PR c++/85963 - -Wunused-but-set with ?: in template.

	* pt.c (tsubst_copy_and_build) [COND_EXPR]: Call mark_rvalue_use.

From-SVN: r261458

gcc/cp/ChangeLog

+2018-06-11  Jason Merrill  <jason@redhat.com>
+
+	PR c++/85963 - -Wunused-but-set with ?: in template.
+	* pt.c (tsubst_copy_and_build) [COND_EXPR]: Call mark_rvalue_use.
+
 2018-06-11  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* decl.c (grok_op_properties): Consistently use the location

gcc/cp/pt.c

@@ -18511,6 +18511,7 @@ tsubst_copy_and_build (tree t,
     case COND_EXPR:
       {
 	tree cond = RECUR (TREE_OPERAND (t, 0));
+	cond = mark_rvalue_use (cond);
 	tree folded_cond = fold_non_dependent_expr (cond);
 	tree exp1, exp2;
 

gcc/testsuite/g++.dg/warn/Wunused-var-34.C

+// PR c++/85963
+// { dg-additional-options -Wall }
+
+template<typename T>
+struct foo {
+  T val, alpha;
+  foo() : val(0), alpha(0) {}
+};
+
+template<typename T>
+inline void bar(const foo<T>& A, const foo<T>& B, foo<T>& C) {
+  const bool use_alpha = true;
+  const T        alpha = use_alpha ? (A.alpha * B.alpha) : T(0);
+  
+  C.val   = A.val * B.val;
+  C.alpha = alpha;
+}
+
+
+int main() {
+  foo<double> A,B,C;
+  
+  bar(A,B,C);
+  
+  return 0;
+}
+