On Mon, 7 Oct 2024, Patrick Palka wrote:
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk only?
> This doesn't seem worth backporting since there should be no
> behavior change.
>
> -- >8 --
>
> This implements the library changes in P0849R8 "auto(x): decay-copy
> in the language" which consist of replacing most uses of the
> exposition-only function decay-copy with auto(x) throughout the library
> wording.
>
> Note the main difference between the two is that decay-copy materializes
> its argument whereas auto(x) doesn't, and so the latter is a no-op when
> its argument is a prvalue. Effectively the former could introduce an
> unnecessary move constructor call in some contexts. In C++20 and earlier
> we could emulate auto(x) with decay_t<decltype((x))>(x).
I should note this patch is treating this paper as a DR against C++20,
which should be fine since there should be no behavior change in
practice (especially in light of LWG 3724 which constrains decay-copy
to make it SFINAE-friendly).
>
> After this paper the only remaining uses of decay-copy in the library
> are in the specification of some range adaptors. In our implementation
> of those range adaptors I believe decay-copy is already implied which
> is why we don't mirror the wording and use __decay_copy explicitly. So
> since it's apparently no longer needed this patch goes ahead and removes
> __decay_copy.
>
> libstdc++-v3/ChangeLog:
>
> * c++config (_GLIBCXX_AUTO_CAST): Define.
> * include/bits/iterator_concepts.h (_Decay_copy, __decay_copy):
> Remove.
> (__member_begin, __adl_begin): Use _GLIBCXX_AUTO_CAST instead of
> __decay_copy as per P0849R8.
> * include/bits/ranges_base.h (_Begin): Likewise.
> (__member_end, __adl_end, _End): Likewise.
> (__member_rbegin, __adl_rbegin, _RBegin): Likewise.
> (__member_rend, __adl_rend, _Rend): Likewise.
> (__member_size, __adl_size, _Size): Likewise.
> (_Data): Likewise.
> ---
> libstdc++-v3/include/bits/c++config | 6 +++
> libstdc++-v3/include/bits/iterator_concepts.h | 13 +-----
> libstdc++-v3/include/bits/ranges_base.h | 40 +++++++++----------
> 3 files changed, 28 insertions(+), 31 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/c++config
> b/libstdc++-v3/include/bits/c++config
> index 29d795f687c..fdbf90e28fc 100644
> --- a/libstdc++-v3/include/bits/c++config
> +++ b/libstdc++-v3/include/bits/c++config
> @@ -265,6 +265,12 @@
> #define _GLIBCXX_NOEXCEPT_QUAL
> #endif
>
> +#if __cpp_auto_cast
> +# define _GLIBCXX_AUTO_CAST(X) auto(X)
> +#else
> +# define _GLIBCXX_AUTO_CAST(X) ::std::__decay_t<decltype((X))>(X)
> +#endif
> +
> // Macro for extern template, ie controlling template linkage via use
> // of extern keyword on template declaration. As documented in the g++
> // manual, it inhibits all implicit instantiations and is used
> diff --git a/libstdc++-v3/include/bits/iterator_concepts.h
> b/libstdc++-v3/include/bits/iterator_concepts.h
> index 490a362cdf1..0fcfed56737 100644
> --- a/libstdc++-v3/include/bits/iterator_concepts.h
> +++ b/libstdc++-v3/include/bits/iterator_concepts.h
> @@ -1003,19 +1003,10 @@ namespace ranges
> {
> using std::__detail::__class_or_enum;
>
> - struct _Decay_copy final
> - {
> - template<typename _Tp>
> - constexpr decay_t<_Tp>
> - operator()(_Tp&& __t) const
> - noexcept(is_nothrow_convertible_v<_Tp, decay_t<_Tp>>)
> - { return std::forward<_Tp>(__t); }
> - } inline constexpr __decay_copy{};
> -
> template<typename _Tp>
> concept __member_begin = requires(_Tp& __t)
> {
> - { __decay_copy(__t.begin()) } -> input_or_output_iterator;
> + { _GLIBCXX_AUTO_CAST(__t.begin()) } -> input_or_output_iterator;
> };
>
> // Poison pill so that unqualified lookup doesn't find std::begin.
