commit 5b8341e2a06c9dead76cba2485a2e0633badbac0
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Aug 11 11:15:13 2016 -0400

    	PR c++/73456 - ICE with constrained parameter pack.
    
    	* logic.cc (non_atomic_constraint_p): Handle EXPR_PACK_EXPANSION.

diff --git a/gcc/cp/logic.cc b/gcc/cp/logic.cc
index dda98df..b86e740 100644
--- a/gcc/cp/logic.cc
+++ b/gcc/cp/logic.cc
@@ -305,6 +305,9 @@ non_atomic_constraint_p (tree t)
     case ICONV_CONSTR:
     case DEDUCT_CONSTR:
     case EXCEPT_CONSTR:
+      /* A pack expansion isn't atomic, but it can't decompose to prove an
+	 atom, so it shouldn't cause analyze_atom to return undecided.  */
+    case EXPR_PACK_EXPANSION:
       return false;
     case CHECK_CONSTR:
     case PARM_CONSTR:
diff --git a/gcc/testsuite/g++.dg/concepts/variadic4.C b/gcc/testsuite/g++.dg/concepts/variadic4.C
new file mode 100644
index 0000000..d20fa7d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/variadic4.C
@@ -0,0 +1,20 @@
+// PR c++/73456
+// { dg-options "-std=c++1z -fconcepts" }
+
+template<typename...> struct list {};
+
+template<typename Seq>
+concept bool Sequence = true;
+
+template<Sequence... Seqs>
+struct zip;
+
+template<Sequence... Seqs>
+    requires requires { typename list<Seqs...>; }
+// main.cpp:12:8: internal compiler error: in non_atomic_constraint_p, at cp/logic.cc:315
+struct zip<Seqs...> {};
+
+int main()
+{
+    zip<list<>, list<int>> {};
+}