Add etl::is_constant_evaluated (#1198)

Co-authored-by: John Wellbelove <[email protected]>

Author: rolandreichweinbmw

include/etl/profiles/determine_builtin_support.h

@@ -55,6 +55,10 @@ SOFTWARE.
   #if !defined(ETL_USING_BUILTIN_UNDERLYING_TYPE)
     #define ETL_USING_BUILTIN_UNDERLYING_TYPE 1
   #endif
+
+  #if !defined(ETL_USING_BUILTIN_IS_CONSTANT_EVALUATED)
+    #define ETL_USING_BUILTIN_IS_CONSTANT_EVALUATED 1
+  #endif
 #endif
 
 #if defined(__has_builtin) && !defined(ETL_COMPILER_MICROSOFT) // Use __has_builtin to check for existence of builtin functions? Fix VS2022 intellisense issue.
@@ -82,6 +86,10 @@ SOFTWARE.
     #define ETL_USING_BUILTIN_UNDERLYING_TYPE __has_builtin(__underlying_type)
   #endif
 
+  #if !defined(ETL_USING_BUILTIN_IS_CONSTANT_EVALUATED)
+    #define ETL_USING_BUILTIN_IS_CONSTANT_EVALUATED __has_builtin(__builtin_is_constant_evaluated)
+  #endif
+
   #if !defined(ETL_USING_BUILTIN_MEMCPY)
     #define ETL_USING_BUILTIN_MEMCPY __has_builtin(__builtin_memcpy)
    #endif
@@ -128,6 +136,10 @@ SOFTWARE.
   #define ETL_USING_BUILTIN_UNDERLYING_TYPE 0
 #endif
 
+#if !defined(ETL_USING_BUILTIN_IS_CONSTANT_EVALUATED)
+  #define ETL_USING_BUILTIN_IS_CONSTANT_EVALUATED 0
+#endif
+
 #if !defined(ETL_USING_BUILTIN_MEMCPY)
   #define ETL_USING_BUILTIN_MEMCPY 0
 #endif
@@ -160,6 +172,7 @@ namespace etl
     static ETL_CONSTANT bool using_builtin_is_trivially_destructible  = (ETL_USING_BUILTIN_IS_TRIVIALLY_DESTRUCTIBLE == 1);
     static ETL_CONSTANT bool using_builtin_is_trivially_copyable      = (ETL_USING_BUILTIN_IS_TRIVIALLY_COPYABLE == 1);
     static ETL_CONSTANT bool using_builtin_underlying_type            = (ETL_USING_BUILTIN_UNDERLYING_TYPE == 1);
+    static ETL_CONSTANT bool using_builtin_is_constant_evaluated      = (ETL_USING_BUILTIN_IS_CONSTANT_EVALUATED == 1);
     static ETL_CONSTANT bool using_builtin_memcpy                     = (ETL_USING_BUILTIN_MEMCPY == 1);
     static ETL_CONSTANT bool using_builtin_memmove                    = (ETL_USING_BUILTIN_MEMMOVE == 1);
     static ETL_CONSTANT bool using_builtin_memset                     = (ETL_USING_BUILTIN_MEMSET == 1);

include/etl/type_traits.h

@@ -2482,6 +2482,28 @@ typedef integral_constant<bool, true>  true_type;
   template <typename T, template <typename...> class Template>
   inline constexpr bool is_specialization_v = etl::is_specialization<T, Template>::value;
 #endif
+
+  //*********************************************
+  // is_constant_evaluated
+  ETL_CONSTEXPR inline bool is_constant_evaluated() ETL_NOEXCEPT
+  {
+#if ETL_USING_CPP23
+    if consteval
+    {
+      return true;
+    }
+    else
+    {
+      return false;
+    }
+#elif ETL_USING_BUILTIN_IS_CONSTANT_EVALUATED == 1
+    // fallback for C++20 on supported compilers
+    return __builtin_is_constant_evaluated();
+#else
+    // default if unsupported
+    return false;
+#endif
+  }
 }
 
 // Helper macros

test/test_type_traits.cpp

@@ -1472,4 +1472,22 @@ namespace
     CHECK_FALSE((etl::is_specialization<other_specialized<int>, specialized>::value));
 #endif
   }
+
+#if ETL_USING_CPP11
+  //*************************************************************************
+  TEST(test_is_constant_evaluated)
+  {
+    const bool c0 = etl::is_constant_evaluated();
+#if !ETL_USING_CPP23 && defined(ETL_COMPILER_MICROSOFT)
+    // Not supported on MSVC via __has_builtin, see determine_builtin_support.h
+    CHECK_FALSE(c0);
+#else
+    CHECK_TRUE(c0);
+#endif
+
+    volatile int i = 1;
+    const bool c1 = (((i == 1) && etl::is_constant_evaluated()) ? true : false);
+    CHECK_FALSE(c1);
+  }
+#endif
 }