On Thu, Nov 4, 2021 at 2:34 PM Jonathan Wakely via Libstdc++
<libstd...@gcc.gnu.org> wrote:
>
> Tested powerpc64le-linux, committed to trunk.
>
>
> This reduces the number of class template instantiations needed for code
> using tuples, by reusing _Nth_type in tuple_element and specializing
> tuple_size_v for tuple, pair and array (and const-qualified versions of
> them).
>
> Also define the _Nth_type primary template as a complete type (but with
> no nested 'type' member). This avoids "invalid use of incomplete type"
> errors for out-of-range specializations of tuple_element. Those errors
> would probably be confusing and unhelpful for users. We already have
> a user-friendly static assert in tuple_element itself.
>
> Also ensure that tuple_size_v is available whenever tuple_size is (as
> proposed by LWG 3387). We already do that for tuple_element_t.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/stl_pair.h (tuple_size_v): Define partial
> specializations for std::pair.
> * include/bits/utility.h (_Nth_type): Move definition here
> and define primary template.
> (tuple_size_v): Move definition here.
> * include/std/array (tuple_size_v): Define partial
> specializations for std::array.
> * include/std/tuple (tuple_size_v): Move primary template to
> <bits/utility.h>. Define partial specializations for
> std::tuple.
> (tuple_element): Change definition to use _Nth_type.
> * include/std/variant (_Nth_type): Move to <bits/utility.h>.
> (variant_alternative, variant): Adjust qualification of
> _Nth_type.
> * testsuite/20_util/tuple/element_access/get_neg.cc: Prune
> additional errors from _Nth_type.
> ---
> libstdc++-v3/include/bits/stl_pair.h | 8 +++
> libstdc++-v3/include/bits/utility.h | 51 +++++++++++++++-
> libstdc++-v3/include/std/array | 8 +++
> libstdc++-v3/include/std/tuple | 39 +++++--------
> libstdc++-v3/include/std/variant | 58 ++-----------------
> .../20_util/tuple/element_access/get_neg.cc | 1 +
> 6 files changed, 84 insertions(+), 81 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/stl_pair.h
> b/libstdc++-v3/include/bits/stl_pair.h
> index 5b400daf97f..6081e0c7fe9 100644
> --- a/libstdc++-v3/include/bits/stl_pair.h
> +++ b/libstdc++-v3/include/bits/stl_pair.h
> @@ -771,6 +771,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> struct tuple_element<1, pair<_Tp1, _Tp2>>
> { typedef _Tp2 type; };
>
> +#if __cplusplus >= 201703L
> + template<typename _Tp1, typename _Tp2>
> + inline constexpr size_t tuple_size_v<pair<_Tp1, _Tp2>> = 2;
> +
> + template<typename _Tp1, typename _Tp2>
> + inline constexpr size_t tuple_size_v<const pair<_Tp1, _Tp2>> = 2;
> +#endif
> +
> /// @cond undocumented
> template<size_t _Int>
> struct __pair_get;
> diff --git a/libstdc++-v3/include/bits/utility.h
> b/libstdc++-v3/include/bits/utility.h
> index fce52a4530d..c9ffa008217 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -70,6 +70,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> struct tuple_size<const volatile __enable_if_has_tuple_size<_Tp>>
> : public tuple_size<_Tp> { };
>
> +#if __cplusplus >= 201703L
> + template<typename _Tp>
> + inline constexpr size_t tuple_size_v = tuple_size<_Tp>::value;
> +#endif
> +
> /// Gives the type of the ith element of a given tuple type.
> template<size_t __i, typename _Tp>
> struct tuple_element;
> @@ -97,8 +102,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> };
>
> #if __cplusplus >= 201402L
> -// The standard says this macro and alias template should be in <tuple>
> -// but we define them here, to be available in <utility> and <array> too.
> +// The standard says this macro and alias template should be in <tuple> but
> we
> +// we define them here, to be available in <array>, <utility> and <ranges>
> too.
