PR c++/53609 - Wrong argument deduction for pack expansion in argument pack

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index ecb013e..313f7a4 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -201,7 +201,6 @@  static void append_type_to_template_for_access_check_1 (tree, tree, tree,
 static tree listify (tree);
 static tree listify_autos (tree, tree);
 static tree template_parm_to_arg (tree t);
-static bool arg_from_parm_pack_p (tree, tree);
 static tree current_template_args (void);
 static tree tsubst_template_parm (tree, tree, tsubst_flags_t);
 static tree instantiate_alias_template (tree, tree, tsubst_flags_t);
@@ -3308,6 +3307,29 @@  make_pack_expansion (tree arg)
   return result;
 }
 
+/* Return NULL_TREE iff T contains *NO* unexpanded parameter packs.
+   Return the TREE_LIST of unexpanded parameter packs otherwise.  */
+
+static tree
+has_bare_parameter_packs (tree t)
+{
+  tree parameter_packs = NULL_TREE;
+  struct find_parameter_pack_data ppd;
+
+  if (!processing_template_decl || !t || t == error_mark_node)
+    return NULL_TREE;
+
+  if (TREE_CODE (t) == TYPE_DECL)
+    t = TREE_TYPE (t);
+
+  ppd.parameter_packs = &parameter_packs;
+  ppd.visited = pointer_set_create ();
+  cp_walk_tree (&t, &find_parameter_packs_r, &ppd, ppd.visited);
+  pointer_set_destroy (ppd.visited);
+
+  return parameter_packs;
+}
+
 /* Checks T for any "bare" parameter packs, which have not yet been
    expanded, and issues an error if any are found. This operation can
    only be done on full expressions or types (e.g., an expression
@@ -3325,19 +3347,7 @@  make_pack_expansion (tree arg)
 bool 
 check_for_bare_parameter_packs (tree t)
 {
-  tree parameter_packs = NULL_TREE;
-  struct find_parameter_pack_data ppd;
-
-  if (!processing_template_decl || !t || t == error_mark_node)
-    return false;
-
-  if (TREE_CODE (t) == TYPE_DECL)
-    t = TREE_TYPE (t);
-
-  ppd.parameter_packs = &parameter_packs;
-  ppd.visited = pointer_set_create ();
-  cp_walk_tree (&t, &find_parameter_packs_r, &ppd, ppd.visited);
-  pointer_set_destroy (ppd.visited);
+  tree parameter_packs = has_bare_parameter_packs (t);
 
   if (parameter_packs) 
     {
@@ -3812,42 +3822,6 @@  template_parm_to_arg (tree t)
   return t;
 }
 
-/* This function returns TRUE if PARM_PACK is a template parameter
-   pack and if ARG_PACK is what template_parm_to_arg returned when
-   passed PARM_PACK.  */
-
-static bool
-arg_from_parm_pack_p (tree arg_pack, tree parm_pack)
-{
-  /* For clarity in the comments below let's use the representation
-     argument_pack<elements>' to denote an argument pack and its
-     elements.
-
-     In the 'if' block below, we want to detect cases where
-     ARG_PACK is argument_pack<PARM_PACK...>.  I.e, we want to
-     check if ARG_PACK is an argument pack which sole element is
-     the expansion of PARM_PACK.  That argument pack is typically
-     created by template_parm_to_arg when passed a parameter
-     pack.  */
-
-  if (arg_pack
-      && TREE_VEC_LENGTH (ARGUMENT_PACK_ARGS (arg_pack)) == 1
-      && PACK_EXPANSION_P (TREE_VEC_ELT (ARGUMENT_PACK_ARGS (arg_pack), 0)))
-    {
-      tree expansion = TREE_VEC_ELT (ARGUMENT_PACK_ARGS (arg_pack), 0);
-      tree pattern = PACK_EXPANSION_PATTERN (expansion);
-      if ((TYPE_P (pattern) && same_type_p (pattern, parm_pack))
-	  || (!TYPE_P (pattern) && cp_tree_equal (parm_pack, pattern)))
-	/* The argument pack that the parameter maps to is just an
-	   expansion of the parameter itself, such as one would
-	   find in the implicit typedef of a class inside the
-	   class itself.  Consider this parameter "unsubstituted",
-	   so that we will maintain the outer pack expansion.  */
-	return true;
-    }
-  return false;
-}
-
 /* Given a set of template parameters, return them as a set of template
    arguments.  The template parameters are represented as a TREE_VEC, in
    the form documented in cp-tree.h for template arguments.  */
@@ -9095,6 +9069,165 @@  make_fnparm_pack (tree spec_parm)
   return extract_fnparm_pack (NULL_TREE, &spec_parm);
 }
 
