commit 0ff050a998827c27f216c373331d643b7875d5a7
Author: jason <jason@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Sat Mar 10 03:35:17 2018 +0000

            PR c++/84785 - ICE with alias template and default targs.
    
            * pt.c (type_unification_real): Set processing_template_decl if
            saw_undeduced == 1.
    
    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/branches/gcc-7-branch@258409 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 9815df63c28..b4032550849 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -19490,7 +19490,13 @@ type_unification_real (tree tparms,
 	  location_t save_loc = input_location;
 	  if (DECL_P (parm))
 	    input_location = DECL_SOURCE_LOCATION (parm);
+
+	  if (saw_undeduced == 1)
+	    ++processing_template_decl;
 	  arg = tsubst_template_arg (arg, full_targs, complain, NULL_TREE);
+	  if (saw_undeduced == 1)
+	    --processing_template_decl;
+
 	  if (arg != error_mark_node && !uses_template_parms (arg))
 	    arg = convert_template_argument (parm, arg, full_targs, complain,
 					     i, NULL_TREE);
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-63.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-63.C
new file mode 100644
index 00000000000..04fb42d9e09
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-63.C
@@ -0,0 +1,18 @@
+// PR c++/84785
+// { dg-do compile { target c++11 } }
+
+template <typename> struct A;
+template <bool> struct B;
+template <bool B, typename> using enable_if_t = typename B<B>::type;
+template <long> using type_pack_element = int;
+struct variant {
+  variant() {}
+  template <typename Arg, long I = Arg::type::value,
+            typename = type_pack_element<I>, enable_if_t<A<Arg>::value, int>>
+  variant(Arg &&);
+};
+
+struct S {
+  variant var;
+};
+int main() { S s; }