diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 42c6a52f033..38b60bc695f 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,11 @@
+2017-02-22  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/79653
+	* parser.c (cp_parser_std_attribute_spec): Don't build the attribute
+	if the alignas expression is erroneous.
+	* pt.c (tsubst_attribute): If tsubst_pack_expansion fails, return
+	error_mark_node.
+
 2017-02-21  Jason Merrill  <jason@redhat.com>
 
 	PR c++/50308 - wrong deprecated warning with ADL
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 3992516de0e..e20257c5bf9 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -24977,12 +24977,17 @@ cp_parser_std_attribute_spec (cp_parser *parser)
       alignas_expr = cxx_alignas_expr (alignas_expr);
       alignas_expr = build_tree_list (NULL_TREE, alignas_expr);
 
+      /* Handle alignas (pack...).  */
       if (cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS))
 	{
 	  cp_lexer_consume_token (parser->lexer);
 	  alignas_expr = make_pack_expansion (alignas_expr);
 	}
 
+      /* Something went wrong, so don't build the attribute.  */
+      if (alignas_expr == error_mark_node)
+	return error_mark_node;
+
       if (cp_parser_require (parser, CPP_CLOSE_PAREN, RT_CLOSE_PAREN) == NULL)
 	{
 	  cp_parser_error (parser, "expected %<)%>");
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 38a01e168a7..5b0f62d3832 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -10012,6 +10012,8 @@ tsubst_attribute (tree t, tree *decl_p, tree args,
       /* An attribute pack expansion.  */
       tree purp = TREE_PURPOSE (t);
       tree pack = tsubst_pack_expansion (val, args, complain, in_decl);
+      if (pack == error_mark_node)
+	return error_mark_node;
       int len = TREE_VEC_LENGTH (pack);
       tree list = NULL_TREE;
       tree *q = &list;
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 395620c6c54..628529b535c 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2017-02-22  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/79653
+	* g++.dg/cpp0x/alignas10.C: New test.
+	* g++.dg/cpp0x/alignas9.C: New test.
+
 2017-02-22  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/70465
diff --git a/gcc/testsuite/g++.dg/cpp0x/alignas10.C b/gcc/testsuite/g++.dg/cpp0x/alignas10.C
new file mode 100644
index 00000000000..164994eb12b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alignas10.C
@@ -0,0 +1,7 @@
+// PR c++/79653
+// { dg-do compile { target c++11 } }
+
+template <typename... A> struct outer {
+  template <typename... B> struct alignas(alignof(A) * alignof(B)...) inner {}; // { dg-error "mismatched argument pack lengths" }
+};
+outer<int>::inner<> mismatched_packs;
diff --git a/gcc/testsuite/g++.dg/cpp0x/alignas9.C b/gcc/testsuite/g++.dg/cpp0x/alignas9.C
new file mode 100644
index 00000000000..98fe7077582
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alignas9.C
@@ -0,0 +1,6 @@
+// PR c++/79653
+// { dg-do compile { target c++11 } }
+
+template <typename... T>
+struct A { alignas(int...) char c; }; // { dg-error "no argument packs|expected" }
+A<int, double> a;