C++ PATCH to class using-declaration ambiguity handling

[Jason Merrill]

While working on the inheriting constructors overhaul I noticed that
we were handling this wrong: we were allowing one using-declaration to
hide another, but they should both be added so that the ambiguity is
seen by overload resolution.

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

commit cda993583ed7eb9270b8758c201894122997c516
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Oct 26 13:15:43 2016 -0400

            * class.c (add_method): Allow using-declarations to coexist.

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index d334b7c..a2a9346 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -1094,6 +1094,11 @@ add_method (tree type, tree method, tree using_decl)
       if (TREE_CODE (fn) != TREE_CODE (method))
        continue;
 
+      /* Two using-declarations can coexist, we'll complain about ambiguity in
+        overload resolution.  */
+      if (using_decl && TREE_CODE (fns) == OVERLOAD && OVL_USED (fns))
+       continue;
+
       /* [over.load] Member function declarations with the
         same name and the same parameter types cannot be
         overloaded if any of them is a static member
diff --git a/gcc/testsuite/g++.dg/overload/using4.C 
b/gcc/testsuite/g++.dg/overload/using4.C
new file mode 100644
index 0000000..e4ee823
--- /dev/null
+++ b/gcc/testsuite/g++.dg/overload/using4.C
@@ -0,0 +1,19 @@
+struct A
+{
+  void f();
+};
+
+struct B
+{
+  void f();
+};
+
+struct C: A,B {
+  using A::f;
+  using B::f;
+};
+
+int main()
+{
+  C().f();                     // { dg-error "ambiguous" }
+}