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

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Submitter Dodji Seketeli
Date Dec. 12, 2012, 1:01 p.m.
Message ID <87k3sn8lvl.fsf@redhat.com>
Download mbox | patch
Permalink /patch/205512/
State New
Headers show

Comments

Dodji Seketeli - Dec. 12, 2012, 1:01 p.m.
Jason Merrill <jason@redhat.com> writes:


> I'd also like to move the scan and PACK_EXPANSION_EXTRA_ARGS code back
> out of the loop.

Like this?

Tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp/

	* pt.c (argument_pack_element_is_expansion_p)
	(make_argument_pack_select, scan_parm_packs_and_args)
	(gen_elem_of_pack_expansion_instantiation): New static functions.
	(has_bare_parameter_packs): Factorized out of ...
	(check_for_bare_parameter_packs): ... here.
	(tsubst): When looking through an ARGUMENT_PACK_SELECT tree node,
	look through the possibly resulting pack expansion as well.
	(tsubst_pack_expansion): Now this function is clearly organized in
	two parts: a part that maps each parameter pack with its matching
	argument pack, and a part that walks that map to build the result
	of substituting each element of the argument packs into the
	parameter packs.  Use gen_elem_of_pack_expansion_instantiation for
	the latter part.
	(arg_from_parm_pack_p): Remove this for it's not used by
	tsubst_pack_expansion anymore.

gcc/testsuite/

	* g++.dg/cpp0x/variadic139.C: New test.
	* g++.dg/cpp0x/variadic140.C: Likewise.
	* g++.dg/cpp0x/variadic141.C: Likewise.
---
 gcc/cp/pt.c                              | 485 ++++++++++++++++++++-----------
 gcc/testsuite/g++.dg/cpp0x/variadic139.C |  14 +
 gcc/testsuite/g++.dg/cpp0x/variadic140.C |  22 ++
 gcc/testsuite/g++.dg/cpp0x/variadic141.C |  22 ++
 4 files changed, 366 insertions(+), 177 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/variadic139.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/variadic140.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/variadic141.C
Jason Merrill - Dec. 17, 2012, 7:02 p.m.
On 12/12/2012 08:01 AM, Dodji Seketeli wrote:
> Jason Merrill <jason@redhat.com> writes:
>> I'd also like to move the scan and PACK_EXPANSION_EXTRA_ARGS code back
>> out of the loop.
>
> Like this?

Let's put this scanning back in the loop over the packs that we already 
have in the main function.

> +	  if (argument_pack_element_is_expansion_p (arg, i))
> +	    {
> +	      has_expansion_arg = true;
> +	      if (TREE_CODE (parm) == PARM_DECL)
> +		parm = DECL_INITIAL (parm);
> +
> +	      int level = 0, idx = 0;
> +	      template_parm_level_and_index (parm, &level, &idx);
> +
> +	      if (*fake_arg_level_to_prune == 0
> +		  || *fake_arg_level_to_prune > level)
> +		*fake_arg_level_to_prune = level;
> +	    }

This is now assuming that any expansion we see is a fake arg, which 
isn't the case for the testcase in this PR.  Do we really need to 
recognize fake args rather than treating them the same as real args?

> +      /* If one pack has an empty argument and another other doesn't,
> +	 or if for a given index one parameter pack has an expansion
> +	 as argument and another one doesn't, then

This seems to assume that any expansions in different argument packs 
will end up expanding to the same number of elements.  I'm not sure 
that's a valid assumption; I think once we see an expansion, any 
non-expansion arguments after that index are also problematic.

> +  /* If we have empty arguments ... */
> +  if (has_empty_args)
> +    /* ... we just return a pack expansion which pattern is PATTERN
> +       into which ARGS has been substituted.  */;

I want to still handle this before the elements loop, too.

