On 9/27/22 15:50, Patrick Palka wrote:
We already have generic support for predicate-like traits that yield a
boolean via TRAIT_EXPR, but we lack the same support for transform-like
traits that yield a type.  Such support would be useful for implementing
efficient built-ins for std::decay and std::remove_cvref and other
conceptually simple type traits that are otherwise relatively expensive
to implement.

This patch implements a generic TRAIT_TYPE type and replaces the
existing hardcoded UNDERLYING_TYPE type to use TRAIT_TYPE instead.

Sounds good, perhaps we also want to convert BASES to e.g. TRAIT_TYPE_PACK at some point...

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::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/ext/underlying_type7.C: Adjust expected error message.
---
  gcc/cp/cp-objcp-common.cc                   |  2 +-
  gcc/cp/cp-tree.def                          |  9 ++--
  gcc/cp/cp-tree.h                            | 18 ++++++++
  gcc/cp/cxx-pretty-print.cc                  | 49 ++++++++++++++-------
  gcc/cp/cxx-pretty-print.h                   |  2 +-
  gcc/cp/error.cc                             | 14 +++---
  gcc/cp/mangle.cc                            |  5 ++-
  gcc/cp/module.cc                            | 24 +++++++++-
  gcc/cp/parser.cc                            | 24 +++++-----
  gcc/cp/pt.cc                                | 26 +++++++----
  gcc/cp/semantics.cc                         | 41 ++++++++++++-----
  gcc/cp/tree.cc                              | 22 ++++++---
  gcc/cp/typeck.cc                            |  7 ++-
  gcc/testsuite/g++.dg/cpp0x/alias-decl-59.C  |  4 +-
  gcc/testsuite/g++.dg/ext/underlying_type7.C |  2 +-
  15 files changed, 171 insertions(+), 78 deletions(-)

diff --git a/gcc/cp/cp-objcp-common.cc b/gcc/cp/cp-objcp-common.cc
index 64975699351..380f288a7f1 100644
--- a/gcc/cp/cp-objcp-common.cc
+++ b/gcc/cp/cp-objcp-common.cc
@@ -518,7 +518,7 @@ cp_common_init_ts (void)
    MARK_TS_TYPE_NON_COMMON (DECLTYPE_TYPE);
    MARK_TS_TYPE_NON_COMMON (TYPENAME_TYPE);
    MARK_TS_TYPE_NON_COMMON (TYPEOF_TYPE);
-  MARK_TS_TYPE_NON_COMMON (UNDERLYING_TYPE);
+  MARK_TS_TYPE_NON_COMMON (TRAIT_TYPE);
    MARK_TS_TYPE_NON_COMMON (BOUND_TEMPLATE_TEMPLATE_PARM);
    MARK_TS_TYPE_NON_COMMON (TEMPLATE_TEMPLATE_PARM);
    MARK_TS_TYPE_NON_COMMON (TEMPLATE_TYPE_PARM);
diff --git a/gcc/cp/cp-tree.def b/gcc/cp/cp-tree.def
index f9cbd339f19..f83b4c54d43 100644
--- a/gcc/cp/cp-tree.def
+++ b/gcc/cp/cp-tree.def
@@ -444,9 +444,12 @@ DEFTREECODE (BIT_CAST_EXPR, "bit_cast_expr", 
tcc_expression, 1)
/** C++ extensions. */ -/* Represents a trait expression during template expansion. */
+/* Represents a templated trait that yields an expression.  */
  DEFTREECODE (TRAIT_EXPR, "trait_expr", tcc_exceptional, 0)
+/* Represents a templated trait that yields a type. */
+DEFTREECODE (TRAIT_TYPE, "trait_type", tcc_type, 0)
+
  /* A lambda expression.  This is a C++0x extension.
     LAMBDA_EXPR_DEFAULT_CAPTURE_MODE is an enum for the default, which may be
     none.
@@ -466,10 +469,6 @@ DEFTREECODE (LAMBDA_EXPR, "lambda_expr", tcc_exceptional, 
0)
     DECLTYPE_FOR_LAMBDA_RETURN is set if we want lambda return deduction.  */
  DEFTREECODE (DECLTYPE_TYPE, "decltype_type", tcc_type, 0)
-/* A type designated by `__underlying_type (type)'.
