c++: Define built-in for std::tuple_element [PR100157]

This adds a new built-in to replace the recursive class template
instantiations done by traits such as std::tuple_element and
std::variant_alternative.  The purpose is to select the Nth type from a
list of types, e.g. __type_pack_element<1, char, int, float> is int.
We implement it as a special kind of TRAIT_TYPE.

For a pathological example tuple_element_t<1000, tuple<2000 types...>>
the compilation time is reduced by more than 90% and the memory used by
the compiler is reduced by 97%.  In realistic examples the gains will be
much smaller, but still relevant.

Unlike the other built-in traits, __type_pack_element uses template-id
syntax instead of call syntax and is SFINAE-enabled, matching Clang's
implementation.  And like the other built-in traits, it's not mangleable
so we can't use it directly in function signatures.

N.B. Clang seems to implement __type_pack_element as a first-class
template that can e.g. be used as a template-template argument.  For
simplicity we implement it in a more ad-hoc way.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

	PR c++/100157

gcc/cp/ChangeLog:

	* cp-trait.def (TYPE_PACK_ELEMENT): Define.
	* cp-tree.h (finish_trait_type): Add complain parameter.
	* cxx-pretty-print.cc (pp_cxx_trait): Handle
	CPTK_TYPE_PACK_ELEMENT.
	* parser.cc (cp_parser_constant_expression): Document default
	arguments.
	(cp_parser_trait): Handle CPTK_TYPE_PACK_ELEMENT.  Pass
	tf_warning_or_error to finish_trait_type.
	* pt.cc (tsubst) <case TRAIT_TYPE>: Handle non-type first
	argument.  Pass complain to finish_trait_type.
	* semantics.cc (finish_type_pack_element): Define.
	(finish_trait_type): Add complain parameter.  Handle
	CPTK_TYPE_PACK_ELEMENT.
	* tree.cc (strip_typedefs): Handle non-type first argument.
	Pass tf_warning_or_error to finish_trait_type.
	* typeck.cc (structural_comptypes) <case TRAIT_TYPE>: Use
	cp_tree_equal instead of same_type_p for the first argument.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (_Nth_type): Conditionally define in
	terms of __type_pack_element if available.
	* testsuite/20_util/tuple/element_access/get_neg.cc: Prune
	additional errors from the new built-in.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/type_pack_element1.C: New test.
	* g++.dg/ext/type_pack_element2.C: New test.
	* g++.dg/ext/type_pack_element3.C: New test.

gcc/cp/cp-trait.def

@@ -91,6 +91,7 @@ DEFTRAIT_TYPE (REMOVE_CV, "__remove_cv", 1)
 DEFTRAIT_TYPE (REMOVE_REFERENCE, "__remove_reference", 1)
 DEFTRAIT_TYPE (REMOVE_CVREF, "__remove_cvref", 1)
 DEFTRAIT_TYPE (UNDERLYING_TYPE,  "__underlying_type", 1)
+DEFTRAIT_TYPE (TYPE_PACK_ELEMENT, "__type_pack_element", -1)
 
 /* These traits yield a type pack, not a type, and are represented by
    cp_parser_trait as a special BASES tree instead of a TRAIT_TYPE tree.  */

gcc/cp/cp-tree.h

@@ -7761,7 +7761,7 @@ 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 finish_trait_type			(enum cp_trait_kind, tree, tree, tsubst_flags_t);
 extern tree build_lambda_expr                   (void);
 extern tree build_lambda_object			(tree);
 extern tree begin_lambda_type                   (tree);

gcc/cp/cxx-pretty-print.cc

@@ -2625,11 +2625,19 @@ pp_cxx_trait (cxx_pretty_printer *pp, tree t)
 #undef DEFTRAIT
     }
 
