PR c++/85006 - -fconcepts ICE with A<auto...> return type

	* pt.c (tsubst_pack_expansion): Allow unsubstituted auto pack.

From-SVN: r259100

gcc/cp/ChangeLog

 2018-04-04  Jason Merrill  <jason@redhat.com>
 
+	PR c++/85006 - -fconcepts ICE with A<auto...> return type
+	* pt.c (tsubst_pack_expansion): Allow unsubstituted auto pack.
+
 	PR c++/85200 - ICE with constexpr if in generic lambda.
 	* tree.c (cp_walk_subtrees): Walk into DECL_EXPR in templates.
 

gcc/cp/pt.c

@@ -11860,7 +11860,7 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 	  /* We can't substitute for this parameter pack.  We use a flag as
 	     well as the missing_level counter because function parameter
 	     packs don't have a level.  */
-	  gcc_assert (processing_template_decl);
+	  gcc_assert (processing_template_decl || is_auto (parm_pack));
 	  unsubstituted_packs = true;
 	}
     }

gcc/testsuite/g++.dg/concepts/auto4.C

+// PR c++/85006
+// { dg-additional-options "-std=c++17 -fconcepts" }
+
+template<typename... Ts> struct A {};
+
+template<typename... Us> A<auto...> foo() { return A{}; }
+
+void bar()
+{
+  foo();
+}