PR c++/83942 - wrong unused warning with static_cast.

	* cvt.c (ocp_convert): Call mark_rvalue_use.

From-SVN: r257155

gcc/cp/ChangeLog

+2018-01-29  Jason Merrill  <jason@redhat.com>
+
+	PR c++/83942 - wrong unused warning with static_cast.
+	* cvt.c (ocp_convert): Call mark_rvalue_use.
+
 2018-01-26  Jason Merrill  <jason@redhat.com>
 
 	PR c++/83956 - wrong dtor error with anonymous union

gcc/cp/cvt.c

@@ -691,7 +691,10 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
 
   /* FIXME remove when moving to c_fully_fold model.  */
   if (!CLASS_TYPE_P (type))
+    {
+      e = mark_rvalue_use (e);
       e = scalar_constant_value (e);
+    }
   if (error_operand_p (e))
     return error_mark_node;
 

gcc/testsuite/g++.dg/warn/Wunused-var30.C

+// PR c++/83942
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-Wall" }
+
+enum class E { E1 };
+int main() {
+    E const e = E::E1;
+    return static_cast<int>(e);
+}