-   UNDERLYING_TYPE_TYPE is the type in question.  */
-DEFTREECODE (UNDERLYING_TYPE, "underlying_type", tcc_type, 0)
-
  /* A type designated by one of the bases type traits.
     BASES_TYPE is the type in question.  */
  DEFTREECODE (BASES, "bases", tcc_type, 0)
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 99b486b8002..c9adf1b3822 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -1436,6 +1436,22 @@ struct GTY (()) tree_trait_expr {
    enum cp_trait_kind kind;
  };
+/* An INTEGER_CST containing the kind of the trait type NODE. */
+#define TRAIT_TYPE_KIND_RAW(NODE) \
+  TYPE_VALUES_RAW (TRAIT_TYPE_CHECK (NODE))
+
+/* The kind of the trait type NODE.  */
+#define TRAIT_TYPE_KIND(NODE) \
+  ((enum cp_trait_kind) TREE_INT_CST_LOW (TRAIT_TYPE_KIND_RAW (NODE)))
+
+/* The first argument of the trait type NODE.  */
+#define TRAIT_TYPE_TYPE1(NODE) \
+  TYPE_MIN_VALUE_RAW (TRAIT_TYPE_CHECK (NODE))
+
+/* The rest of the arguments of the trait type NODE.  */
+#define TRAIT_TYPE_TYPE2(NODE) \
+  TYPE_MAX_VALUE_RAW (TRAIT_TYPE_CHECK (NODE))

Can we also store the location of the trait use?

+
  /* Identifiers used for lambda types are almost anonymous.  Use this
     spare flag to distinguish them (they also have the anonymous flag).  */
  #define IDENTIFIER_LAMBDA_P(NODE) \
@@ -2225,6 +2241,7 @@ enum languages { lang_c, lang_cplusplus };
     || TREE_CODE (T) == TYPEOF_TYPE                    \
     || TREE_CODE (T) == BOUND_TEMPLATE_TEMPLATE_PARM   \
     || TREE_CODE (T) == DECLTYPE_TYPE                  \
+   || TREE_CODE (T) == TRAIT_TYPE                      \
     || TREE_CODE (T) == DEPENDENT_OPERATOR_TYPE)
/* Nonzero if T is a class (or struct or union) type. Also nonzero
@@ -7730,6 +7747,7 @@ extern tree finish_decltype_type                (tree, 
bool, tsubst_flags_t);
  extern tree fold_builtin_is_corresponding_member (location_t, int, tree *);
  extern tree fold_builtin_is_pointer_inverconvertible_with_class (location_t, 
int, tree *);
  extern tree finish_trait_expr                 (location_t, enum 
cp_trait_kind, tree, tree);
+extern tree finish_trait_type                  (enum cp_trait_kind, tree, 
tree);
  extern tree build_lambda_expr                   (void);
  extern tree build_lambda_object                       (tree);
  extern tree begin_lambda_type                   (tree);
diff --git a/gcc/cp/cxx-pretty-print.cc b/gcc/cp/cxx-pretty-print.cc
index e18143e39a9..d484019a539 100644
--- a/gcc/cp/cxx-pretty-print.cc
+++ b/gcc/cp/cxx-pretty-print.cc
@@ -483,7 +483,7 @@ cxx_pretty_printer::primary_expression (tree t)
        break;
case TRAIT_EXPR:
-      pp_cxx_trait_expression (this, t);
+      pp_cxx_trait (this, t);
        break;
case VA_ARG_EXPR:
@@ -1240,7 +1240,7 @@ cxx_pretty_printer::expression (tree t)
        break;
case TRAIT_EXPR:
-      pp_cxx_trait_expression (this, t);
+      pp_cxx_trait (this, t);
        break;
case ATOMIC_CONSTR:
@@ -1876,7 +1876,7 @@ cxx_pretty_printer::type_id (tree t)
      case TEMPLATE_PARM_INDEX:
      case TEMPLATE_DECL:
      case TYPEOF_TYPE:
-    case UNDERLYING_TYPE:
+    case TRAIT_TYPE:

Does this ever end up calling pp_cxx_trait? Is this code even reachable? It looks like we were already missing support for UNDERLYING_TYPE in cxx_pretty_printer::simple_type_specifier.