> +// _GLIBCXX_RESOLVE_LIB_DEFECTS
> +// 3378. tuple_size_v/tuple_element_t should be available when
> +// tuple_size/tuple_element are
> #define __cpp_lib_tuple_element_t 201402L
>
> template<size_t __i, typename _Tp>
> @@ -195,6 +203,45 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif // C++17
> #endif // C++14
>
> + template<size_t _Np, typename... _Types>
> + struct _Nth_type
> + { };
> +
> + template<typename _Tp0, typename... _Rest>
> + struct _Nth_type<0, _Tp0, _Rest...>
> + { using type = _Tp0; };
> +
> + template<typename _Tp0, typename _Tp1, typename... _Rest>
> + struct _Nth_type<1, _Tp0, _Tp1, _Rest...>
> + { using type = _Tp1; };
> +
> + template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> + struct _Nth_type<2, _Tp0, _Tp1, _Tp2, _Rest...>
> + { using type = _Tp2; };
> +
> + template<size_t _Np, typename _Tp0, typename _Tp1, typename _Tp2,
> + typename... _Rest>
> +#if __cpp_concepts
> + requires (_Np >= 3)
> +#endif
> + struct _Nth_type<_Np, _Tp0, _Tp1, _Tp2, _Rest...>
> + : _Nth_type<_Np - 3, _Rest...>
> + { };
> +
> +#if ! __cpp_concepts // Need additional specializations to avoid ambiguities.
> + template<typename _Tp0, typename _Tp1, typename... _Rest>
> + struct _Nth_type<0, _Tp0, _Tp1, _Rest...>
> + { using type = _Tp0; };
> +
> + template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> + struct _Nth_type<0, _Tp0, _Tp1, _Tp2, _Rest...>
> + { using type = _Tp0; };
> +
> + template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> + struct _Nth_type<1, _Tp0, _Tp1, _Tp2, _Rest...>
> + { using type = _Tp1; };
> +#endif
> +
> _GLIBCXX_END_NAMESPACE_VERSION
> } // namespace
>
> diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
> index 3e12d35157c..413f8e2be01 100644
> --- a/libstdc++-v3/include/std/array
> +++ b/libstdc++-v3/include/std/array
> @@ -481,6 +481,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> using type = _Tp;
> };
>
> +#if __cplusplus >= 201703L
> + template<typename _Tp, size_t _Nm>
> + inline constexpr size_t tuple_size_v<array<_Tp, _Nm>> = _Nm;
> +
> + template<typename _Tp, size_t _Nm>
> + inline constexpr size_t tuple_size_v<const array<_Tp, _Nm>> = _Nm;
> +#endif
> +
> template<typename _Tp, size_t _Nm>
> struct __is_tuple_like_impl<array<_Tp, _Nm>> : true_type
> { };
> diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
> index aaee0b8826a..b82cdf12569 100644
> --- a/libstdc++-v3/include/std/tuple
> +++ b/libstdc++-v3/include/std/tuple
> @@ -1344,36 +1344,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> struct tuple_size<tuple<_Elements...>>
> : public integral_constant<size_t, sizeof...(_Elements)> { };
>
> -#if __cplusplus > 201402L
> - template <typename _Tp>
> - inline constexpr size_t tuple_size_v = tuple_size<_Tp>::value;
> +#if __cplusplus >= 201703L
> + template<typename... _Types>
> + inline constexpr size_t tuple_size_v<tuple<_Types>>
Missing ... after _Types?
> + = sizeof...(_Types);
> +
> + template<typename... _Types>
> + inline constexpr size_t tuple_size_v<const tuple<_Types>>
Same here. (I guess now's a good time to fix PR100652!)
> + = sizeof...(_Types);
> #endif
>
> - /**
> - * Recursive case for tuple_element: strip off the first element in
> - * the tuple and retrieve the (i-1)th element of the remaining tuple.
> - */
> - template<size_t __i, typename _Head, typename... _Tail>
> - struct tuple_element<__i, tuple<_Head, _Tail...> >
> - : tuple_element<__i - 1, tuple<_Tail...> > { };
> -
> - /**
> - * Basis case for tuple_element: The first element is the one we're
> seeking.
> - */
> - template<typename _Head, typename... _Tail>
> - struct tuple_element<0, tuple<_Head, _Tail...> >
> + /// Trait to get the Ith element type from a tuple.
> + template<size_t __i, typename... _Types>
> + struct tuple_element<__i, tuple<_Types...>>
> {
> - typedef _Head type;
> - };
> + static_assert(__i < sizeof...(_Types), "tuple index must be in range");
>
> - /**
> - * Error case for tuple_element: invalid index.