+/* Return true iff the Ith element of the argument pack ARG_PACK is a
+   pack expansion.  */
+
+static bool
+argument_pack_element_is_expansion_p (tree arg_pack, int i)
+{
+  tree vec = ARGUMENT_PACK_ARGS (arg_pack);
+  if (i >= TREE_VEC_LENGTH (vec))
+    return false;
+  return PACK_EXPANSION_P (TREE_VEC_ELT (vec, i));
+}
+
+
+/* Creates and return an ARGUMENT_PACK_SELECT tree node.  */
+
+static tree
+make_argument_pack_select (tree arg_pack, unsigned index)
+{
+  tree aps = make_node (ARGUMENT_PACK_SELECT);
+
+  ARGUMENT_PACK_SELECT_FROM_PACK (aps) = arg_pack;
+  ARGUMENT_PACK_SELECT_INDEX (aps) = index;
+
+  return aps;
+}
+
+/*  This is a subroutine of tsubst_pack_expansion.
+
+    It returns TRUE if we need to use the PACK_EXPANSION_EXTRA_ARGS
+    mechanism to store the (non complete list of) arguments of the
+    substitution and return a non substituted pack expansion, in order
+    to wait for when we have enough arguments to really perform the
+    substitution.  */
+
+static bool
+use_pack_expansion_extra_args_p (tree parm_packs,
+				 int arg_pack_len,
+				 bool has_empty_arg)
+{
+  if (parm_packs == NULL_TREE)
+    return false;
+
+  bool has_expansion_arg = false;
+  for (int i = 0 ; i < arg_pack_len; ++i)
+    {
+      bool has_non_expansion_arg = false;
+      for (tree parm_pack = parm_packs;
+	   parm_pack;
+	   parm_pack = TREE_CHAIN (parm_pack))
+	{
+	  tree arg = TREE_VALUE (parm_pack);
+
+	  if (argument_pack_element_is_expansion_p (arg, i))
+	    has_expansion_arg = true;
+	  else
+	    has_non_expansion_arg = true;
+	}
+
+      /* If one pack has an expansion and another pack has a normal
+	 argument or if one pack has an empty argument another one
+	 hasn't then tsubst_pack_expansion cannot perform the
+	 substitution and need to fall back on the
+	 PACK_EXPANSION_EXTRA mechanism.  */
+      if ((has_expansion_arg && has_non_expansion_arg)
+	  || (has_empty_arg && (has_expansion_arg || has_non_expansion_arg)))
+	return true;
+    }
+  return false;
+}
+
+/* [temp.variadic]/6 says that:
+
+       The instantiation of a pack expansion [...]
+       produces a list E1,E2, ..., En, where N is the number of elements
+       in the pack expansion parameters.
+
+   This subroutine of tsubst_pack_expansion produces one of these Ei.
+
+   PATTERN is the pattern of the pack expansion.  PARM_PACKS is a
+   TREE_LIST in which each TREE_PURPOSE is a parameter pack of
+   PATTERN, and each TREE_VALUE is its corresponding argument pack.
+   INDEX is the index 'i' of the element Ei to produce.  ARGS,
+   COMPLAIN, and IN_DECL are the same parameters as for the
+   tsubst_pack_expansion function.
+
+   The function returns the resulting Ei upon successful completion,
+   or error_mark_node.
+
+   Note that this function possibly modifies the ARGS parameter, so
+   it's the responsibility of the caller to restore it.  */
+
+static tree
+gen_elem_of_pack_expansion_instantiation (tree pattern,
+					  tree parm_packs,
+					  unsigned index,
+					  tree args /* This parm gets
+						       modified.  */,
+					  tsubst_flags_t complain,
+					  tree in_decl)
+{
+  tree t;
+
+  /* For each parameter pack, change the substitution of the parameter
+     pack to the ith argument in its argument pack, then expand the
+     pattern.  */
+  for (tree pack = parm_packs; pack; pack = TREE_CHAIN (pack))
+    {
+      tree parm = TREE_PURPOSE (pack);
+      tree arg_pack = TREE_VALUE (pack);
+      tree aps;			/* instance of ARGUMENT_PACK_SELECT
+				   tree.  */
+
+      /* Select the Ith argument from the pack.  */
+      if (TREE_CODE (parm) == PARM_DECL)
+	{
+	  if (index == 0)
+	    {
+	      aps = make_argument_pack_select (arg_pack, index);
+	      mark_used (parm);
+	      register_local_specialization (aps, parm);
+	    }
+	  else
+	    aps = retrieve_local_specialization (parm);
+	}
+      else
+	{
+	  int idx, level;
+	  template_parm_level_and_index (parm, &level, &idx);
+
+	  if (index == 0)
+	    {
+	      aps = make_argument_pack_select (arg_pack, index);
+	      /* Update the corresponding argument.  */
+	      TMPL_ARG (args, level, idx) = aps;
+	    }
+	  else
+	    /* Re-use the ARGUMENT_PACK_SELECT.  */
+	    aps = TMPL_ARG (args, level, idx);
+	}
+      ARGUMENT_PACK_SELECT_INDEX (aps) = index;
+    }
+
+  /* Substitute into the PATTERN with the (possibly altered)
+     arguments.  */
+  if (!TYPE_P (pattern))
+    t = tsubst_expr (pattern, args, complain, in_decl,
+		     /*integral_constant_expression_p=*/false);
+  else
+    t = tsubst (pattern, args, complain, in_decl);
+
+  /*  If the Ith argument pack element is a pack expansion, then
+      the Ith element resulting from the substituting is going to
+      be a pack expansion as well.  */
+  if (has_bare_parameter_packs (t))
+    t = make_pack_expansion (t);
+
+  return t;
+}
+
 /* Substitute ARGS into T, which is an pack expansion
    (i.e. TYPE_PACK_EXPANSION or EXPR_PACK_EXPANSION). Returns a
    TREE_VEC with the substituted arguments, a PACK_EXPANSION_* node
@@ -9107,8 +9240,6 @@  tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
   tree pattern;
   tree pack, packs = NULL_TREE;
   bool unsubstituted_packs = false;
-  bool real_packs = false;
-  int missing_level = 0;
   int i, len = -1;
   tree result;
   struct pointer_map_t *saved_local_specializations = NULL;
@@ -9187,14 +9318,6 @@  tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 	  return result;
 	}
 
