From 5d4d0be8132cb837ce075e28f14d7ef8dcc02767 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Mon, 18 Jun 2018 14:16:38 -0400
Subject: [PATCH] PR c++/86171 - ICE with recursive alias instantiation.

	* pt.c (tsubst_decl): Handle recursive alias instantiation.

From-SVN: r261709
---
 gcc/cp/ChangeLog                           |  5 +++++
 gcc/cp/pt.c                                | 10 +++++++++-
 gcc/testsuite/g++.dg/cpp0x/alias-decl-65.C | 10 ++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-65.C

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index c3b5bc628b0..b1a449f91e2 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,8 @@
+2018-06-18  Jason Merrill  <jason@redhat.com>
+
+	PR c++/86171 - ICE with recursive alias instantiation.
+	* pt.c (tsubst_decl): Handle recursive alias instantiation.
+
 2018-06-18  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* decl.c (duplicate_decls): Consistently use DECL_SOURCE_LOCATION
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 4ee84b201e9..6e590d4d342 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13639,7 +13639,6 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 	  }
 
 	/* Create a new node for the specialization we need.  */
-	r = copy_decl (t);
 	if (type == NULL_TREE)
 	  {
 	    if (is_typedef_decl (t))
@@ -13664,7 +13663,16 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 		  sub_args = strip_innermost_template_args (args, extra);
 	      }
 	    type = tsubst (type, sub_args, complain, in_decl);
+	    /* Substituting the type might have recursively instantiated this
+	       same alias (c++/86171).  */
+	    if (gen_tmpl && DECL_ALIAS_TEMPLATE_P (gen_tmpl)
+		&& (spec = retrieve_specialization (gen_tmpl, argvec, hash)))
+	      {
+		r = spec;
+		break;
+	      }
 	  }
+	r = copy_decl (t);
 	if (VAR_P (r))
 	  {
 	    DECL_INITIALIZED_P (r) = 0;
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-65.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-65.C
new file mode 100644
index 00000000000..e320f3eedd0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-65.C
@@ -0,0 +1,10 @@
+// PR c++/86171
+// { dg-do compile { target c++11 } }
+
+template <class> struct A;
+template <class T> using B = typename A<T>::X;
+template <class T> struct A {
+  typedef int X;
+  typedef B<T> U;
+};
+B<short> b;
-- 
2.30.2