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

PiperOrigin-RevId: 503437019
Change-Id: I3630fec690f1472130fef21b16dfcd3c5208aa69

absl/base/config.h

@@ -928,4 +928,15 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_INTERNAL_HAVE_ARM_NEON 1
 #endif
 
+// ABSL_HAVE_CONSTANT_EVALUATED is used for compile-time detection of
+// constant evaluation support through `absl::is_constant_evaluated`.
+#ifdef ABSL_HAVE_CONSTANT_EVALUATED
+#error ABSL_HAVE_CONSTANT_EVALUATED cannot be directly set
+#endif
+#ifdef __cpp_lib_is_constant_evaluated
+#define ABSL_HAVE_CONSTANT_EVALUATED 1
+#elif ABSL_HAVE_BUILTIN(__builtin_is_constant_evaluated)
+#define ABSL_HAVE_CONSTANT_EVALUATED 1
+#endif
+
 #endif  // ABSL_BASE_CONFIG_H_

absl/meta/BUILD.bazel

@@ -42,7 +42,9 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":type_traits",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/time",
         "@com_google_googletest//:gtest_main",
     ],
 )

absl/meta/CMakeLists.txt

@@ -34,6 +34,8 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::config
+    absl::time
     absl::core_headers
     absl::type_traits
     GTest::gmock_main

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
 

absl/meta/type_traits_test.cc

@@ -22,6 +22,9 @@
 
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
 
 namespace {
 
@@ -1413,4 +1416,33 @@ TEST(TrivallyRelocatable, Sanity) {
   EXPECT_TRUE(absl::is_trivially_relocatable<TrivialAbi>::value);
 }
 
+#ifdef ABSL_HAVE_CONSTANT_EVALUATED
+
+constexpr int64_t NegateIfConstantEvaluated(int64_t i) {
+  if (absl::is_constant_evaluated()) {
+    return -i;
+  } else {
+    return i;
+  }
+}
+
+#endif  // ABSL_HAVE_CONSTANT_EVALUATED
+
+TEST(TrivallyRelocatable, is_constant_evaluated) {
+#ifdef ABSL_HAVE_CONSTANT_EVALUATED
+  constexpr int64_t constant = NegateIfConstantEvaluated(42);
+  EXPECT_EQ(constant, -42);
+
+  int64_t now = absl::ToUnixSeconds(absl::Now());
+  int64_t not_constant = NegateIfConstantEvaluated(now);
+  EXPECT_EQ(not_constant, now);
+
+  static int64_t const_init = NegateIfConstantEvaluated(42);
+  EXPECT_EQ(const_init, -42);
+#else
+  GTEST_SKIP() << "absl::is_constant_evaluated is not defined";
+#endif  // ABSL_HAVE_CONSTANT_EVALUATED
+}
+
+
 }  // namespace