-      if (arg_from_parm_pack_p (arg_pack, parm_pack))
-	/* The argument pack that the parameter maps to is just an
-	   expansion of the parameter itself, such as one would find
-	   in the implicit typedef of a class inside the class itself.
-	   Consider this parameter "unsubstituted", so that we will
-	   maintain the outer pack expansion.  */
-	arg_pack = NULL_TREE;
-          
       if (arg_pack)
         {
           int my_len = 
@@ -9222,13 +9345,6 @@  tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
               return error_mark_node;
             }
 
-	  if (TREE_VEC_LENGTH (ARGUMENT_PACK_ARGS (arg_pack)) == 1
-	      && PACK_EXPANSION_P (TREE_VEC_ELT (ARGUMENT_PACK_ARGS (arg_pack),
-						 0)))
-	    /* This isn't a real argument pack yet.  */;
-	  else
-	    real_packs = true;
-
           /* Keep track of the parameter packs and their corresponding
              argument packs.  */
           packs = tree_cons (parm_pack, arg_pack, packs);
@@ -9240,38 +9356,30 @@  tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 	     well as the missing_level counter because function parameter
 	     packs don't have a level.  */
 	  unsubstituted_packs = true;
-	  if (!missing_level || missing_level > level)
-	    missing_level = level;
 	}
     }
 