      case DECLTYPE_TYPE:
      case NULLPTR_TYPE:
      case TEMPLATE_ID_EXPR:
@@ -2594,9 +2594,22 @@ pp_cxx_binary_fold_expression (cxx_pretty_printer *pp, 
tree t)
  }
void
-pp_cxx_trait_expression (cxx_pretty_printer *pp, tree t)
+pp_cxx_trait (cxx_pretty_printer *pp, tree t)
  {
-  cp_trait_kind kind = TRAIT_EXPR_KIND (t);
+  cp_trait_kind kind;
+  tree type1, type2;
+  if (TREE_CODE (t) == TRAIT_EXPR)
+    {
+      kind = TRAIT_EXPR_KIND (t);
+      type1 = TRAIT_EXPR_TYPE1 (t);
+      type2 = TRAIT_EXPR_TYPE2 (t);
+    }
+  else
+    {
+      kind = TRAIT_TYPE_KIND (t);
+      type1 = TRAIT_TYPE_TYPE1 (t);
+      type2 = TRAIT_TYPE_TYPE2 (t);
+    }
switch (kind)
      {
@@ -2708,23 +2721,29 @@ pp_cxx_trait_expression (cxx_pretty_printer *pp, tree t)
      case CPTK_REF_CONVERTS_FROM_TEMPORARY:
        pp_cxx_ws_string (pp, "__reference_converts_from_temporary");
        break;
-
+    case CPTK_UNDERLYING_TYPE:
+      pp_cxx_ws_string (pp, "__underlying_type");
+      break;
      default:
        gcc_unreachable ();
      }
pp_cxx_left_paren (pp);
-  pp->type_id (TRAIT_EXPR_TYPE1 (t));
-
-  if (kind == CPTK_IS_BASE_OF
-      || kind == CPTK_IS_SAME_AS
-      || kind == CPTK_IS_LAYOUT_COMPATIBLE
-      || kind == CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF)
+  pp->type_id (type1);
+  if (type2)
      {
-      pp_cxx_separate_with (pp, ',');
-      pp->type_id (TRAIT_EXPR_TYPE2 (t));
+      if (TREE_CODE (type2) != TREE_LIST)
+       {
+         pp_cxx_separate_with (pp, ',');
+         pp->type_id (type2);
+       }
+      else
+       for (tree arg = type2; arg; arg = TREE_CHAIN (arg))
+         {
+           pp_cxx_separate_with (pp, ',');
+           pp->type_id (TREE_VALUE (arg));
+         }
      }
-
    pp_cxx_right_paren (pp);
  }
diff --git a/gcc/cp/cxx-pretty-print.h b/gcc/cp/cxx-pretty-print.h
index 593bd91d4f7..25a2c7c8d4a 100644
--- a/gcc/cp/cxx-pretty-print.h
+++ b/gcc/cp/cxx-pretty-print.h
@@ -90,7 +90,7 @@ void pp_cxx_colon_colon (cxx_pretty_printer *);
  void pp_cxx_separate_with (cxx_pretty_printer *, int);
void pp_cxx_canonical_template_parameter (cxx_pretty_printer *, tree);
-void pp_cxx_trait_expression (cxx_pretty_printer *, tree);
+void pp_cxx_trait (cxx_pretty_printer *, tree);
  void pp_cxx_va_arg_expression (cxx_pretty_printer *, tree);
  void pp_cxx_offsetof_expression (cxx_pretty_printer *, tree);
  void pp_cxx_addressof_expression (cxx_pretty_printer *, tree);
diff --git a/gcc/cp/error.cc b/gcc/cp/error.cc
index 0389f35d731..4bf9a83f20b 100644
--- a/gcc/cp/error.cc
+++ b/gcc/cp/error.cc
@@ -698,12 +698,8 @@ dump_type (cxx_pretty_printer *pp, tree t, int flags)
        pp_cxx_right_paren (pp);
        break;
- case UNDERLYING_TYPE:
-      pp_cxx_ws_string (pp, "__underlying_type");
-      pp_cxx_whitespace (pp);
-      pp_cxx_left_paren (pp);
-      dump_expr (pp, UNDERLYING_TYPE_TYPE (t), flags & ~TFF_EXPR_IN_PARENS);
-      pp_cxx_right_paren (pp);
+    case TRAIT_TYPE:
+      pp_cxx_trait (pp, t);
        break;
case TYPE_PACK_EXPANSION:
@@ -971,7 +967,7 @@ dump_type_prefix (cxx_pretty_printer *pp, tree t, int flags)
