diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index fc7b6b9ce13..b7a4f4381e4 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2018-03-28  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/85028
+	* pt.c (tsubst_default_argument): Early return if the type of the
+	parameter is erroneous.
+
 2018-03-28  Alexandre Oliva <aoliva@redhat.com>
 
 	PR c++/84973
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index d6cce3e67da..07dad996504 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -12337,6 +12337,9 @@ tsubst_default_argument (tree fn, int parmnum, tree type, tree arg,
   tree parmtype = TREE_TYPE (parm);
   if (DECL_BY_REFERENCE (parm))
     parmtype = TREE_TYPE (parmtype);
+  if (parmtype == error_mark_node)
+    return error_mark_node;
+
   gcc_assert (same_type_ignoring_top_level_qualifiers_p (type, parmtype));
 
   tree *slot;
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 62111d459d3..d8ec226391c 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2018-03-28  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/85028
+	* g++.dg/other/default13.C: New.
+
 2018-03-28  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/85095
diff --git a/gcc/testsuite/g++.dg/other/default13.C b/gcc/testsuite/g++.dg/other/default13.C
new file mode 100644
index 00000000000..eae23ffdf2d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/other/default13.C
@@ -0,0 +1,11 @@
+// PR c++/85028
+
+struct A;
+
+template < typename > struct B
+{
+  B (int, A = A()) : f (0) {}  // { dg-error "incomplete type" }
+  int f;
+};
+
+B < int > b (0);