> @@ -1025,7 +1016,7 @@ namespace ranges
> concept __adl_begin = __class_or_enum<remove_reference_t<_Tp>>
> && requires(_Tp& __t)
> {
> - { __decay_copy(begin(__t)) } -> input_or_output_iterator;
> + { _GLIBCXX_AUTO_CAST(begin(__t)) } -> input_or_output_iterator;
> };
>
> // Simplified version of std::ranges::begin that only supports lvalues,
> diff --git a/libstdc++-v3/include/bits/ranges_base.h
> b/libstdc++-v3/include/bits/ranges_base.h
> index cb2eba1f841..80ff1e300ce 100644
> --- a/libstdc++-v3/include/bits/ranges_base.h
> +++ b/libstdc++-v3/include/bits/ranges_base.h
> @@ -115,9 +115,9 @@ namespace ranges
> if constexpr (is_array_v<remove_reference_t<_Tp>>)
> return true;
> else if constexpr (__member_begin<_Tp>)
> - return noexcept(__decay_copy(std::declval<_Tp&>().begin()));
> + return noexcept(_GLIBCXX_AUTO_CAST(std::declval<_Tp&>().begin()));
> else
> - return noexcept(__decay_copy(begin(std::declval<_Tp&>())));
> + return noexcept(_GLIBCXX_AUTO_CAST(begin(std::declval<_Tp&>())));
> }
>
> public:
> @@ -142,7 +142,7 @@ namespace ranges
> template<typename _Tp>
> concept __member_end = requires(_Tp& __t)
> {
> - { __decay_copy(__t.end()) } -> sentinel_for<__range_iter_t<_Tp>>;
> + { _GLIBCXX_AUTO_CAST(__t.end()) } ->
> sentinel_for<__range_iter_t<_Tp>>;
> };
>
> // Poison pill so that unqualified lookup doesn't find std::end.
> @@ -152,7 +152,7 @@ namespace ranges
> concept __adl_end = __class_or_enum<remove_reference_t<_Tp>>
> && requires(_Tp& __t)
> {
> - { __decay_copy(end(__t)) } -> sentinel_for<__range_iter_t<_Tp>>;
> + { _GLIBCXX_AUTO_CAST(end(__t)) } -> sentinel_for<__range_iter_t<_Tp>>;
> };
>
> struct _End
> @@ -165,9 +165,9 @@ namespace ranges
> if constexpr (is_bounded_array_v<remove_reference_t<_Tp>>)
> return true;
> else if constexpr (__member_end<_Tp>)
> - return noexcept(__decay_copy(std::declval<_Tp&>().end()));
> + return noexcept(_GLIBCXX_AUTO_CAST(std::declval<_Tp&>().end()));
> else
> - return noexcept(__decay_copy(end(std::declval<_Tp&>())));
> + return noexcept(_GLIBCXX_AUTO_CAST(end(std::declval<_Tp&>())));
> }
>
> public:
> @@ -192,7 +192,7 @@ namespace ranges
> template<typename _Tp>
> concept __member_rbegin = requires(_Tp& __t)
> {
> - { __decay_copy(__t.rbegin()) } -> input_or_output_iterator;
> + { _GLIBCXX_AUTO_CAST(__t.rbegin()) } -> input_or_output_iterator;
> };
>
> void rbegin() = delete;
> @@ -201,7 +201,7 @@ namespace ranges
> concept __adl_rbegin = __class_or_enum<remove_reference_t<_Tp>>
> && requires(_Tp& __t)
> {
> - { __decay_copy(rbegin(__t)) } -> input_or_output_iterator;
> + { _GLIBCXX_AUTO_CAST(rbegin(__t)) } -> input_or_output_iterator;
> };
>
> template<typename _Tp>
> @@ -219,9 +219,9 @@ namespace ranges
> _S_noexcept()
> {
> if constexpr (__member_rbegin<_Tp>)
> - return noexcept(__decay_copy(std::declval<_Tp&>().rbegin()));
> + return noexcept(_GLIBCXX_AUTO_CAST(std::declval<_Tp&>().rbegin()));
> else if constexpr (__adl_rbegin<_Tp>)