      case COMPLEX_TYPE:
      case VECTOR_TYPE:
      case TYPEOF_TYPE:
-    case UNDERLYING_TYPE:
+    case TRAIT_TYPE:
      case DECLTYPE_TYPE:
      case TYPE_PACK_EXPANSION:
      case FIXED_POINT_TYPE:
@@ -1095,7 +1091,7 @@ dump_type_suffix (cxx_pretty_printer *pp, tree t, int 
flags)
      case COMPLEX_TYPE:
      case VECTOR_TYPE:
      case TYPEOF_TYPE:
-    case UNDERLYING_TYPE:
+    case TRAIT_TYPE:
      case DECLTYPE_TYPE:
      case TYPE_PACK_EXPANSION:
      case FIXED_POINT_TYPE:
@@ -2956,7 +2952,7 @@ dump_expr (cxx_pretty_printer *pp, tree t, int flags)
        break;
case TRAIT_EXPR:
-      pp_cxx_trait_expression (pp, t);
+      pp_cxx_trait (pp, t);
        break;
case VA_ARG_EXPR:
diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index 00d283fff8c..fc750fc5d8e 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -2389,8 +2389,9 @@ write_type (tree type)
              sorry ("mangling %<typeof%>, use %<decltype%> instead");
              break;
- case UNDERLYING_TYPE:
-             sorry ("mangling %<__underlying_type%>");
+           case TRAIT_TYPE:
+             error ("use of built-in trait %qT in function signature; "
+                    "use library traits instead", type);
              break;
case LANG_TYPE:
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 7496df5e843..25741e5d827 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -8910,7 +8910,6 @@ 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)
@@ -8920,6 +8919,12 @@ 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;
@@ -9434,7 +9439,6 @@ trees_in::tree_node (bool is_use)
case DECLTYPE_TYPE:
          case TYPEOF_TYPE:
-         case UNDERLYING_TYPE:
          case DEPENDENT_OPERATOR_TYPE:
            {
              tree expr = tree_node ();
@@ -9449,6 +9453,22 @@ 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);

This needs a rationale for structural equality rather than canonicalization.

+               }
+           }
+           break;
+
          case TYPE_ARGUMENT_PACK:
            if (!get_overrun ())
              {
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index d501178634a..9f5e2c292b3 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -2783,7 +2783,7 @@ static void cp_parser_late_parsing_default_args
    (cp_parser *, tree);
  static tree cp_parser_sizeof_operand
    (cp_parser *, enum rid);
-static cp_expr cp_parser_trait_expr
+static cp_expr cp_parser_trait
    (cp_parser *, enum rid);
  static bool cp_parser_declares_only_class_p
    (cp_parser *);
@@ -5928,7 +5928,7 @@ cp_parser_primary_expression (cp_parser *parser,
        case RID_IS_NOTHROW_CONVERTIBLE:
        case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
        case RID_REF_CONVERTS_FROM_TEMPORARY:
-         return cp_parser_trait_expr (parser, token->keyword);
+         return cp_parser_trait (parser, token->keyword);
// C++ concepts
        case RID_REQUIRES:
@@ -10882,18 +10882,16 @@ cp_parser_builtin_offsetof (cp_parser *parser)
    return expr;
  }
-/* Parse a trait expression.