> - */
> - template<size_t __i>
> - struct tuple_element<__i, tuple<>>
> - {
> - static_assert(__i < tuple_size<tuple<>>::value,
> - "tuple index must be in range");
> + using type = typename _Nth_type<__i, _Types...>::type;
> };
>
> template<size_t __i, typename _Head, typename... _Tail>
> diff --git a/libstdc++-v3/include/std/variant
> b/libstdc++-v3/include/std/variant
> index c4c307b7bb2..993ce3dba91 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -54,51 +54,6 @@ namespace std _GLIBCXX_VISIBILITY(default)
> {
> _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> -namespace __detail
> -{
> -namespace __variant
> -{
> - template<size_t _Np, typename... _Types>
> - struct _Nth_type;
> -
> - template<typename _Tp0, typename... _Rest>
> - struct _Nth_type<0, _Tp0, _Rest...>
> - { using type = _Tp0; };
> -
> - template<typename _Tp0, typename _Tp1, typename... _Rest>
> - struct _Nth_type<1, _Tp0, _Tp1, _Rest...>
> - { using type = _Tp1; };
> -
> - template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> - struct _Nth_type<2, _Tp0, _Tp1, _Tp2, _Rest...>
> - { using type = _Tp2; };
> -
> - template<size_t _Np, typename _Tp0, typename _Tp1, typename _Tp2,
> - typename... _Rest>
> -#if __cpp_concepts
> - requires (_Np >= 3)
> -#endif
> - struct _Nth_type<_Np, _Tp0, _Tp1, _Tp2, _Rest...>
> - : _Nth_type<_Np - 3, _Rest...>
> - { };
> -
> -#if ! __cpp_concepts // Need additional specializations to avoid ambiguities.
> - template<typename _Tp0, typename _Tp1, typename... _Rest>
> - struct _Nth_type<0, _Tp0, _Tp1, _Rest...>
> - { using type = _Tp0; };
> -
> - template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> - struct _Nth_type<0, _Tp0, _Tp1, _Tp2, _Rest...>
> - { using type = _Tp0; };
> -
> - template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> - struct _Nth_type<1, _Tp0, _Tp1, _Tp2, _Rest...>
> - { using type = _Tp1; };
> -#endif
> -
> -} // namespace __variant
> -} // namespace __detail
> -
> #if __cplusplus >= 202002L && __cpp_concepts
> // P2231R1 constexpr needs constexpr unions and constrained destructors.
> # define __cpp_lib_variant 202106L
> @@ -145,8 +100,7 @@ namespace __variant
> {
> static_assert(_Np < sizeof...(_Types));
>
> - using type
> - = typename __detail::__variant::_Nth_type<_Np, _Types...>::type;
> + using type = typename _Nth_type<_Np, _Types...>::type;
> };
>
> template<size_t _Np, typename _Variant>
> @@ -1442,8 +1396,7 @@ namespace __variant
> = __detail::__variant::__accepted_index<_Tp, variant>::value;
>
> template<size_t _Np, typename = enable_if_t<(_Np < sizeof...(_Types))>>
> - using __to_type
> - = typename __detail::__variant::_Nth_type<_Np, _Types...>::type;
> + using __to_type = typename _Nth_type<_Np, _Types...>::type;
>
> template<typename _Tp, typename = enable_if_t<__not_self<_Tp>>>
> using __accepted_type = __to_type<__accepted_index<_Tp>>;
> @@ -1580,7 +1533,7 @@ namespace __variant
> emplace(_Args&&... __args)
> {
> namespace __variant = std::__detail::__variant;
> - using type = typename __variant::_Nth_type<_Np, _Types...>::type;
> + using type = typename _Nth_type<_Np, _Types...>::type;
> // Provide the strong exception-safety guarantee when possible,
> // to avoid becoming valueless.
> if constexpr (is_nothrow_constructible_v<type, _Args...>)
> @@ -1620,7 +1573,7 @@ namespace __variant
> emplace(initializer_list<_Up> __il, _Args&&... __args)
> {
> namespace __variant = std::__detail::__variant;
> - using type = typename __variant::_Nth_type<_Np, _Types...>::type;
> + using type = typename _Nth_type<_Np, _Types...>::type;
> // Provide the strong exception-safety guarantee when possible,
> // to avoid becoming valueless.
> if constexpr (is_nothrow_constructible_v<type,
> @@ -1803,8 +1756,7 @@ namespace __variant
> constexpr size_t __max = 11; // "These go to eleven."
>
> // The type of the first variant in the pack.
> - using _V0
> - = typename __detail::__variant::_Nth_type<0, _Variants...>::type;
> + using _V0 = typename _Nth_type<0, _Variants...>::type;
> // The number of alternatives in that first variant.
> constexpr auto __n = variant_size_v<remove_reference_t<_V0>>;
>
> diff --git a/libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc
> b/libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc
> index 225bb6245a6..113a7fd62de 100644
> --- a/libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc
> +++ b/libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc
> @@ -60,3 +60,4 @@ test03()
> }
>
> // { dg-error "tuple index must be in range" "" { target *-*-* } 0 }
> +// { dg-prune-output "no type named 'type' in .*_Nth_type" }
> --
> 2.31.1
>
