re PR c++/85827 (false positive for -Wunused-but-set-variable because of constexpr-if)

PR c++/85827
g++.dg/cpp1z/constexpr-if29.C: New test.

From-SVN: r274587

gcc/cp/ChangeLog

+2019-08-16  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/85827
+	* g++.dg/cpp1z/constexpr-if29.C: New test.
+
 2019-08-15  Jason Merrill  <jason@redhat.com>
 
 	PR c++/90393 - ICE with thow in ?:

gcc/testsuite/g++.dg/cpp1z/constexpr-if29.C

+// PR c++/85827
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wunused-variable -Wunused-parameter" }
+
+template <int N> int f()
+{
+    constexpr bool _1 = N == 1;
+    constexpr bool _2 = N == 2;
+    constexpr bool _3 = N == 3;
+    
+    if constexpr (_1) {
+        return 5;
+    } else if constexpr (_2) {
+        return 1;
+    } else if constexpr (_3) {
+        return 7;
+    }
+}
+
+int a() {
+    return f<1>();
+}
+int b() {
+    return f<2>();
+}
+int c() {
+    return f<3>();
+}