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?
>
> > -- >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 <jwak...@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
>
>