+  /* We could not find any argument packs that work, so we'll just
+     return an unsubstituted pack expansion.  The caller must be
+     prepared to deal with this.  */
+  if (len < 0)
+    len = 1;
+
+  /*  Do we need to use the PACK_EXPANSION_EXTRA_ARGS mechanism?  */
+  bool use_pack_expansion_extra_args =
+    use_pack_expansion_extra_args_p (packs, len, unsubstituted_packs);
+
   /* We cannot expand this expansion expression, because we don't have
      all of the argument packs we need.  */
-  if (unsubstituted_packs)
+  if (unsubstituted_packs || use_pack_expansion_extra)
     {
-      if (real_packs)
+      if (use_pack_expansion_extra)
 	{
 	  /* We got some full packs, but we can't substitute them in until we
 	     have values for all the packs.  So remember these until then.  */
-	  tree save_args;
 
 	  t = make_pack_expansion (pattern);
-
-	  /* The call to add_to_template_args above assumes no overlap
-	     between saved args and new args, so prune away any fake
-	     args, i.e. those that satisfied arg_from_parm_pack_p above.  */
-	  if (missing_level && levels >= missing_level)
-	    {
-	      gcc_assert (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (args)
-			  && missing_level > 1);
-	      TREE_VEC_LENGTH (args) = missing_level - 1;
-	      save_args = copy_node (args);
-	      TREE_VEC_LENGTH (args) = levels;
-	    }
-	  else
-	    save_args = args;
-
-	  PACK_EXPANSION_EXTRA_ARGS (t) = save_args;
+	  PACK_EXPANSION_EXTRA_ARGS (t) = args;
 	}
       else
 	{
@@ -9289,10 +9397,6 @@  tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
       return t;
     }
 
-  /* We could not find any argument packs that work.  */
-  if (len < 0)
-    return error_mark_node;
-
   if (need_local_specializations)
     {
       /* We're in a late-specified return type, so create our own local
@@ -9306,60 +9410,20 @@  tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
   /* For each argument in each argument pack, substitute into the
      pattern.  */
   result = make_tree_vec (len);
-  for (i = 0; i < len; ++i)
+  if (len > 0)
     {
-      /* For parameter pack, change the substitution of the parameter
-         pack to the ith argument in its argument pack, then expand
-         the pattern.  */
-      for (pack = packs; pack; pack = TREE_CHAIN (pack))
-        {
-          tree parm = TREE_PURPOSE (pack);
-	  tree arg;
-
-	  /* Select the Ith argument from the pack.  */
-          if (TREE_CODE (parm) == PARM_DECL)
-            {
-	      if (i == 0)
-		{
-		  arg = make_node (ARGUMENT_PACK_SELECT);
-		  ARGUMENT_PACK_SELECT_FROM_PACK (arg) = TREE_VALUE (pack);
-		  mark_used (parm);
-		  register_local_specialization (arg, parm);
-		}
-	      else
-		arg = retrieve_local_specialization (parm);
-            }
-          else
-            {
-              int idx, level;
-              template_parm_level_and_index (parm, &level, &idx);
-
-	      if (i == 0)
-		{
-		  arg = make_node (ARGUMENT_PACK_SELECT);
-		  ARGUMENT_PACK_SELECT_FROM_PACK (arg) = TREE_VALUE (pack);
-		  /* Update the corresponding argument.  */
-		  TMPL_ARG (args, level, idx) = arg;
-		}
-	      else
-		/* Re-use the ARGUMENT_PACK_SELECT.  */
-		arg = TMPL_ARG (args, level, idx);
-            }
-	  ARGUMENT_PACK_SELECT_INDEX (arg) = i;
-        }
-
-      /* Substitute into the PATTERN with the altered arguments.  */
-      if (!TYPE_P (pattern))
-        TREE_VEC_ELT (result, i) = 
-          tsubst_expr (pattern, args, complain, in_decl,
-                       /*integral_constant_expression_p=*/false);
-      else
-        TREE_VEC_ELT (result, i) = tsubst (pattern, args, complain, in_decl);
-
-      if (TREE_VEC_ELT (result, i) == error_mark_node)
+      for (i = 0; i < len; ++i)
 	{
-	  result = error_mark_node;
-	  break;
+	  t = gen_elem_of_pack_expansion_instantiation (pattern, packs,
+							i,
+							args, complain,
+							in_decl);
+	  TREE_VEC_ELT (result, i) = t;
+      	  if (t == error_mark_node)
+	    {
+	      result = error_mark_node;
+	      break;
+	    }
 	}
     }
 
