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

[Jason Merrill <jason@redhat.com>]

On 1/25/23 15:35, Patrick Palka wrote:
> On Tue, 17 Jan 2023, Jason Merrill wrote:
> 
>> 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?
> 
> If we used cp_parser_enclosed_template_argument_list we would then need
> to convert the returned TREE_VEC into a TREE_LIST and also diagnose
> argument kind mismatches (i.e. verify the first argument is an
> expression and the rest are types).  It seemed like more complexity
> overall then just duplicating the >> splitting logic, but I can do that
> if you prefer?

I think I would prefer that, parser stuff can be pretty subtle.

Instead of turning the TREE_VEC into a TREE_LIST, we could handle 
TREE_VEC as a trait operand?

>>> -- >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
>>
>>
>