PR c++/86485 - -Wmaybe-unused with empty class ?:

The problem in this testcase is that the front end expects an rvalue
COND_EXPR to initialize a single temporary from one of the arms.  But
because gimplify_cond_expr used MODIFY_EXPR, instead the arms would each
create their own temporary and then copy that into the common temporary.

So, let's use INIT_EXPR instead.

	* gimplify.c (gimplify_cond_expr): Use INIT_EXPR.

From-SVN: r269403

gcc/ChangeLog

+2019-03-02  Jason Merrill  <jason@redhat.com>
+
+	PR c++/86485 - -Wmaybe-unused with empty class ?:
+	* gimplify.c (gimplify_cond_expr): Use INIT_EXPR.
+
 2019-03-05  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/89587

gcc/gimplify.c

@@ -4024,11 +4024,11 @@ gimplify_cond_expr (tree *expr_p, gimple_seq *pre_p, fallback_t fallback)
       /* Build the new then clause, `tmp = then_;'.  But don't build the
 	 assignment if the value is void; in C++ it can be if it's a throw.  */
       if (!VOID_TYPE_P (TREE_TYPE (then_)))
-	TREE_OPERAND (expr, 1) = build2 (MODIFY_EXPR, type, tmp, then_);
+	TREE_OPERAND (expr, 1) = build2 (INIT_EXPR, type, tmp, then_);
 
       /* Similarly, build the new else clause, `tmp = else_;'.  */
       if (!VOID_TYPE_P (TREE_TYPE (else_)))
-	TREE_OPERAND (expr, 2) = build2 (MODIFY_EXPR, type, tmp, else_);
+	TREE_OPERAND (expr, 2) = build2 (INIT_EXPR, type, tmp, else_);
 
       TREE_TYPE (expr) = void_type_node;
       recalculate_side_effects (expr);

gcc/testsuite/g++.dg/init/empty2.C

+// PR c++/86485
+// { dg-additional-options -Wmaybe-uninitialized }
+
+struct E {};
+struct S { S () {} E e; };
+void foo (S);
+
+void
+bar (bool b)
+{
+  foo (b ? S () : S ());
+}