--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if17.C	Thu Oct 25 07:37:49 2018 +0900
@@ -0,0 +1,32 @@
+// PR c++/85149
+// { dg-do run { target c++17 } }
+
+template <typename T> struct is_void { static constexpr bool value = false; };
+template <> struct is_void<void> { static constexpr bool value = true; };
+
+template<typename S, typename T>
+constexpr decltype(auto) pipeline(S source, T target)
+{
+  return [=](auto... args)
+    {
+      if constexpr(false
+		   && is_void<decltype(source(args...))>::value)
+	{
+	  source(args...);
+	  return target();
+	}
+      else
+	{
+	  return target(source(args...));
+        }
+    };
+}
+
+int main() {
+  int i = 10;
+  int j = 42;
+  auto p = pipeline([&]{ return j; },
+		    [=](int val){ return val * i; });
+  if (p() != 420)
+    __builtin_abort();
+}