::type_list<TFromHead, TFromTail...>, etl::type_list<TToHead, TToTail...>> 
+    : public etl::bool_constant<etl::is_convertible<TFromHead, TToHead>::value && 
+                                etl::type_lists_are_convertible<etl::type_list<TFromTail...>, etl::type_list<TToTail...>>::value>
+  {
+    static_assert(sizeof...(TFromTail) == sizeof...(TToTail), "Type lists are not the same length");
+  };
+
+#if ETL_USING_CPP17
+  template <typename TFromList, typename TToList>
+  inline constexpr bool type_lists_are_convertible_v = etl::type_lists_are_convertible<TFromList, TToList>::value;
+#endif
 }
 #endif
 
diff --git a/test/test_type_list.cpp b/test/test_type_list.cpp
index de9be9fc..5cd253d0 100644
--- a/test/test_type_list.cpp
+++ b/test/test_type_list.cpp
@@ -5,7 +5,7 @@ Embedded Template Library.
 https://github.com/ETLCPP/etl
 https://www.etlcpp.com
 
-Copyright(c) 2025 BMW AG
+Copyright(c) 2025 BMW AG, John Wellbelove
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files(the "Software"), to deal
@@ -180,6 +180,33 @@ namespace
 #endif
     }
 
+    //*************************************************************************
+    TEST(test_type_lists_are_convertible)
+    {
+      typedef etl::type_list<char,  int,  uint32_t> t1;
+      typedef etl::type_list<short, long, uint64_t> t2;
+      typedef etl::type_list<char,  std::string, uint32_t> t3;
+      typedef etl::type_list<> t4;
+
+      // Uncomment to generate static_assert error.
+      //typedef etl::type_list<char,  int> t5;
+
+      CHECK_TRUE((etl::type_lists_are_convertible<t1, t2>::value));
+      CHECK_FALSE((etl::type_lists_are_convertible<t1, t3>::value));
+      CHECK_TRUE((etl::type_lists_are_convertible<t4, t4>::value));
+
+      // Uncomment to generate static_assert error.
+      //CHECK_FALSE((etl::type_lists_are_convertible<t1, t5>::value));
+
+#if ETL_USING_CPP17
+      CHECK_TRUE((etl::type_lists_are_convertible_v<t1, t2>));
+      CHECK_FALSE((etl::type_lists_are_convertible_v<t1, t3>));
+      CHECK_TRUE((etl::type_lists_are_convertible_v<t4, t4>));
+
+      // Uncomment to generate static_assert error.
+      //CHECK_FALSE((etl::type_lists_are_convertible_v<t1, t5>));
+#endif
+    }
   };
 #endif
 }