-
-   Returns a representation of the expression, the underlying type
-   of the type at issue when KEYWORD is RID_UNDERLYING_TYPE.  */
+/* Parse a builtin trait expression or type.  */
static cp_expr
-cp_parser_trait_expr (cp_parser* parser, enum rid keyword)
+cp_parser_trait (cp_parser* parser, enum rid keyword)
  {
    cp_trait_kind kind;
    tree type1, type2 = NULL_TREE;
    bool binary = false;
    bool variadic = false;
+  bool type = false;
switch (keyword)
      {
@@ -10989,6 +10987,7 @@ cp_parser_trait_expr (cp_parser* parser, enum rid 
keyword)
        break;
      case RID_UNDERLYING_TYPE:
        kind = CPTK_UNDERLYING_TYPE;
+      type = true;
        break;
      case RID_BASES:
        kind = CPTK_BASES;
@@ -11092,14 +11091,15 @@ cp_parser_trait_expr (cp_parser* parser, enum rid 
keyword)
       the trait expr now or saving it for template instantiation.  */
    switch (kind)
      {
-    case CPTK_UNDERLYING_TYPE:
-      return cp_expr (finish_underlying_type (type1), trait_loc);
      case CPTK_BASES:
        return cp_expr (finish_bases (type1, false), trait_loc);
      case CPTK_DIRECT_BASES:
        return cp_expr (finish_bases (type1, true), trait_loc);
      default:
-      return finish_trait_expr (trait_loc, kind, type1, type2);
+      if (type)
+       return finish_trait_type (kind, type1, type2);
+      else
+       return finish_trait_expr (trait_loc, kind, type1, type2);
      }
  }
@@ -19867,7 +19867,7 @@ cp_parser_simple_type_specifier (cp_parser* parser,
        return type;
case RID_UNDERLYING_TYPE:
-      type = cp_parser_trait_expr (parser, RID_UNDERLYING_TYPE);
+      type = cp_parser_trait (parser, token->keyword);
        if (decl_specs)
        cp_parser_set_decl_spec_type (decl_specs, type,
                                      token,
@@ -19877,7 +19877,7 @@ cp_parser_simple_type_specifier (cp_parser* parser,
case RID_BASES:
      case RID_DIRECT_BASES:
-      type = cp_parser_trait_expr (parser, token->keyword);
+      type = cp_parser_trait (parser, token->keyword);
        if (decl_specs)
         cp_parser_set_decl_spec_type (decl_specs, type,
                                       token,
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 30c6994bae1..1a4491d3556 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10619,7 +10619,6 @@ for_each_template_parm_r (tree *tp, int *walk_subtrees, 
void *d)
case TYPEOF_TYPE:
      case DECLTYPE_TYPE:
-    case UNDERLYING_TYPE:
        if (pfd->include_nondeduced_p
          && for_each_template_parm (TYPE_VALUES_RAW (t), fn, data,
                                     pfd->visited,
@@ -10629,6 +10628,15 @@ for_each_template_parm_r (tree *tp, int 
*walk_subtrees, void *d)
        *walk_subtrees = false;
        break;
+ case TRAIT_TYPE:
+      if (pfd->include_nondeduced_p)
+       {
+         WALK_SUBTREE (TRAIT_TYPE_TYPE1 (t));
+         WALK_SUBTREE (TRAIT_TYPE_TYPE2 (t));
+       }
+      *walk_subtrees = false;
+      break;
+
      case FUNCTION_DECL:
      case VAR_DECL:
        if (DECL_LANG_SPECIFIC (t) && DECL_TEMPLATE_INFO (t))
@@ -16515,11 +16523,11 @@ tsubst (tree t, tree args, tsubst_flags_t complain, 
tree in_decl)
                                        complain | tf_ignore_bad_quals);
        }
- case UNDERLYING_TYPE:
+    case TRAIT_TYPE:
        {
-       tree type = tsubst (UNDERLYING_TYPE_TYPE (t), args,
-                           complain, in_decl);
-       return finish_underlying_type (type);
