libstdc++: Prefer const T to std::add_const_t<T>

  Does anybody see a problem with this change? The point is to avoid
unnecessary class template instantiations.

Tested aarch64-linux.

-- >8 --

For any typedef-name or template parameter, T, add_const_t<T> is
equivalent to T const, so we can avoid instantiating the std::add_const
class template and just say T const (or const T).

This isn't true for a non-typedef like int&, where int& const would be
ill-formed, but we shouldn't be using add_const_t<int&> anyway, because
we know what that type is.

The only place we need to continue using std::add_const is in the
std::bind implementation where it's used as a template template
parameter to be applied as a metafunction elsewhere.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (__iter_to_alloc_t): Replace
	add_const_t with const-qualifier.
	* include/bits/utility.h (tuple_element<N, cv T>): Likewise for
	all cv-qualifiers.
	* include/std/type_traits (add_const, add_volatile): Replace
	typedef-declaration with using-declaration.
	(add_cv): Replace add_const and add_volatile with cv-qualifiers.
	* include/std/variant (variant_alternative<N, cv T>): Replace
	add_const_t, add_volatile_t and add_cv_t etc. with cv-qualifiers.
---
 libstdc++-v3/include/bits/stl_iterator.h | 11 +++++------
 libstdc++-v3/include/bits/utility.h      |  6 +++---
 libstdc++-v3/include/std/type_traits     |  9 +++------
 libstdc++-v3/include/std/variant         |  6 +++---
 4 files changed, 14 insertions(+), 18 deletions(-)
  

On Thu, 7 Jul 2022 at 17:55, Jonathan Wakely via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Does anybody see a problem with this change? The point is to avoid
> unnecessary class template instantiations.
>
> Tested aarch64-linux.

Pushed to trunk now.

>
> -- >8 --
>
> For any typedef-name or template parameter, T, add_const_t<T> is
> equivalent to T const, so we can avoid instantiating the std::add_const
> class template and just say T const (or const T).
>
> This isn't true for a non-typedef like int&, where int& const would be
> ill-formed, but we shouldn't be using add_const_t<int&> anyway, because
> we know what that type is.
>
> The only place we need to continue using std::add_const is in the
> std::bind implementation where it's used as a template template
> parameter to be applied as a metafunction elsewhere.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/stl_iterator.h (__iter_to_alloc_t): Replace
>         add_const_t with const-qualifier.
>         * include/bits/utility.h (tuple_element<N, cv T>): Likewise for
>         all cv-qualifiers.
>         * include/std/type_traits (add_const, add_volatile): Replace
>         typedef-declaration with using-declaration.
>         (add_cv): Replace add_const and add_volatile with cv-qualifiers.
>         * include/std/variant (variant_alternative<N, cv T>): Replace
>         add_const_t, add_volatile_t and add_cv_t etc. with cv-qualifiers.
> ---
>  libstdc++-v3/include/bits/stl_iterator.h | 11 +++++------
>  libstdc++-v3/include/bits/utility.h      |  6 +++---
>  libstdc++-v3/include/std/type_traits     |  9 +++------
>  libstdc++-v3/include/std/variant         |  6 +++---
>  4 files changed, 14 insertions(+), 18 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
> index 12a89ab229f..049cb02a4c4 100644
> --- a/libstdc++-v3/include/bits/stl_iterator.h
> +++ b/libstdc++-v3/include/bits/stl_iterator.h
> @@ -2536,19 +2536,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    // of associative containers.
>    template<typename _InputIterator>
>      using __iter_key_t = remove_const_t<
> -    typename iterator_traits<_InputIterator>::value_type::first_type>;
> +      typename iterator_traits<_InputIterator>::value_type::first_type>;
>
>    template<typename _InputIterator>
> -    using __iter_val_t =
> -    typename iterator_traits<_InputIterator>::value_type::second_type;
> +    using __iter_val_t
> +      = typename iterator_traits<_InputIterator>::value_type::second_type;
>
>    template<typename _T1, typename _T2>
>      struct pair;
>
>    template<typename _InputIterator>
> -    using __iter_to_alloc_t =
> -    pair<add_const_t<__iter_key_t<_InputIterator>>,
> -        __iter_val_t<_InputIterator>>;
> +    using __iter_to_alloc_t
> +      = pair<const __iter_key_t<_InputIterator>, __iter_val_t<_InputIterator>>;
>  #endif // __cpp_deduction_guides
>
>  _GLIBCXX_END_NAMESPACE_VERSION
> diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
> index e0e40309a6d..6a192e27836 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -86,19 +86,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    template<size_t __i, typename _Tp>
>      struct tuple_element<__i, const _Tp>
>      {
> -      typedef typename add_const<__tuple_element_t<__i, _Tp>>::type type;
> +      using type = const __tuple_element_t<__i, _Tp>;
>      };
>
>    template<size_t __i, typename _Tp>
>      struct tuple_element<__i, volatile _Tp>
>      {
> -      typedef typename add_volatile<__tuple_element_t<__i, _Tp>>::type type;
> +      using type = volatile __tuple_element_t<__i, _Tp>;
>      };
>
>    template<size_t __i, typename _Tp>
>      struct tuple_element<__i, const volatile _Tp>
>      {
> -      typedef typename add_cv<__tuple_element_t<__i, _Tp>>::type type;
> +      using type = const volatile __tuple_element_t<__i, _Tp>;
>      };
>
>  #if __cplusplus >= 201402L
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index 2572d8edd69..e5f58bc2e3f 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -1577,20 +1577,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    /// add_const
>    template<typename _Tp>
>      struct add_const
> -    { typedef _Tp const     type; };
> +    { using type = _Tp const; };
>
>    /// add_volatile
>    template<typename _Tp>
>      struct add_volatile
> -    { typedef _Tp volatile     type; };
> +    { using type = _Tp volatile; };
>
>    /// add_cv
>    template<typename _Tp>
>      struct add_cv
> -    {
> -      typedef typename
> -      add_const<typename add_volatile<_Tp>::type>::type     type;
> -    };
> +    { using type = _Tp const volatile; };
>
>  #if __cplusplus > 201103L
>
> diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> index 5ff1e3edcdf..f8f15665433 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -107,15 +107,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>    template<size_t _Np, typename _Variant>
>      struct variant_alternative<_Np, const _Variant>
> -    { using type = add_const_t<variant_alternative_t<_Np, _Variant>>; };
> +    { using type = const variant_alternative_t<_Np, _Variant>; };
>
>    template<size_t _Np, typename _Variant>
>      struct variant_alternative<_Np, volatile _Variant>
> -    { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
> +    { using type = volatile variant_alternative_t<_Np, _Variant>; };
>
>    template<size_t _Np, typename _Variant>
>      struct variant_alternative<_Np, const volatile _Variant>
> -    { using type = add_cv_t<variant_alternative_t<_Np, _Variant>>; };
> +    { using type = const volatile variant_alternative_t<_Np, _Variant>; };
>
>    inline constexpr size_t variant_npos = -1;
>
> --
> 2.36.1
>