C++ PATCH for c++/70844, wrong -Wuseless-cast with inheriting constructor

[Jason Merrill]

Let's use warning_sentinel to suppress -Wuseless-cast for this conversion.

Tested x86_64-pc-linux-gnu, applying to trunk and 7.

commit 74a5be8e0ca159667f4b3e87a0b329345afaf0fa
Author: Jason Merrill <ja...@redhat.com>
Date:   Fri Jun 16 23:06:17 2017 -0400

            PR c++/70844 - -Wuseless-cast and inheriting constructor.
    
            * method.c (forward_parm): Suppress warn_useless_cast.

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index 9541fcb..fe4b2af 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -486,6 +486,7 @@ forward_parm (tree parm)
     type = PACK_EXPANSION_PATTERN (type);
   if (TREE_CODE (type) != REFERENCE_TYPE)
     type = cp_build_reference_type (type, /*rval=*/true);
+  warning_sentinel w (warn_useless_cast);
   exp = build_static_cast (type, exp, tf_warning_or_error);
   if (DECL_PACK_P (parm))
     exp = make_pack_expansion (exp);
diff --git a/gcc/testsuite/g++.dg/cpp0x/inh-ctor27.C 
b/gcc/testsuite/g++.dg/cpp0x/inh-ctor27.C
new file mode 100644
index 0000000..ef2ada1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/inh-ctor27.C
@@ -0,0 +1,13 @@
+// PR c++/70844
+// { dg-options -Wuseless-cast }
+// { dg-do compile { target c++11 } }
+
+struct base {
+    base (int const &);
+};
+
+struct derived : public base {
+    using base::base;
+};
+
+derived d(0);