+       tree type1 = tsubst (TRAIT_TYPE_TYPE1 (t), args, complain, in_decl);
+       tree type2 = tsubst (TRAIT_TYPE_TYPE2 (t), args, complain, in_decl);
+       return finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2);
        }
case TYPE_ARGUMENT_PACK:
@@ -24929,9 +24937,9 @@ unify (tree tparms, tree targs, tree parm, tree arg, 
int strict,
case TYPEOF_TYPE:
      case DECLTYPE_TYPE:
-    case UNDERLYING_TYPE:
+    case TRAIT_TYPE:
        /* Cannot deduce anything from TYPEOF_TYPE, DECLTYPE_TYPE,
-        or UNDERLYING_TYPE nodes.  */
+        or TRAIT_TYPE nodes.  */
        return unify_success (explain_p);
case ERROR_MARK:
@@ -27506,12 +27514,12 @@ dependent_type_p_r (tree type)
               (INNERMOST_TEMPLATE_ARGS (CLASSTYPE_TI_ARGS (type)))))
      return true;
- /* All TYPEOF_TYPEs, DECLTYPE_TYPEs, and UNDERLYING_TYPEs are
+  /* All TYPEOF_TYPEs, DECLTYPE_TYPEs, and TRAIT_TYPEs are
       dependent; if the argument of the `typeof' expression is not
       type-dependent, then it should already been have resolved.  */
    if (TREE_CODE (type) == TYPEOF_TYPE
        || TREE_CODE (type) == DECLTYPE_TYPE
-      || TREE_CODE (type) == UNDERLYING_TYPE)
+      || TREE_CODE (type) == TRAIT_TYPE)
      return true;
/* A template argument pack is dependent if any of its packed
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index e8cd50558d6..ea00805c97d 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -4397,17 +4397,6 @@ finish_typeof (tree expr)
  tree
  finish_underlying_type (tree type)
  {
-  tree underlying_type;
-
-  if (processing_template_decl)
-    {
-      underlying_type = cxx_make_type (UNDERLYING_TYPE);
-      UNDERLYING_TYPE_TYPE (underlying_type) = type;
-      SET_TYPE_STRUCTURAL_EQUALITY (underlying_type);
-
-      return underlying_type;
-    }
-
    if (!complete_type_or_else (type, NULL_TREE))
      return error_mark_node;
@@ -4417,7 +4406,7 @@ finish_underlying_type (tree type)
        return error_mark_node;
      }
- underlying_type = ENUM_UNDERLYING_TYPE (type);
+  tree underlying_type = ENUM_UNDERLYING_TYPE (type);
/* Fixup necessary in this case because ENUM_UNDERLYING_TYPE
       includes TYPE_MIN_VALUE and TYPE_MAX_VALUE information.
@@ -12224,6 +12213,34 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, 
tree type1, tree type2)
    return maybe_wrap_with_location (val, loc);
  }
+/* Process a trait type. */
+
+tree
+finish_trait_type (cp_trait_kind kind, tree type1, tree type2)
+{
+  if (type1 == error_mark_node
+      || type2 == error_mark_node)
+    return error_mark_node;
+
+  if (processing_template_decl)
+    {
+      tree type = cxx_make_type (TRAIT_TYPE);
+      TRAIT_TYPE_TYPE1 (type) = type1;
+      TRAIT_TYPE_TYPE2 (type) = type2;
+      TRAIT_TYPE_KIND_RAW (type) = build_int_cstu (integer_type_node, kind);
+      SET_TYPE_STRUCTURAL_EQUALITY (type);
+      return type;
+    }
+
+  switch (kind)
+    {
+    case CPTK_UNDERLYING_TYPE:
+      return finish_underlying_type (type1);
+    default:
+      gcc_unreachable ();
+    }
+}
+
  /* Do-nothing variants of functions to handle pragma FLOAT_CONST_DECIMAL64,
     which is ignored for C++.  */
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index d0bd41ae5a0..eef694689cc 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -1776,10 +1776,17 @@ strip_typedefs (tree t, bool *remove_attributes /* = 
NULL */,
                   DECLTYPE_TYPE_ID_EXPR_OR_MEMBER_ACCESS_P (t),
                   tf_none));
        break;
-    case UNDERLYING_TYPE:
