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

[Jason Merrill <jason@redhat.com>]

On 1/9/23 14:25, Patrick Palka via Gcc-patches wrote:
> On Mon, 9 Jan 2023, Patrick Palka wrote:
> 
>> On Wed, 5 Oct 2022, Patrick Palka wrote:
>>
>>> On Thu, 7 Jul 2022, Jonathan Wakely via Gcc-patches wrote:
>>>
>>>> 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. __builtin_type_pack_element(1, char, int, float) is
>>>> int.
>>>>
>>>> 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.
>>>>
>>>> Clang has a similar built-in, __type_pack_element<N, T...>, but that's a
>>>> "magic template" built-in using <> syntax, which GCC doesn't support. So
>>>> this provides an equivalent feature, but as a built-in function using
>>>> parens instead of <>. I don't really like the name "type pack element"
>>>> (it gives you an element from a pack of types) but the semi-consistency
>>>> with Clang seems like a reasonable argument in favour of keeping the
>>>> name. I'd be open to alternative names though, e.g. __builtin_nth_type
>>>> or __builtin_type_at_index.
>>>
>>> Rather than giving the trait a different name from __type_pack_element,
>>> I wonder if we could just special case cp_parser_trait to expect <>
>>> instead of parens for this trait?
>>>
>>> Btw the frontend recently got a generic TRAIT_TYPE tree code, which gets
>>> rid of much of the boilerplate of adding a new type-yielding built-in
>>> trait, see e.g. cp-trait.def.
>>
>> Here's a tested patch based on Jonathan's original patch that implements
>> the built-in in terms of TRAIT_TYPE, names it __type_pack_element
>> instead of __builtin_type_pack_element, and treats invocations of it
>> like a template-id instead of a call (to match Clang).
>>
>> -- >8 --
>>
>> Subject: [PATCH] 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.
>>
>> Some caveats:
>>
>>    * Clang's version of the built-in seems to act like a "magic template"
>>      that can e.g. be used as a template template argument.  For simplicity
>>      we implement it in a more ad-hoc way.
>>    * Our parsing of the <>'s in __type_pack_element<...> is currently
>>      rudimentary and doesn't try to disambiguate a trailing >> vs > >
>>      as cp_parser_enclosed_template_argument_list does.
> 
> Hmm, this latter caveat turns out to be inconvenient (for code such as
> type_pack_element3.C) and admits an easy workaround inspired by what
> cp_parser_enclosed_template_argument_list does.
> 
> v2: Consider the >> in __type_pack_element<0, int, char>> to be two >'s.
>      Handle non-type TRAIT_TYPE_TYPE1 in strip_typedefs (for sake of
>      CPTK_TYPE_PACK_ELEMENT).

Why not use cp_parser_enclosed_template_argument_list directly?

