c++: Fix member alias template in C++17 and up. [PR96805]

Here we're trying to push into a<T>::c<N> in order to instantiate t<N>, but
were building a TYPENAME_TYPE for it because a<T> isn't open yet.  Don't
do that when we know we're trying to enter the scope.

gcc/cp/ChangeLog:

	PR c++/96805
	PR c++/96199
	* pt.c (tsubst_aggr_type): Don't build a TYPENAME_TYPE when
	entering_scope.
	(tsubst_template_decl): Use tsubst_aggr_type.

gcc/testsuite/ChangeLog:

	PR c++/96805
	* g++.dg/cpp0x/alias-decl-pr96805.C: New test.

2 files changed, 16 insertions, 2 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 9a83810..555dc47 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13451,7 +13451,8 @@ tsubst_aggr_type (tree t,
 					 complain, in_decl);
 	  if (argvec == error_mark_node)
 	    r = error_mark_node;
-	  else if (cxx_dialect >= cxx17 && dependent_scope_p (context))
+	  else if (!entering_scope
+		   && cxx_dialect >= cxx17 && dependent_scope_p (context))
 	    {
 	      /* See maybe_dependent_member_ref.  */
 	      tree name = TYPE_IDENTIFIER (t);
@@ -14160,7 +14161,11 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
 	  class_p = true;
 	  inner = TREE_TYPE (inner);
 	}
-      inner = tsubst (inner, args, complain, in_decl);
+      if (class_p)
+	inner = tsubst_aggr_type (inner, args, complain,
+				  in_decl, /*entering*/1);
+      else
+	inner = tsubst (inner, args, complain, in_decl);
     }
   --processing_template_decl;
   if (inner == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-pr96805.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-pr96805.C
new file mode 100644
index 0000000..c784f27
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-pr96805.C
@@ -0,0 +1,9 @@
+// PR c++/96805
+// { dg-do compile { target c++11 } }
+
+template <typename T> class a {
+  template <int N> struct c {
+    template <bool B> using t = int;
+    t<N> m;
+  };
+};