-  pp_cxx_left_paren (pp);
-  if (TYPE_P (type1))
-    pp->type_id (type1);
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    {
+      pp_cxx_begin_template_argument_list (pp);
+      pp->expression (type1);
+    }
   else
-    pp->expression (type1);
+    {
+      pp_cxx_left_paren (pp);
+      if (TYPE_P (type1))
+	pp->type_id (type1);
+      else
+	pp->expression (type1);
+    }
   if (type2)
     {
       if (TREE_CODE (type2) != TREE_LIST)
@@ -2644,7 +2652,10 @@ pp_cxx_trait (cxx_pretty_printer *pp, tree t)
 	    pp->type_id (TREE_VALUE (arg));
 	  }
     }
-  pp_cxx_right_paren (pp);
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    pp_cxx_end_template_argument_list (pp);
+  else
+    pp_cxx_right_paren (pp);
 }
 
 // requires-clause:

gcc/cp/parser.cc

@@ -10730,9 +10730,9 @@ cp_parser_expression (cp_parser* parser, cp_id_kind * pidk,
 
 static cp_expr
 cp_parser_constant_expression (cp_parser* parser,
-			       int allow_non_constant_p,
-			       bool *non_constant_p,
-			       bool strict_p)
+			       int allow_non_constant_p /* = 0 */,
+			       bool *non_constant_p /* = NULL */,
+			       bool strict_p /* = false */)
 {
   bool saved_integral_constant_expression_p;
   bool saved_allow_non_integral_constant_expression_p;
@@ -10959,7 +10959,10 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
   cp_lexer_consume_token (parser->lexer);
 
   matching_parens parens;
-  parens.require_open (parser);
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    cp_parser_require (parser, CPP_LESS, RT_LESS);
+  else
+    parens.require_open (parser);
 
   if (kind == CPTK_IS_DEDUCIBLE)
     {
@@ -10972,6 +10975,12 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
 				       /*optional_p=*/false);
       type1 = cp_parser_lookup_name_simple (parser, type1, token->location);
     }
+  else if (kind == CPTK_TYPE_PACK_ELEMENT)
+    /* __type_pack_element takes an expression as its first argument and uses
+       template-id syntax instead of function call syntax (for consistency
+       with Clang).  We special case these properties of __type_pack_element
+       here and elsewhere.  */
+    type1 = cp_parser_constant_expression (parser);
   else
     {
       type_id_in_expr_sentinel s (parser);
@@ -10981,7 +10990,24 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
   if (type1 == error_mark_node)
     return error_mark_node;
 
-  if (binary)
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    {
+      cp_parser_require (parser, CPP_COMMA, RT_COMMA);
+      tree rest = cp_parser_enclosed_template_argument_list (parser);
+      for (tree elt : tree_vec_range (rest))
+	{
+	  if (!TYPE_P (elt))
+	    {
+	      error_at (cp_expr_loc_or_input_loc (elt),
+			"trailing argument to %<__type_pack_element%> "
+			"is not a type");
+	      return error_mark_node;
+	    }
+	  type2 = tree_cons (NULL_TREE, elt, type2);
+	}
+      type2 = nreverse (type2);
+    }
+  else if (binary)
     {
       cp_parser_require (parser, CPP_COMMA, RT_COMMA);
 
@@ -11012,7 +11038,11 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
     }
 
   location_t finish_loc = cp_lexer_peek_token (parser->lexer)->location;
-  parens.require_close (parser);
+  if (kind == CPTK_TYPE_PACK_ELEMENT)
+    /* cp_parser_enclosed_template_argument_list above already took care
+       of parsing the closing '>'.  */;
+  else
+    parens.require_close (parser);
 
   /* Construct a location of the form:
        __is_trivially_copyable(_Tp)
@@ -11030,7 +11060,7 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
       return cp_expr (finish_bases (type1, true), trait_loc);
     default:
       if (type)
-	return finish_trait_type (kind, type1, type2);
+	return finish_trait_type (kind, type1, type2, tf_warning_or_error);
       else
 	return finish_trait_expr (trait_loc, kind, type1, type2);
     }