> -- >8 --
> 
> Subject: [PATCH] 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
> 	CPTK_TYPE_PACK_ELEMENT.
> 	* 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 TRAIT_TYPE_TYPE1.
> 	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 TRAIT_TYPE_TYPE1.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/utility.h (_Nth_type): Conditionally define in
> 	terms of __type_pack_element if available.
> 
> 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                           |  1 +
>   gcc/cp/cp-tree.h                              |  2 +-
>   gcc/cp/cxx-pretty-print.cc                    | 17 +++++--
>   gcc/cp/parser.cc                              | 46 +++++++++++++++----
>   gcc/cp/pt.cc                                  |  8 +++-
>   gcc/cp/semantics.cc                           | 39 +++++++++++++++-
>   gcc/cp/tree.cc                                | 10 ++--
>   gcc/cp/typeck.cc                              |  2 +-
>   gcc/testsuite/g++.dg/ext/type_pack_element1.C | 19 ++++++++
>   gcc/testsuite/g++.dg/ext/type_pack_element2.C | 14 ++++++
>   gcc/testsuite/g++.dg/ext/type_pack_element3.C | 22 +++++++++
>   libstdc++-v3/include/bits/utility.h           |  6 +++
>   12 files changed, 165 insertions(+), 21 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/ext/type_pack_element1.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/type_pack_element2.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/type_pack_element3.C
> 
> diff --git a/gcc/cp/cp-trait.def b/gcc/cp/cp-trait.def
> index 823899a26c5..63f6b101eeb 100644
> --- a/gcc/cp/cp-trait.def
> +++ b/gcc/cp/cp-trait.def
> @@ -89,6 +89,7 @@ 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.  */
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 1f4967c2ba0..f3cc2c5874c 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -7764,7 +7764,7 @@ 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);
> diff --git a/gcc/cp/cxx-pretty-print.cc b/gcc/cp/cxx-pretty-print.cc
> index 8ca1b8f234a..07a0406e491 100644
> --- a/gcc/cp/cxx-pretty-print.cc
> +++ b/gcc/cp/cxx-pretty-print.cc
> @@ -2625,8 +2625,16 @@ pp_cxx_trait (cxx_pretty_printer *pp, tree t)
>   #undef DEFTRAIT
>       }
>   
> -  pp_cxx_left_paren (pp);
> -  pp->type_id (type1);
> +  if (kind == CPTK_TYPE_PACK_ELEMENT)
> +    {
> +      pp_cxx_begin_template_argument_list (pp);
> +      pp->expression (type1);
> +    }
> +  else
> +    {
> +      pp_cxx_left_paren (pp);
> +      pp->type_id (type1);
> +    }
>     if (type2)
>       {
>         if (TREE_CODE (type2) != TREE_LIST)
> @@ -2641,7 +2649,10 @@ 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:
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index 8b1658decba..0a88d83d955 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -10683,9 +10683,9 @@ 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;
> @@ -10912,12 +10912,25 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
>     cp_lexer_consume_token (parser->lexer);
>   
>     matching_parens parens;
> -  parens.require_open (parser);
>   
> -  {
> -    type_id_in_expr_sentinel s (parser);
> -    type1 = cp_parser_type_id (parser);
> -  }
> +  /* Unlike the other built-in traits, __type_pack_element is unique in that
> +     it takes an expression as its first argument and it uses template-id
> +     syntax instead of function call syntax (for compatibility with Clang).
> +     We special case these properties of __type_pack_element here and elsewhere.
> +
> +     TODO: Generalize the trait infrastructure so that __type_pack_element
> +     is no longer a special case.  */
> +  if (kind == CPTK_TYPE_PACK_ELEMENT)
> +    {
> +      cp_parser_require (parser, CPP_LESS, RT_LESS);
> +      type1 = cp_parser_constant_expression (parser);
> +    }
> +  else
> +    {
> +      parens.require_open (parser);
> +      type_id_in_expr_sentinel s (parser);
> +      type1 = cp_parser_type_id (parser);
> +    }
>   
>     if (type1 == error_mark_node)
>       return error_mark_node;
> @@ -10953,7 +10966,20 @@ 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)
> +    {
> +      /* As in cp_parser_enclosed_template_argument_list, a trailing
> +	 '>>' here is considered to be two separate '>' tokens.  */
> +      if (cp_lexer_next_token_is (parser->lexer, CPP_RSHIFT))
> +	{
> +	  cp_token *token = cp_lexer_peek_token (parser->lexer);
> +	  token->type = CPP_GREATER;
> +	}
> +      else
> +	cp_parser_require (parser, CPP_GREATER, RT_GREATER);
> +    }
> +  else
> +    parens.require_close (parser);
>   
>     /* Construct a location of the form:
>          __is_trivially_copyable(_Tp)
> @@ -10971,7 +10997,7 @@ 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);
>       }
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index cbe5898b553..2e83970a252 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -16576,9 +16576,13 @@ 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 (TRAIT_TYPE_KIND (t) != CPTK_TYPE_PACK_ELEMENT)
> +	  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);
> diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> index ef5bf2430b1..af9edf59b3b 100644
> --- a/gcc/cp/semantics.cc
> +++ b/gcc/cp/semantics.cc
> @@ -4457,6 +4457,36 @@ 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.  */
>   
> @@ -12213,7 +12243,8 @@ 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)
> @@ -12237,17 +12268,23 @@ 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"
> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> index faf01616f87..d834a0fd38b 100644
> --- a/gcc/cp/tree.cc
> +++ b/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:
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 69b1268cfec..f5548737e4c 100644
> --- a/gcc/cp/typeck.cc
> +++ b/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;
> diff --git a/gcc/testsuite/g++.dg/ext/type_pack_element1.C b/gcc/testsuite/g++.dg/ext/type_pack_element1.C
> new file mode 100644
> index 00000000000..46858555502
> --- /dev/null
> +++ b/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>;
> diff --git a/gcc/testsuite/g++.dg/ext/type_pack_element2.C b/gcc/testsuite/g++.dg/ext/type_pack_element2.C
> new file mode 100644
> index 00000000000..363cfa92514
> --- /dev/null
> +++ b/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<-1, int>; // { dg-error "negative" }
> +using type = __type_pack_element<2, int, char>; // { dg-error "out of range" }
> +
> +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" }
> diff --git a/gcc/testsuite/g++.dg/ext/type_pack_element3.C b/gcc/testsuite/g++.dg/ext/type_pack_element3.C
> new file mode 100644
> index 00000000000..269f84f464f
> --- /dev/null
> +++ b/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>;
> diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
> index 6a192e27836..5d524d33b4c 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/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
>   
>   _GLIBCXX_END_NAMESPACE_VERSION
>   } // namespace