--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,79 @@
+// PR c++/81676 - bogus -Wunused warnings in constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wall -Wextra" }
+
+template <typename T> int
+f1 (T v)
+{
+  T x = 0;
+  if constexpr(sizeof(T) == sizeof(int))
+    return v + x;
+  else
+    return 0;
+}
+
+template <typename T> int
+f2 (T v) // { dg-warning "unused parameter .v." }
+{
+  T x = 0;
+  if constexpr(sizeof(T) == sizeof(int))
+    return x;
+  else
+    return 0;
+}
+
+template <typename T> int
+f3 (T v)
+{
+  T x = 0; // { dg-warning "unused variable .x." }
+  if constexpr(sizeof(T) == sizeof(int))
+    return v;
+  else
+    return 0;
+}
+
+template <typename T> int
+f4 (T v)
+{
+  T x = 0;
+  if constexpr(sizeof(T) == sizeof(int))
+    return 0;
+  else
+    return v + x;
+}
+
+template <typename T> int
+f5 (T v) // { dg-warning "unused parameter .v." }
+{
+  T x = 0;
+  if constexpr(sizeof(T) == sizeof(int))
+    return 0;
+  else
+    return x;
+}
+
+template <typename T> int
+f6 (T v)
+{
+  T x = 0; // { dg-warning "unused variable .x." }
+  if constexpr(sizeof(T) == sizeof(int))
+    return 0;
+  else
+    return v;
+}
+
+int main()
+{
+  f1(0);
+  f1('a');
+  f2(0);
+  f2('a');
+  f3(0);
+  f3('a');
+  f4(0);
+  f4('a');
+  f5(0);
+  f5('a');
+  f6(0);
+  f6('a');
+}