--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-decltype3.C	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,25 @@
+// PR c++/88815
+// { dg-do compile { target c++11 } }
+
+struct true_type {
+    constexpr operator bool() const { return true; }
+};
+
+struct false_type {
+    constexpr operator bool() const { return false; }
+};
+
+template<int (*p)()>
+true_type is_constexpr_impl(decltype(int{(p(), 0U)}));
+
+template<int (*p)()>
+false_type is_constexpr_impl(...);
+
+template<int (*p)()>
+using is_constexpr = decltype(is_constexpr_impl<p>(0));
+
+constexpr int f() { return 0; }
+int g() { return 0; }
+
+static_assert(is_constexpr<f>(), "");
+static_assert(!is_constexpr<g>(), "");