Add absl::is_constant_evaluated for pre-c++20 support

PiperOrigin-RevId: 503437019
Change-Id: I3630fec690f1472130fef21b16dfcd3c5208aa69

1 files changed, 39 insertions, 0 deletions

diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index dd9de42f..b1656c39 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -844,6 +844,45 @@ template <class T>
 struct is_trivially_relocatable : std::integral_constant<bool, false> {};
 #endif
 
+// absl::is_constant_evaluated()
+//
+// Detects whether the function call occurs within a constant-evaluated context.
+// Returns true if the evaluation of the call occurs within the evaluation of an
+// expression or conversion that is manifestly constant-evaluated; otherwise
+// returns false.
+//
+// This function is implemented in terms of `std::is_constant_evaluated` for
+// c++20 and up. For older c++ versions, the function is implemented in terms
+// of `__builtin_is_constant_evaluated` if available, otherwise the function
+// will fail to compile.
+//
+// Applications can inspect `ABSL_HAVE_CONSTANT_EVALUATED` at compile time
+// to check if this function is supported.
+//
+// Example:
+//
+// constexpr MyClass::MyClass(int param) {
+// #ifdef ABSL_HAVE_CONSTANT_EVALUATED
+//   if (!absl::is_constant_evaluated()) {
+//     ABSL_LOG(INFO) << "MyClass(" << param << ")";
+//   }
+// #endif  // ABSL_HAVE_CONSTANT_EVALUATED
+// }
+//
+// Upstream documentation:
+//
+// http://en.cppreference.com/w/cpp/types/is_constant_evaluated
+// http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#:~:text=__builtin_is_constant_evaluated
+//
+#if defined(ABSL_HAVE_CONSTANT_EVALUATED)
+constexpr bool is_constant_evaluated() noexcept {
+#ifdef __cpp_lib_is_constant_evaluated
+  return std::is_constant_evaluated();
+#elif ABSL_HAVE_BUILTIN(__builtin_is_constant_evaluated)
+  return __builtin_is_constant_evaluated();
+#endif
+}
+#endif  // ABSL_HAVE_CONSTANT_EVALUATED
 ABSL_NAMESPACE_END
 }  // namespace absl