--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated10.C	Mon May 25 07:50:57 2020 +0900
@@ -0,0 +1,30 @@
+// { dg-do compile { target c++2a } }
+// { dg-options "-Wtautological-compare" }
+
+namespace std {
+  constexpr inline bool
+  is_constant_evaluated () noexcept
+  {
+    return __builtin_is_constant_evaluated ();
+  }
+}
+
+template<typename>
+constexpr int
+foo(int i)
+{
+  if constexpr (std::is_constant_evaluated ()) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
+    return 42;
+  else
+    return i;
+}
+
+template<typename>
+constexpr int
+foo2(int i)
+{
+  if constexpr (__builtin_is_constant_evaluated ()) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
+    return 42;
+  else
+    return i;
+}