[base] Add base::is_constant_evaluated()

This change adds base::is_constant_evaluated(), a backport of C++20's
std::is_constant_evaluated(). Its intended use is within constexpr
functions, that can change their implementation depending on whether
or not they are constant evaluated.

Bug: None
Change-Id: I14a01c350d10fc8411fc5574889703af89b4c610
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2831596
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#873925}

base/template_util.h

@@ -12,6 +12,7 @@
 #include <utility>
 #include <vector>
 
+#include "base/compiler_specific.h"
 #include "build/build_config.h"
 
 // Some versions of libstdc++ have partial support for type_traits, but misses
@@ -322,6 +323,19 @@ struct remove_cvref {
 template <typename T>
 using remove_cvref_t = typename remove_cvref<T>::type;
 
+// Implementation of C++20's std::is_constant_evaluated.
+//
+// References:
+// - https://en.cppreference.com/w/cpp/types/is_constant_evaluated
+// - https://wg21.link/meta.const.eval
+constexpr bool is_constant_evaluated() noexcept {
+#if HAS_BUILTIN(__builtin_is_constant_evaluated)
+  return __builtin_is_constant_evaluated();
+#else
+  return false;
+#endif
+}
+
 // Simplified implementation of C++20's std::iter_value_t.
 // As opposed to std::iter_value_t, this implementation does not restrict
 // the type of `Iter` and does not consider specializations of

base/template_util_unittest.cc

@@ -275,6 +275,13 @@ TEST(TemplateUtil, RemoveCvRefT) {
       std::is_same<void (*)(int), remove_cvref_t<void (*)(int)>>::value, "");
 }
 
+TEST(TemplateUtil, IsConstantEvaluated) {
+  // base::is_constant_evaluated() should return whether it is evaluated as part
+  // of a constant expression.
+  static_assert(is_constant_evaluated(), "");
+  EXPECT_FALSE(is_constant_evaluated());
+}
+
 }  // namespace
 
 }  // namespace base