gcc/cp/pt.cc

@@ -16696,9 +16696,13 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
     case TRAIT_TYPE:
       {
-	tree type1 = tsubst (TRAIT_TYPE_TYPE1 (t), args, complain, in_decl);
+	tree type1 = TRAIT_TYPE_TYPE1 (t);
+	if (TYPE_P (type1))
+	  type1 = tsubst (type1, args, complain, in_decl);
+	else
+	  type1 = tsubst_copy_and_build (type1, args, complain, in_decl);
 	tree type2 = tsubst (TRAIT_TYPE_TYPE2 (t), args, complain, in_decl);
-	type = finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2);
+	type = finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2, complain);
 	return cp_build_qualified_type (type,
 					cp_type_quals (t) | cp_type_quals (type),
 					complain | tf_ignore_bad_quals);

gcc/cp/semantics.cc

@@ -4470,6 +4470,36 @@ finish_underlying_type (tree type)
   return underlying_type;
 }
 
+/* Implement the __type_pack_element keyword: Return the type
+   at index IDX within TYPES.  */
+
+static tree
+finish_type_pack_element (tree idx, tree types, tsubst_flags_t complain)
+{
+  idx = maybe_constant_value (idx);
+  if (TREE_CODE (idx) != INTEGER_CST || !INTEGRAL_TYPE_P (TREE_TYPE (idx)))
+    {
+      if (complain & tf_error)
+	error ("%<__type_pack_element%> index is not an integral constant");
+      return error_mark_node;
+    }
+  HOST_WIDE_INT val = tree_to_shwi (idx);
+  if (val < 0)
+    {
+      if (complain & tf_error)
+	error ("%<__type_pack_element%> index is negative");
+      return error_mark_node;
+    }
+  tree result = chain_index (val, types);
+  if (!result)
+    {
+      if (complain & tf_error)
+	error ("%<__type_pack_element%> index is out of range");
+      return error_mark_node;
+    }
+  return TREE_VALUE (result);
+}
+
 /* Implement the __direct_bases keyword: Return the direct base classes
    of type.  */
 
@@ -12245,7 +12275,8 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
 /* Process a trait type.  */
 
 tree
-finish_trait_type (cp_trait_kind kind, tree type1, tree type2)
+finish_trait_type (cp_trait_kind kind, tree type1, tree type2,
+		   tsubst_flags_t complain)
 {
   if (type1 == error_mark_node
       || type2 == error_mark_node)
@@ -12269,17 +12300,23 @@ finish_trait_type (cp_trait_kind kind, tree type1, tree type2)
     {
     case CPTK_UNDERLYING_TYPE:
       return finish_underlying_type (type1);
+
     case CPTK_REMOVE_CV:
       return cv_unqualified (type1);
+
     case CPTK_REMOVE_REFERENCE:
       if (TYPE_REF_P (type1))
 	type1 = TREE_TYPE (type1);
       return type1;
+
     case CPTK_REMOVE_CVREF:
       if (TYPE_REF_P (type1))
 	type1 = TREE_TYPE (type1);
       return cv_unqualified (type1);
 
+    case CPTK_TYPE_PACK_ELEMENT:
+      return finish_type_pack_element (type1, type2, complain);
+
 #define DEFTRAIT_EXPR(CODE, NAME, ARITY) \
     case CPTK_##CODE:
 #include "cp-trait.def"

gcc/cp/tree.cc

@@ -1792,14 +1792,18 @@ strip_typedefs (tree t, bool *remove_attributes /* = NULL */,
       break;
     case TRAIT_TYPE:
       {
-	tree type1 = strip_typedefs (TRAIT_TYPE_TYPE1 (t),
-				     remove_attributes, flags);
+	tree type1 = TRAIT_TYPE_TYPE1 (t);
+	if (TYPE_P (type1))
+	  type1 = strip_typedefs (type1, remove_attributes, flags);
+	else
+	  type1 = strip_typedefs_expr (type1, 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);
+	  result = finish_trait_type (TRAIT_TYPE_KIND (t), type1, type2,
+				      tf_warning_or_error);
       }
       break;
     case TYPE_PACK_EXPANSION:

