Re: [PATCH 2/3] libstdc++: Fix std::bind_front perfect forwarding [PR111327]

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 12 Sept 2023 at 02:09, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> In order to properly implement a perfect forwarding call wrapper
> (before 'deducing this' at least) we need a total of 8 operator()
> overloads, 4 main ones and 4 deleted ones for each const/ref qual pair,
> as described in section 5.5 of P0847R6.  Otherwise the wrapper may
> not perfectly forward according to the value category and constness
> of the wrapped object.  This patch fixes this bug in std::bind_front.

OK for trunk, thanks.

>
>         PR libstdc++/111327
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/functional (_Bind_front::operator()): Add deleted
>         fallback overloads for each const/ref qualifier pair.  Give the
>         main overloads dummy constraints to make them more specialized
>         than the deleted overloads.
>         * testsuite/20_util/function_objects/bind_front/111327.cc: New test.
> ---
>  libstdc++-v3/include/std/functional           | 16 ++++++++
>  .../function_objects/bind_front/111327.cc     | 41 +++++++++++++++++++
>  2 files changed, 57 insertions(+)
>  create mode 100644 libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
>
> diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
> index 7d1b890bb4e..c50b9e4d365 100644
> --- a/libstdc++-v3/include/std/functional
> +++ b/libstdc++-v3/include/std/functional
> @@ -938,6 +938,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        ~_Bind_front() = default;
>
>        template<typename... _CallArgs>
> +       requires true
>         constexpr
>         invoke_result_t<_Fd&, _BoundArgs&..., _CallArgs...>
>         operator()(_CallArgs&&... __call_args) &
> @@ -948,6 +949,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         }
>
>        template<typename... _CallArgs>
> +       requires true
>         constexpr
>         invoke_result_t<const _Fd&, const _BoundArgs&..., _CallArgs...>
>         operator()(_CallArgs&&... __call_args) const &
> @@ -959,6 +961,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         }
>
>        template<typename... _CallArgs>
> +       requires true
>         constexpr
>         invoke_result_t<_Fd, _BoundArgs..., _CallArgs...>
>         operator()(_CallArgs&&... __call_args) &&
> @@ -969,6 +972,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         }
>
>        template<typename... _CallArgs>
> +       requires true
>         constexpr
>         invoke_result_t<const _Fd, const _BoundArgs..., _CallArgs...>
>         operator()(_CallArgs&&... __call_args) const &&
> @@ -979,6 +983,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               std::forward<_CallArgs>(__call_args)...);
>         }
>
> +      template<typename... _CallArgs>
> +       void operator()(_CallArgs&&...) & = delete;
> +
> +      template<typename... _CallArgs>
> +       void operator()(_CallArgs&&...) const & = delete;
> +
> +      template<typename... _CallArgs>
> +       void operator()(_CallArgs&&...) && = delete;
> +
> +      template<typename... _CallArgs>
> +       void operator()(_CallArgs&&...) const && = delete;
> +
>      private:
>        using _BoundIndices = index_sequence_for<_BoundArgs...>;
>
> diff --git a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
> new file mode 100644
> index 00000000000..6eb51994476
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
> @@ -0,0 +1,41 @@
> +// PR libstdc++/111327 - std::bind_front doesn't perfectly forward according
> +// to value category of the call wrapper object
> +// { dg-options "-std=gnu++20" }
> +// { dg-do compile { target c++20 } }
> +
> +#include <functional>
> +#include <utility>
> +
> +struct F {
> +  void operator()(...) & = delete;
> +  void operator()(...) const &;
> +};
> +
> +struct G {
> +  void operator()(...) && = delete;
> +  void operator()(...) const &&;
> +};
> +
> +int main() {
> +  auto f0 = std::bind_front(F{});
> +  f0(); // { dg-error "deleted" }
> +  std::move(f0)();
> +  std::as_const(f0)();
> +  std::move(std::as_const(f0))();
> +
> +  auto g0 = std::bind_front(G{});
> +  g0(); // { dg-error "deleted" }
> +  std::move(g0)(); // { dg-error "deleted" }
> +  std::move(std::as_const(g0))();
> +
> +  auto f1 = std::bind_front(F{}, 42);
> +  f1(); // { dg-error "deleted" }
> +  std::move(f1)();
> +  std::as_const(f1)();
> +  std::move(std::as_const(f1))();
> +
> +  auto g1 = std::bind_front(G{}, 42);
> +  g1(); // { dg-error "deleted" }
> +  std::move(g1)(); // { dg-error "deleted" }
> +  std::move(std::as_const(g1))();
> +}
> --
> 2.42.0.158.g94e83dcf5b
>