c++: cv-qualified dependent name of alias tmpl [PR100592]

Here, the dependent template name in the return type of f() resolves to
an alias of int& after substitution, and we end up complaining about
qualifying this reference type with 'const' from cp_build_qualified_type
rather than just silently dropping the qualification as per [dcl.ref]/1.

The problem is ultimately that make_typename_type ignores the
tf_keep_type_decl flag when the dependent name is a template-id.  This
in turn causes the TYPE_DECL check within tsubst <case TYPENAME_TYPE>
to fail, and so we end up not passing tf_ignore_bad_quals to
cp_build_qualified_type.  This patch fixes this by making
make_typename_type respect the tf_keep_type_decl flag in this situation.

	PR c++/100592

gcc/cp/ChangeLog:

	* decl.c (make_typename_type): After calling
	lookup_template_class, adjust the result to its TYPE_NAME and
	then consider the tf_keep_type_decl flag.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-71.C: New test.

2 files changed, 22 insertions, 4 deletions

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index fb21a3a..a3687db 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -4136,10 +4136,15 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
     return error_mark_node;
 
   if (want_template)
-    return lookup_template_class (t, TREE_OPERAND (fullname, 1),
-				  NULL_TREE, context,
-				  /*entering_scope=*/0,
-				  complain | tf_user);
+    {
+      t = lookup_template_class (t, TREE_OPERAND (fullname, 1),
+				 NULL_TREE, context,
+				 /*entering_scope=*/0,
+				 complain | tf_user);
+      if (t == error_mark_node)
+	return error_mark_node;
+      t = TYPE_NAME (t);
+    }
   
   if (DECL_ARTIFICIAL (t) || !(complain & tf_keep_type_decl))
     t = TREE_TYPE (t);
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-71.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-71.C
new file mode 100644
index 0000000..6a61f93
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-71.C
@@ -0,0 +1,13 @@
+// PR c++/100592
+// { dg-do compile { target c++11 } }
+
+template<bool>
+struct meta {
+  template<class> using if_c = int&;
+};
+
+template<bool B>
+typename meta<B>::template if_c<void> const f();
+
+using type = decltype(f<true>());
+using type = int&;