PR c++/86485 - simple_empty_class_p

Yet another tweak that would have fixed this bug: we should treat INIT_EXPR
and MODIFY_EXPR differently for determining whether this is a simple empty
class copy, since a TARGET_EXPR on the RHS is direct initialization if
INIT_EXPR but copy if MODIFY_EXPR.

	* cp-gimplify.c (simple_empty_class_p): Also true for MODIFY_EXPR.

From-SVN: r271521

gcc/cp/ChangeLog

+2019-05-22  Jason Merrill  <jason@redhat.com>
+
+	PR c++/86485 - -Wmaybe-unused with empty class ?:
+	* cp-gimplify.c (simple_empty_class_p): Also true for MODIFY_EXPR.
+
 2019-05-21  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* parser.c (cp_parser_template_declaration_after_parameters): Use

gcc/cp/cp-gimplify.c

@@ -594,19 +594,20 @@ gimplify_must_not_throw_expr (tree *expr_p, gimple_seq *pre_p)
    return slot optimization alone because it isn't a copy.  */
 
 static bool
-simple_empty_class_p (tree type, tree op)
+simple_empty_class_p (tree type, tree op, tree_code code)
 {
+  if (TREE_CODE (op) == COMPOUND_EXPR)
+    return simple_empty_class_p (type, TREE_OPERAND (op, 1), code);
   return
-    ((TREE_CODE (op) == COMPOUND_EXPR
-      && simple_empty_class_p (type, TREE_OPERAND (op, 1)))
-     || TREE_CODE (op) == EMPTY_CLASS_EXPR
+    (TREE_CODE (op) == EMPTY_CLASS_EXPR
+     || code == MODIFY_EXPR
      || is_gimple_lvalue (op)
      || INDIRECT_REF_P (op)
      || (TREE_CODE (op) == CONSTRUCTOR
-	 && CONSTRUCTOR_NELTS (op) == 0
-	 && !TREE_CLOBBER_P (op))
+	 && CONSTRUCTOR_NELTS (op) == 0)
      || (TREE_CODE (op) == CALL_EXPR
 	 && !CALL_EXPR_RETURN_SLOT_OPT (op)))
+    && !TREE_CLOBBER_P (op)
     && is_really_empty_class (type, /*ignore_vptr*/true);
 }
 
@@ -715,7 +716,7 @@ cp_gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
 	  TREE_OPERAND (*expr_p, 1) = build1 (VIEW_CONVERT_EXPR,
 					      TREE_TYPE (op0), op1);
 
-	else if (simple_empty_class_p (TREE_TYPE (op0), op1))
+	else if (simple_empty_class_p (TREE_TYPE (op0), op1, code))
 	  {
 	    /* Remove any copies of empty classes.  Also drop volatile
 	       variables on the RHS to avoid infinite recursion from