c++: introduce TRAIT_TYPE alongside TRAIT_EXPR

We already have generic support for predicate-like traits that yield a
boolean value via TRAIT_EXPR, but we lack the same support for traits
that yield a type instead of a value.  Such support would streamline
implementing efficient builtins for the standard library type traits.

To that end this patch implements a generic TRAIT_TYPE type alongside
TRAIT_EXPR, and reimplements the existing UNDERLYING_TYPE builtin trait
using this new TRAIT_TYPE.

gcc/cp/ChangeLog:

	* cp-objcp-common.cc (cp_common_init_ts): Replace
	UNDERLYING_TYPE with TRAIT_TYPE.
	* cp-tree.def (TRAIT_TYPE): Define.
	(UNDERLYING_TYPE): Remove.
	* cp-tree.h (TRAIT_TYPE_KIND_RAW): Define.
	(TRAIT_TYPE_KIND): Define.
	(TRAIT_TYPE_TYPE1): Define.
	(TRAIT_TYPE_TYPE2): Define.
	(WILDCARD_TYPE_P): Return true for TRAIT_TYPE.
	(finish_trait_type): Declare.
	* cxx-pretty-print.cc (cxx_pretty_printer::primary_expression):
	Adjust after renaming pp_cxx_trait_expression.
	(cxx_pretty_printer::simple_type_specifier) <case TRAIT_TYPE>:
	New.
	(cxx_pretty_printer::type_id): Replace UNDERLYING_TYPE with
	TRAIT_TYPE.
	(pp_cxx_trait_expression): Rename to ...
	(pp_cxx_trait): ... this.  Handle TRAIT_TYPE as well.  Correct
	pretty printing of the trailing arguments.
	* cxx-pretty-print.h (pp_cxx_trait_expression): Rename to ...
	(pp_cxx_trait_type): ... this.
	* error.cc (dump_type) <case UNDERLYING_TYPE>: Remove.
	<case TRAIT_TYPE>: New.
	(dump_type_prefix): Replace UNDERLYING_WITH with TRAIT_TYPE.
	(dump_type_suffix): Likewise.
	* mangle.cc (write_type) <case UNDERLYING_TYPE>: Remove.
	<case TRAIT_TYPE>: New.
	* module.cc (trees_out::type_node) <case UNDERLYING_TYPE>:
	Remove.
	<case TRAIT_TYPE>: New.
	(trees_in::tree_node): Likewise.
	* parser.cc (cp_parser_primary_expression): Adjust after
	renaming cp_parser_trait_expr.
	(cp_parser_trait_expr): Rename to ...
	(cp_parser_trait): ... this.  Call finish_trait_type for traits
	that yield a type.
	(cp_parser_simple_type_specifier): Adjust after renaming
	cp_parser_trait_expr.
	* pt.cc (for_each_template_parm_r) <case UNDERLYING_TYPE>:
	Remove.
	<case TRAIT_TYPE>: New.
	(tsubst): Likewise.
	(unify): Replace UNDERLYING_TYPE with TRAIT_TYPE.
	(dependent_type_p_r): Likewise.
	* semantics.cc (finish_underlying_type): Don't return
	UNDERLYING_TYPE anymore when processing_template_decl.
	(finish_trait_type): Define.
	* tree.cc (strip_typedefs) <case UNDERLYING_TYPE>: Remove.
	<case TRAIT_TYPE>: New.
	(cp_walk_subtrees): Likewise.
	* typeck.cc (structural_comptypes): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-59.C: Adjust expected error message.
	* g++.dg/ext/underlying_type7.C: Likewise.
	* g++.dg/ext/underlying_type13.C: New test.
	* g++.dg/ext/underlying_type14.C: New test.

1 files changed, 22 insertions, 2 deletions

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index cbf3a77..500ac06 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -8926,7 +8926,6 @@ trees_out::type_node (tree type)
 
     case DECLTYPE_TYPE:
     case TYPEOF_TYPE:
-    case UNDERLYING_TYPE:
     case DEPENDENT_OPERATOR_TYPE:
       tree_node (TYPE_VALUES_RAW (type));
       if (TREE_CODE (type) == DECLTYPE_TYPE)
@@ -8936,6 +8935,12 @@ trees_out::type_node (tree type)
 	  tree_node_bools (type);
       break;
 
+    case TRAIT_TYPE:
+      tree_node (TRAIT_TYPE_KIND_RAW (type));
+      tree_node (TRAIT_TYPE_TYPE1 (type));
+      tree_node (TRAIT_TYPE_TYPE2 (type));
+      break;
+
     case TYPE_ARGUMENT_PACK:
       /* No additional data.  */
       break;
@@ -9450,7 +9455,6 @@ trees_in::tree_node (bool is_use)
 
 	  case DECLTYPE_TYPE:
 	  case TYPEOF_TYPE:
-	  case UNDERLYING_TYPE:
 	  case DEPENDENT_OPERATOR_TYPE:
 	    {
 	      tree expr = tree_node ();
@@ -9465,6 +9469,22 @@ trees_in::tree_node (bool is_use)
 	    }
 	    break;
 
+	  case TRAIT_TYPE:
+	    {
+	      tree kind = tree_node ();
+	      tree type1 = tree_node ();
+	      tree type2 = tree_node ();
+	      if (!get_overrun ())
+		{
+		  res = cxx_make_type (TRAIT_TYPE);
+		  TRAIT_TYPE_KIND_RAW (res) = kind;
+		  TRAIT_TYPE_TYPE1 (res) = type1;
+		  TRAIT_TYPE_TYPE2 (res) = type2;
+		  SET_TYPE_STRUCTURAL_EQUALITY (res);
+		}
+	    }
+	    break;
+
 	  case TYPE_ARGUMENT_PACK:
 	    if (!get_overrun ())
 	      {