-      type = strip_typedefs (UNDERLYING_TYPE_TYPE (t),
-                            remove_attributes, flags);
-      result = finish_underlying_type (type);
+    case TRAIT_TYPE:
+      {
+       tree type1 = strip_typedefs (TRAIT_TYPE_TYPE1 (t),
+                                    remove_attributes, flags);
+       tree type2 = strip_typedefs (TRAIT_TYPE_TYPE2 (t),
+                                    remove_attributes, flags);
+       if (type1 == TRAIT_TYPE_TYPE1 (t) && type2 == TRAIT_TYPE_TYPE2 (t))
+         result = NULL_TREE;
+       else
+         result = finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2);
+      }
        break;
      case TYPE_PACK_EXPANSION:
        {
@@ -5383,7 +5390,6 @@ cp_walk_subtrees (tree *tp, int *walk_subtrees_p, 
walk_tree_fn func,
      case UNBOUND_CLASS_TEMPLATE:
      case TEMPLATE_PARM_INDEX:
      case TYPEOF_TYPE:
-    case UNDERLYING_TYPE:
        /* None of these have subtrees other than those already walked
         above.  */
        *walk_subtrees_p = 0;
@@ -5472,6 +5478,12 @@ cp_walk_subtrees (tree *tp, int *walk_subtrees_p, 
walk_tree_fn func,
        *walk_subtrees_p = 0;
        break;
+ case TRAIT_TYPE:
+      WALK_SUBTREE (TRAIT_TYPE_TYPE1 (*tp));
+      WALK_SUBTREE (TRAIT_TYPE_TYPE2 (*tp));
+      *walk_subtrees_p = 0;
+      break;
+
      case DECLTYPE_TYPE:
        ++cp_unevaluated_operand;
        /* We can't use WALK_SUBTREE here because of the goto.  */
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 4854b983765..5064a009af0 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -1621,8 +1621,11 @@ structural_comptypes (tree t1, tree t2, int strict)
          return false;
        break;
- case UNDERLYING_TYPE:
-      if (!same_type_p (UNDERLYING_TYPE_TYPE (t1), UNDERLYING_TYPE_TYPE (t2)))
+    case TRAIT_TYPE:
+      if (TRAIT_TYPE_KIND (t1) != TRAIT_TYPE_KIND (t2))
+       return false;
+      if (!same_type_p (TRAIT_TYPE_TYPE1 (t1), TRAIT_TYPE_TYPE1 (t2))
+         || !cp_tree_equal (TRAIT_TYPE_TYPE2 (t1), TRAIT_TYPE_TYPE2 (t2)))
        return false;
        break;
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-59.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-59.C
index 1f5e94f6d83..50946576f74 100644
--- a/gcc/testsuite/g++.dg/cpp0x/alias-decl-59.C
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-59.C
@@ -5,7 +5,7 @@ template<typename>
  struct A {};
template<typename T>
-using B = A<__underlying_type(T) [[gnu::aligned(4)]]>; // { dg-warning "ignoring 
attributes on template argument" }
+using B = A<__underlying_type(T) [[gnu::aligned(4)]]>; // { dg-warning "ignoring 
attributes applied to dependent type" }
template<typename T>
-using B = A<__underlying_type(T) [[gnu::packed]]>; // { dg-warning "ignoring 
attributes on template argument" }
+using B = A<__underlying_type(T) [[gnu::packed]]>; // { dg-warning "ignoring 
attributes applied to dependent type" }
diff --git a/gcc/testsuite/g++.dg/ext/underlying_type7.C 
b/gcc/testsuite/g++.dg/ext/underlying_type7.C
index 2d6ec51792c..137a0f08547 100644
--- a/gcc/testsuite/g++.dg/ext/underlying_type7.C
+++ b/gcc/testsuite/g++.dg/ext/underlying_type7.C
@@ -9,7 +9,7 @@ enum class E6 : long { c = __LONG_MAX__ };
template<typename T>
    void
-  test(T, __underlying_type(T)) // { dg-message "sorry, unimplemented: 
mangling" }
+  test(T, __underlying_type(T)) // { dg-error "built-in trait 
'__underlying_type\\(T\\)'" }
    { }
int main()

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