gcc/cp/typeck.cc

@@ -1632,7 +1632,7 @@ structural_comptypes (tree t1, tree t2, int strict)
     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))
+      if (!cp_tree_equal (TRAIT_TYPE_TYPE1 (t1), TRAIT_TYPE_TYPE1 (t2))
 	  || !cp_tree_equal (TRAIT_TYPE_TYPE2 (t1), TRAIT_TYPE_TYPE2 (t2)))
 	return false;
       break;

gcc/testsuite/g++.dg/ext/type_pack_element1.C

@@ -0,0 +1,19 @@
+// { dg-do compile { target c++11 } }
+
+using ty0 = __type_pack_element<0, int>;
+using ty0 = __type_pack_element<0, int, char>;
+using ty0 = int;
+
+using ty1 = __type_pack_element<1, int, char>;
+using ty1 = __type_pack_element<(6 - 5) * 1, int, char>;
+using ty1 = char;
+
+template<int N, class... Ts>
+using __const_type_pack_element_t = const __type_pack_element<N, Ts...>;
+
+using ty2 = __const_type_pack_element_t<2, int, char, long>;
+using ty2 = const long;
+
+template<class T> struct A { };
+using ty3 = __type_pack_element<3, int, int, int, A<int>>;
+using ty3 = A<int>;

gcc/testsuite/g++.dg/ext/type_pack_element2.C

@@ -0,0 +1,14 @@
+// { dg-do compile { target c++11 } }
+
+int p;
+
+using type = __type_pack_element<&p, int>;      // { dg-error "not an integral constant" }
+using type = __type_pack_element<1, int>;       // { dg-error "out of range" }
+using type = __type_pack_element<2, int, char>; // { dg-error "out of range" }
+using type = __type_pack_element<-1, int>;      // { dg-error "negative" }
+
+template<int N, class... Ts>
+using __type_pack_element_t = __type_pack_element<N, Ts...>;
+// { dg-error "out of range" "" { target *-*-* } .-1 }
+
+using type = __type_pack_element_t<3, int, char, long>; // { dg-message "here" }

gcc/testsuite/g++.dg/ext/type_pack_element3.C

@@ -0,0 +1,22 @@
+// { dg-do compile { target c++11 } }
+
+template<class T, T N, class... Ts, class = __type_pack_element<N, Ts...>>
+constexpr int f(int) { return 1; }
+
+template<class T, T N, class... Ts>
+constexpr int f(...) { return 2; };
+
+int p;
+
+static_assert(f<int, 0, void, char>(0) == 1, "");
+static_assert(f<int, 1, void, char>(0) == 1, "");
+static_assert(f<int, 2, void, char>(0) == 2, "");
+static_assert(f<int*, &p, void, char>(0) == 2, "");
+
+template<class T, class U> struct A;
+template<class T> struct A<T, __type_pack_element<sizeof(T), void, long>> { };
+template struct A<char, long>;
+
+template<class T, class U> struct B;
+template<class T> struct B<T, __type_pack_element<0, T, short>> { };
+template struct B<int, int>;

libstdc++-v3/include/bits/utility.h

@@ -224,6 +224,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // C++17
 #endif // C++14
 
+#if __has_builtin(__type_pack_element)
+  template<size_t _Np, typename... _Types>
+    struct _Nth_type
+    { using type = __type_pack_element<_Np, _Types...>; };
+#else
   template<size_t _Np, typename... _Types>
     struct _Nth_type
     { };
@@ -262,6 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct _Nth_type<1, _Tp0, _Tp1, _Tp2, _Rest...>
     { using type = _Tp1; };
 #endif
+#endif
 
 #if __cplusplus > 202002L
 #define __cpp_lib_ranges_zip 202110L // for <tuple> and <utility>

libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc

@@ -61,3 +61,4 @@ test03()
 
 // { dg-error "tuple index must be in range" "" { target *-*-* } 0 }
 // { dg-prune-output "no type named 'type' in .*_Nth_type" }
+// { dg-prune-output "'__type_pack_element' index is out of range" }