tree: introduce range adaptor for TREE_VEC

This patch implements a simple tree wrapper, named tree_vec_range, which
lets us idiomatically loop over all the elements of a TREE_VEC using a
C++11 range-based for loop:

  // v is a TREE_VEC
  for (tree e : tree_vec_range (v))
    ...

This is similar to the existing tree-based range adaptors ovl_range and
lkp_range added to the C++ FE in r12-340-g3307b9a07a3c51.

This patch also converts some existing loops over TREE_VEC within the
C++ FE to use tree_vec_range / range-for.

gcc/cp/ChangeLog:

	* constraint.cc (tsubst_parameter_mapping): Convert loop over
	TREE_VEC into a range-based for loop using tree_vec_range.
	* pt.cc (iterative_hash_template_arg): Likewise.
	(template_parms_level_to_args): Likewise.
	(deducible_template_args): Likewise.
	(check_undeduced_parms): Likewise.
	(dependent_type_p_r): Likewise.
	(value_dependent_expression_p) <case NONTYPE_ARGUMENT_PACK>:
	Likewise.
	(dependent_template_arg_p): Likewise.
	* tree.cc (cp_walk_subtrees) <case NONTYPE_ARGUMENT_PACK>:
	Likewise.

gcc/ChangeLog:

	* tree.h (TREE_VEC_BEGIN): Define.
	(TREE_VEC_END): Correct 'length' member access.
	(class tree_vec_range): Define.

1 files changed, 3 insertions, 6 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 94f6222..591155c 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2348,12 +2348,9 @@ tsubst_parameter_mapping (tree map, tree args, subst_info info)
       if (TREE_CODE (new_arg) == TYPE_ARGUMENT_PACK)
 	{
 	  tree pack_args = ARGUMENT_PACK_ARGS (new_arg);
-	  for (int i = 0; i < TREE_VEC_LENGTH (pack_args); i++)
-	    {
-	      tree& pack_arg = TREE_VEC_ELT (pack_args, i);
-	      if (TYPE_P (pack_arg))
-		pack_arg = canonicalize_type_argument (pack_arg, complain);
-	    }
+	  for (tree& pack_arg : tree_vec_range (pack_args))
+	    if (TYPE_P (pack_arg))
+	      pack_arg = canonicalize_type_argument (pack_arg, complain);
 	}
       if (new_arg == error_mark_node)
 	return error_mark_node;