From e89ebd3e896f27d4afc400044d5a2b69cb524bcb Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Thu, 8 Oct 2020 15:43:26 -0400
Subject: [PATCH] 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.
---
 gcc/cp/pt.c                                     | 9 +++++++--
 gcc/testsuite/g++.dg/cpp0x/alias-decl-pr96805.C | 9 +++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-pr96805.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 2a0f2527d8c5..de7cbaa1d65b 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13397,7 +13397,8 @@ tsubst_aggr_type (tree t,
 					 complain, in_decl);
 	  if (argvec == error_mark_node)
 	    r = error_mark_node;
-	  else if (cxx_dialect >= cxx2a && dependent_scope_p (context))
+	  else if (!entering_scope
+		   && cxx_dialect >= cxx2a && dependent_scope_p (context))
 	    {
 	      /* See maybe_dependent_member_ref.  */
 	      tree name = TYPE_IDENTIFIER (t);
@@ -14082,7 +14083,11 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
     {
       tree new_type;
       ++processing_template_decl;
-      new_type = tsubst (TREE_TYPE (t), args, complain, in_decl);
+      if (CLASS_TYPE_P (TREE_TYPE (t)))
+	new_type = tsubst_aggr_type (TREE_TYPE (t), args, complain,
+				     in_decl, /*entering*/1);
+      else
+	new_type = tsubst (TREE_TYPE (t), args, complain, in_decl);
       --processing_template_decl;
       if (new_type == error_mark_node)
 	return 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 000000000000..c784f2734417
--- /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;
+  };
+};
-- 
2.47.2