PR c++/86171 - ICE with recursive alias instantiation.

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

From-SVN: r261709

gcc/cp/ChangeLog

+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

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;

gcc/testsuite/g++.dg/cpp0x/alias-decl-65.C

+// 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;