> - return noexcept(__decay_copy(rbegin(std::declval<_Tp&>())));
> + return noexcept(_GLIBCXX_AUTO_CAST(rbegin(std::declval<_Tp&>())));
> else
> {
> if constexpr (noexcept(_End{}(std::declval<_Tp&>())))
> @@ -254,7 +254,7 @@ namespace ranges
> template<typename _Tp>
> concept __member_rend = requires(_Tp& __t)
> {
> - { __decay_copy(__t.rend()) }
> + { _GLIBCXX_AUTO_CAST(__t.rend()) }
> -> sentinel_for<decltype(_RBegin{}(std::forward<_Tp>(__t)))>;
> };
>
> @@ -264,7 +264,7 @@ namespace ranges
> concept __adl_rend = __class_or_enum<remove_reference_t<_Tp>>
> && requires(_Tp& __t)
> {
> - { __decay_copy(rend(__t)) }
> + { _GLIBCXX_AUTO_CAST(rend(__t)) }
> -> sentinel_for<decltype(_RBegin{}(std::forward<_Tp>(__t)))>;
> };
>
> @@ -276,9 +276,9 @@ namespace ranges
> _S_noexcept()
> {
> if constexpr (__member_rend<_Tp>)
> - return noexcept(__decay_copy(std::declval<_Tp&>().rend()));
> + return noexcept(_GLIBCXX_AUTO_CAST(std::declval<_Tp&>().rend()));
> else if constexpr (__adl_rend<_Tp>)
> - return noexcept(__decay_copy(rend(std::declval<_Tp&>())));
> + return noexcept(_GLIBCXX_AUTO_CAST(rend(std::declval<_Tp&>())));
> else
> {
> if constexpr (noexcept(_Begin{}(std::declval<_Tp&>())))
> @@ -312,7 +312,7 @@ namespace ranges
> concept __member_size = !disable_sized_range<remove_cvref_t<_Tp>>
> && requires(_Tp& __t)
> {
> - { __decay_copy(__t.size()) } -> __detail::__is_integer_like;
> + { _GLIBCXX_AUTO_CAST(__t.size()) } -> __detail::__is_integer_like;
> };
>
> void size() = delete;
> @@ -322,7 +322,7 @@ namespace ranges
> && !disable_sized_range<remove_cvref_t<_Tp>>
> && requires(_Tp& __t)
> {
> - { __decay_copy(size(__t)) } -> __detail::__is_integer_like;
> + { _GLIBCXX_AUTO_CAST(size(__t)) } -> __detail::__is_integer_like;
> };
>
> template<typename _Tp>
> @@ -347,9 +347,9 @@ namespace ranges
> if constexpr (is_bounded_array_v<remove_reference_t<_Tp>>)
> return true;
> else if constexpr (__member_size<_Tp>)
> - return noexcept(__decay_copy(std::declval<_Tp&>().size()));
> + return noexcept(_GLIBCXX_AUTO_CAST(std::declval<_Tp&>().size()));
> else if constexpr (__adl_size<_Tp>)
> - return noexcept(__decay_copy(size(std::declval<_Tp&>())));
> + return noexcept(_GLIBCXX_AUTO_CAST(size(std::declval<_Tp&>())));
> else if constexpr (__sentinel_size<_Tp>)
> return noexcept(_End{}(std::declval<_Tp&>())
> - _Begin{}(std::declval<_Tp&>()));
> @@ -459,7 +459,7 @@ namespace ranges
> template<typename _Tp>
> concept __member_data = requires(_Tp& __t)
> {
> - { __decay_copy(__t.data()) } -> __pointer_to_object;
> + { _GLIBCXX_AUTO_CAST(__t.data()) } -> __pointer_to_object;
> };
>
> template<typename _Tp>
> @@ -473,7 +473,7 @@ namespace ranges
> _S_noexcept()
> {
> if constexpr (__member_data<_Tp>)
> - return noexcept(__decay_copy(std::declval<_Tp&>().data()));
> + return noexcept(_GLIBCXX_AUTO_CAST(std::declval<_Tp&>().data()));
> else
> return noexcept(_Begin{}(std::declval<_Tp&>()));
> }
> --
> 2.47.0.rc1.33.g90fe3800b9
>
>