@@ -9374,6 +9438,10 @@  tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
       else
         {
           int idx, level;
+
+	  if (TREE_VALUE (pack) == NULL_TREE)
+	    continue;
+
           template_parm_level_and_index (parm, &level, &idx);
           
           /* Update the corresponding argument.  */
@@ -11102,8 +11170,24 @@  tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	    arg = TMPL_ARG (args, level, idx);
 
 	    if (arg && TREE_CODE (arg) == ARGUMENT_PACK_SELECT)
-	      /* See through ARGUMENT_PACK_SELECT arguments. */
-	      arg = ARGUMENT_PACK_SELECT_ARG (arg);
+	      {
+		/* See through ARGUMENT_PACK_SELECT arguments. */
+		arg = ARGUMENT_PACK_SELECT_ARG (arg);
+		/* If the selected argument is an expansion E, that most
+		   likely means we were called from
+		   gen_elem_of_pack_expansion_instantiation during the
+		   substituting of pack an argument pack (which Ith
+		   element is a pack expansion, where I is
+		   ARGUMENT_PACK_SELECT_INDEX) into a pack expansion.
+		   In this case, the Ith element resulting from this
+		   substituting is going to be a pack expansion, which
+		   pattern is the pattern of E.  Let's return the
+		   pattern of E, and
+		   gen_elem_of_pack_expansion_instantiation will
+		   build the resulting pack expansion from it.  */
+		if (PACK_EXPANSION_P (arg))
+		  arg = PACK_EXPANSION_PATTERN (arg);
+	      }
 	  }
 
 	if (arg == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic139.C b/gcc/testsuite/g++.dg/cpp0x/variadic139.C
new file mode 100644
index 0000000..a1c64f3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/variadic139.C
@@ -0,0 +1,14 @@ 
+// Origin: PR c++/53609
+// { dg-do compile { target c++11 } }
+
+template<class...I> struct List {};
+template<int T> struct Z {static const int value = T;};
+template<int...T> using LZ = List<Z<T>...>;
+
+template<class...U>
+struct F
+{
+  using N = LZ<U::value...>;
+};
+
+F<Z<1>, Z<2> >::N A;
diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic140.C b/gcc/testsuite/g++.dg/cpp0x/variadic140.C
new file mode 100644
index 0000000..17ca9e5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/variadic140.C
@@ -0,0 +1,22 @@ 
+// Origin: PR c++/53609
+// { dg-do compile { target c++11 } }
+
+template<class...I> struct List{ static const bool is_ok = false;};
+template<int T> struct Z
+{
+  static const int value = T;
+  static const int value_square = T * T;
+};
+
+template<template<int> class U>
+struct List<U<2>, U<3>, U<4>, U<9>> { static const bool is_ok = true;};
+
+template<int...T> using LZ = List<Z<T>...>;
+
+template<class...T>
+struct F
+{
+  using N = LZ<T::value..., T::value_square...>;
+};
+
+static_assert (F<Z<2>, Z<3>>::N::is_ok, "");
diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic141.C b/gcc/testsuite/g++.dg/cpp0x/variadic141.C
new file mode 100644
index 0000000..6b893a7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/variadic141.C
@@ -0,0 +1,22 @@ 
+// Origin: PR c++/53609
+// { dg-do compile { target c++11 } }
+
+template<class...I> struct List{ static const bool is_ok = false;};
+template<int T> struct Z
+{
+  static const int value = T;
+  static const int value_square = T * T;
+};
+
+template<template<int> class U>
+struct List<U<2>, U<3>, U<4>, U<9>> { static const bool is_ok = true;};
+
+template<int...T> using LZ = List<Z<T>...>;
+
+template<class...T>
+struct F
+{
+  using N = LZ<T::value..., Z<4>::value, Z<9>::value>;
+};
+
+static_assert (F<Z<2>, Z<3>>::N::is_ok, "");