Jason

Patch

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index ecb013e..221323c 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 +9069,216 @@  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 scans the list of parameter packs PARM_PACKS and reports some
+    interesting properties about them.  ARG_PACK_LEN is the length of
+    the argument packs.  Note that PARM_PACKS must be an instance of
+    TREE_LIST where TREE_PURPOSE is a parameter pack, and TREE_VALUE
+    is the argument for that pack.
+
+    The properties returned about the scanned parameter packs
+    are the following:
+
+	- HAS_EXPANSION_ARG_P: Set to TRUE iff at least one parameter
+	  pack has an argument that is an expansion.
+	- HAS_NON_EXPANSION_ARG_P: Set to TRUE iff at least one
+	  parameter pack has an argument that is not an expansion.
+	- USE_PACK_EXPANSION_EXTRA_P: Set to TRUE iff the
+	  tsubst_pack_expansion cannot build the final substitution
+	  piecewise by calling
+	  gen_elem_of_pack_expansion_instantiation, and so must return
+	  an unsubstituted pack expansion using the
+	  PACK_EXPANSION_EXTRA mecanism.
+	- HAS_EMPTY_ARG_P: Set to TRUE iff at least one parameter pack has
+	  no argument.
+	- FAKE_ARG_LEVEL_TO_PRUNE: is non-zero iff the argument of
+	  the parameter pack is e.g the result of calling template_parm_to_arg
+	  on the pack.  In that case, this is set to the parameter level
+	  of that pack.  */
+
+static void
+scan_parm_packs_and_args (tree parm_packs,
+			  int arg_pack_len,
+			  bool *use_pack_expansion_extra_p,
+			  bool *has_empty_arg_p,
+			  int *fake_arg_level_to_prune)
+{
+  if (parm_packs == NULL_TREE)
+    {
+      *has_empty_arg_p = true;
+      return ;
+    }
+
+  for (int i = 0 ; i < arg_pack_len; ++i)
+    {
+      bool has_expansion_arg = false, 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);
+	  tree parm = TREE_PURPOSE (parm_pack);
+
+	  if (arg == NULL_TREE)
+	    {
+	      *has_empty_arg_p = true;
+	      continue;
+	    }
+
+	  if (argument_pack_element_is_expansion_p (arg, i))
+	    {
+	      has_expansion_arg = true;
+	      if (TREE_CODE (parm) == PARM_DECL)
+		parm = DECL_INITIAL (parm);
+
+	      int level = 0, idx = 0;
+	      template_parm_level_and_index (parm, &level, &idx);
+
+	      if (*fake_arg_level_to_prune == 0
+		  || *fake_arg_level_to_prune > level)
+		*fake_arg_level_to_prune = level;
+	    }
+	  else
+	    has_non_expansion_arg = true;
+	}
+
+      /* If one pack has an empty argument and another other doesn't,
+	 or if for a given index one parameter pack has an expansion
+	 as argument and another one doesn'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_p && (has_expansion_arg || has_non_expansion_arg)))
+	*use_pack_expansion_extra_p = true;
+    }
+}
+
+/* [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.
+   HAS_EMPTY_ARG should be set to TRUE if any of the parameter packs
+   in PARM_PACKS has an empty argument.  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,
+					  bool has_empty_args,
+					  unsigned index,
+					  tree args /* This parm gets
+						       modified.  */,
+					  tsubst_flags_t complain,
+					  tree in_decl)
+{
+  tree t;
+
+  /* If we have empty arguments ... */
+  if (has_empty_args)
+    /* ... we just return a pack expansion which pattern is PATTERN
+       into which ARGS has been substituted.  */;
+  else
+    {
+      /* 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
@@ -9106,9 +9290,6 @@  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 +9368,6 @@  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 = 
@@ -9221,77 +9394,55 @@  tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
                        pattern);
               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);
-          TREE_TYPE (packs) = orig_arg;
         }
-      else
-	{
-	  /* We can't substitute for this parameter pack.  We use a flag as
-	     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;
-	}
+
+      /* Keep track of the parameter packs and their corresponding
+	 argument packs.  */
+      packs = tree_cons (parm_pack, arg_pack, packs);
+      TREE_TYPE (packs) = orig_arg;
     }
 
-  /* We cannot expand this expansion expression, because we don't have
-     all of the argument packs we need.  */
-  if (unsubstituted_packs)
-    {
-      if (real_packs)
-	{
-	  /* 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;
+  /* 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;
 
-	  t = make_pack_expansion (pattern);
+  bool use_pack_expansion_extra = false, has_empty_args = false;
+  int fake_arg_level_to_prune = 0;
+  scan_parm_packs_and_args (packs, len, &use_pack_expansion_extra,
+			    &has_empty_args, &fake_arg_level_to_prune);
 
-	  /* 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;
+  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.  */
 
-	  PACK_EXPANSION_EXTRA_ARGS (t) = save_args;
-	}
-      else
+      int levels = TMPL_ARGS_DEPTH (args);
+      tree save_args;
+
+      /* The call to add_to_template_args in
+	 tsubst_pack_expansion assumes no overlap between
+	 saved args and new args, so prune away any fake args,
+	 i.e, those resulting from template_parm_to_arg.  */
+      if (fake_arg_level_to_prune != 0
+	  && levels >= fake_arg_level_to_prune)
 	{
-	  /* There were no real arguments, we're just replacing a parameter
-	     pack with another version of itself. Substitute into the
-	     pattern and return a PACK_EXPANSION_*. The caller will need to
-	     deal with that.  */
-	  if (TREE_CODE (t) == EXPR_PACK_EXPANSION)
-	    t = tsubst_expr (pattern, args, complain, in_decl,
-			     /*integral_constant_expression_p=*/false);
-	  else
-	    t = tsubst (pattern, args, complain, in_decl);
-	  t = make_pack_expansion (t);
+	  gcc_assert (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (args)
+		      && fake_arg_level_to_prune > 1);
+	  TREE_VEC_LENGTH (args) = fake_arg_level_to_prune - 1;
+	  save_args = copy_node (args);
+	  TREE_VEC_LENGTH (args) = levels;
 	}
-      return t;
-    }
+      else
+	save_args = args;
 
-  /* We could not find any argument packs that work.  */
-  if (len < 0)
-    return error_mark_node;
+      result = make_pack_expansion (pattern);
+      PACK_EXPANSION_EXTRA_ARGS (result) = save_args;
+
+      return result;
+    }
 
   if (need_local_specializations)
     {
@@ -9306,60 +9457,20 @@  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,
+							has_empty_args, i,
+							args, complain,
+							in_decl);
+	  TREE_VEC_ELT (result, i) = t;
+      	  if (t == error_mark_node)
+	    {
+	      result = error_mark_node;
+	      break;
+	    }
 	}
     }
 
@@ -9374,6 +9485,10 @@  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 +11217,24 @@  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, "");