c++: alias with same name as base fn [PR91706]

This is a bit complex.  Looking up c<T> in the definition of D::c finds
C::c, OK.  Looking up c in the definition of E finds D::c, OK.  Since the
alias is not dependent, we strip it from the template argument, leaving

using E = A<decltype(c<T>())>;

where 'c' still refers to C::c.  But instantiating E looks up 'c' again and
finds D::c, which isn't a function, and sadness ensues.

I think the bug here is looking up 'c' in D at instantiation time; the
declaration we found before is not dependent.  This seems to happen because
baselink_for_fns gets BASELINK_BINFO wrong; it is supposed to be the base
where lookup found the functions, C in this case.

gcc/cp/ChangeLog:

	PR c++/91706
	* semantics.c (baselink_for_fns): Fix BASELINK_BINFO.

gcc/testsuite/ChangeLog:

	PR c++/91706
	* g++.dg/template/lookup17.C: New test.

2 files changed, 22 insertions, 2 deletions

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index d08c1dd..f506a23 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -3663,8 +3663,10 @@ baselink_for_fns (tree fns)
   cl = currently_open_derived_class (scope);
   if (!cl)
     cl = scope;
-  cl = TYPE_BINFO (cl);
-  return build_baselink (cl, cl, fns, /*optype=*/NULL_TREE);
+  tree access_path = TYPE_BINFO (cl);
+  tree conv_path = (cl == scope ? access_path
+		    : lookup_base (cl, scope, ba_any, NULL, tf_none));
+  return build_baselink (conv_path, access_path, fns, /*optype=*/NULL_TREE);
 }
 
 /* Returns true iff DECL is a variable from a function outside
diff --git a/gcc/testsuite/g++.dg/template/lookup17.C b/gcc/testsuite/g++.dg/template/lookup17.C
new file mode 100644
index 0000000..b8571b9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/lookup17.C
@@ -0,0 +1,18 @@
+// PR c++/91706
+// { dg-do compile { target c++11 } }
+// { dg-additional-options -g }
+
+template <bool> struct A;
+
+struct B { static constexpr bool g = false; };
+
+struct C {
+  template <typename> static B c ();
+};
+
+template <class T> struct D : C {
+  using c = decltype (c<T>());
+  using E = A<c::g>;
+};
+
+D<int> g;