From 555d710f706e31751def9e26a1af266c4d5268fd Mon Sep 17 00:00:00 2001
From: "Chiang, Yi" <yi.chiang@bmw.de>
Date: Wed, 5 Mar 2025 11:03:53 +0100
Subject: [PATCH] Support etl::underlying_type with compiler builtin

msvc is unsupported currently
---
 .../etl/generators/type_traits_generator.h    | 35 ++++++++++++++++
 .../etl/profiles/determine_builtin_support.h  | 12 ++++++
 include/etl/type_traits.h                     | 35 ++++++++++++++++
 test/test_type_traits.cpp                     | 42 ++++++++++++++++++-
 4 files changed, 123 insertions(+), 1 deletion(-)

diff --git a/include/etl/generators/type_traits_generator.h b/include/etl/generators/type_traits_generator.h
index e55d12217..e09432f60 100644
--- a/include/etl/generators/type_traits_generator.h
+++ b/include/etl/generators/type_traits_generator.h
@@ -2302,6 +2302,41 @@ typedef integral_constant<bool, true>  true_type;
   using type_identity_t = typename type_identity<T>::type;
 #endif
 
+#if ETL_USING_BUILTIN_UNDERLYING_TYPE
+namespace private_type_traits
+{
+  template <typename T, bool = is_enum<T>::value>
+  struct __underlying_type_impl;
+
+  template <typename T>
+  struct __underlying_type_impl<T, false>
+  {
+  };
+
+  template <typename T>
+  struct __underlying_type_impl<T, true>
+  {
+    using type = __underlying_type(T);
+  };
+}
+
+template <typename T>
+struct underlying_type : private_type_traits::__underlying_type_impl<T, is_enum<T>::value>
+{
+};
+
+#if ETL_USING_CPP11
+template <typename T>
+using underlying_type_t = typename underlying_type<T>::type;
+
+template <typename T>
+ETL_CONSTEXPR underlying_type_t<T> to_underlying(T val) ETL_NOEXCEPT
+{
+  return static_cast<underlying_type_t<T>>(val);
+}
+#endif
+#endif
+
 #if ETL_USING_CPP11
   //*********************************************
   // has_duplicates
diff --git a/include/etl/profiles/determine_builtin_support.h b/include/etl/profiles/determine_builtin_support.h
index 725927f32..313a18a20 100644
--- a/include/etl/profiles/determine_builtin_support.h
+++ b/include/etl/profiles/determine_builtin_support.h
@@ -51,6 +51,10 @@ SOFTWARE.
   #if !defined(ETL_USING_BUILTIN_IS_TRIVIALLY_COPYABLE)
     #define ETL_USING_BUILTIN_IS_TRIVIALLY_COPYABLE 1
   #endif
+
+  #if !defined(ETL_USING_BUILTIN_UNDERLYING_TYPE)
+    #define ETL_USING_BUILTIN_UNDERLYING_TYPE 1
+  #endif
 #endif
 
 #if defined(__has_builtin) // Use __has_builtin to check for existence of builtin functions?
@@ -73,6 +77,10 @@ SOFTWARE.
   #if !defined(ETL_USING_BUILTIN_IS_TRIVIALLY_COPYABLE)
     #define ETL_USING_BUILTIN_IS_TRIVIALLY_COPYABLE (__has_builtin(__has_trivial_copy) || __has_builtin(__is_trivially_copyable))
   #endif
+
+  #if !defined(ETL_USING_BUILTIN_UNDERLYING_TYPE)
+    #define ETL_USING_BUILTIN_UNDERLYING_TYPE __has_builtin(__underlying_type)
+  #endif
 #endif
 
 // The default. Set to 0, if not already set.
@@ -96,6 +104,10 @@ SOFTWARE.
   #define ETL_USING_BUILTIN_IS_TRIVIALLY_COPYABLE 0
 #endif
 
+#if !defined(ETL_USING_BUILTIN_UNDERLYING_TYPE)
+  #define ETL_USING_BUILTIN_UNDERLYING_TYPE 0
+#endif
+
 namespace etl
 {
   namespace traits
diff --git a/include/etl/type_traits.h b/include/etl/type_traits.h
index 327f164be..88cf5590c 100644
--- a/include/etl/type_traits.h
+++ b/include/etl/type_traits.h
@@ -2295,6 +2295,41 @@ typedef integral_constant<bool, true>  true_type;
   using type_identity_t = typename type_identity<T>::type;
 #endif
 
+#if ETL_USING_BUILTIN_UNDERLYING_TYPE
+namespace private_type_traits
+{
+  template <typename T, bool = is_enum<T>::value>
+  struct __underlying_type_impl;
+
+  template <typename T>
+  struct __underlying_type_impl<T, false>
+  {
+  };
+
+  template <typename T>
+  struct __underlying_type_impl<T, true>
+  {
+    using type = __underlying_type(T);
+  };
+}
+
+template <typename T>
+struct underlying_type : private_type_traits::__underlying_type_impl<T, is_enum<T>::value>
+{
+};
+
+#if ETL_USING_CPP11
+template <typename T>
+using underlying_type_t = typename underlying_type<T>::type;
+
+template <typename T>
+ETL_CONSTEXPR underlying_type_t<T> to_underlying(T val) ETL_NOEXCEPT
+{
+  return static_cast<underlying_type_t<T>>(val);
+}
+#endif
+#endif
+
 #if ETL_USING_CPP11
   //*********************************************
   // has_duplicates
diff --git a/test/test_type_traits.cpp b/test/test_type_traits.cpp
index 4316a697f..b6466aa53 100644
--- a/test/test_type_traits.cpp
+++ b/test/test_type_traits.cpp
@@ -1329,11 +1329,51 @@ namespace
   }
 
   //*************************************************************************
-  TEST(test_type_identity) 
+  TEST(test_type_identity)
   {
     CHECK_CLOSE(type_identity_test_add(1.5f, 2), 3.5f, 0.01f);
   }
 
+  //*************************************************************************
+#if ETL_USING_BUILTIN_UNDERLYING_TYPE
+  TEST(test_underlying_type)
+  {
+    enum enum0_t : char
+    {
+    };
+
+    enum enum1_t : uint32_t
+    {
+    };
+
+    enum class enum2_t : short
+    {
+    };
+
+    enum class enum3_t : size_t
+    {
+    };
+
+    using enum4_t = enum1_t;
+    using enum5_t = std::add_const<enum2_t>::type;
+
+    CHECK_TRUE((std::is_same<etl::underlying_type<enum0_t>::type, char>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type<enum1_t>::type, uint32_t>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type<enum2_t>::type, short>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type<enum3_t>::type, size_t>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type<enum4_t>::type, uint32_t>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type<enum5_t>::type, short>::value));
+#if ETL_USING_CPP11
+    CHECK_TRUE((std::is_same<etl::underlying_type_t<enum0_t>, char>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type_t<enum1_t>, uint32_t>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type_t<enum2_t>, short>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type_t<enum3_t>, size_t>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type_t<enum4_t>, uint32_t>::value));
+    CHECK_TRUE((std::is_same<etl::underlying_type_t<enum5_t>, short>::value));
+#endif
+  }
+#endif
+
   //*************************************************************************
   TEST(test_has_duplicates)
   {