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

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 12 Sept 2023 at 02:11, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> The previous patch fixed perfect forwarding for std::bind_front.
> This patch fixes the same issue for std::not_fn.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk and
> perhaps 13?

Yes for both, thanks.

>
>         PR libstdc++/111327
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/functional (_GLIBCXX_NOT_FN_CALL_OP): Also define
>         a deleted fallback operator() overload.  Constrain both the
>         main and deleted overloads accordingly.
>         * testsuite/20_util/function_objects/not_fn/111327.cc: New test.
> ---
>  libstdc++-v3/include/std/functional           | 10 +++++--
>  .../20_util/function_objects/not_fn/111327.cc | 29 +++++++++++++++++++
>  2 files changed, 37 insertions(+), 2 deletions(-)
>  create mode 100644 libstdc++-v3/testsuite/20_util/function_objects/not_fn/111327.cc
>
> diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
> index c50b9e4d365..9551e38dfdb 100644
> --- a/libstdc++-v3/include/std/functional
> +++ b/libstdc++-v3/include/std/functional
> @@ -1061,7 +1061,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        // forwarding _M_fn and the function arguments with the same qualifiers,
>        // and deducing the return type and exception-specification.
>  #define _GLIBCXX_NOT_FN_CALL_OP( _QUALS )                              \
> -      template<typename... _Args>                                      \
> +      template<typename... _Args,                                      \
> +              typename = enable_if_t<__is_invocable<_Fn _QUALS, _Args...>::value>> \
>         _GLIBCXX20_CONSTEXPR                                            \
>         decltype(_S_not<__inv_res_t<_Fn _QUALS, _Args...>>())           \
>         operator()(_Args&&... __args) _QUALS                            \
> @@ -1070,7 +1071,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         {                                                               \
>           return !std::__invoke(std::forward< _Fn _QUALS >(_M_fn),      \
>                                 std::forward<_Args>(__args)...);        \
> -       }
> +       }                                                               \
> +                                                                       \
> +      template<typename... _Args,                                      \
> +              typename = enable_if_t<!__is_invocable<_Fn _QUALS, _Args...>::value>> \
> +       void operator()(_Args&&... __args) _QUALS = delete;
> +
>        _GLIBCXX_NOT_FN_CALL_OP( & )
>        _GLIBCXX_NOT_FN_CALL_OP( const & )
>        _GLIBCXX_NOT_FN_CALL_OP( && )
> diff --git a/libstdc++-v3/testsuite/20_util/function_objects/not_fn/111327.cc b/libstdc++-v3/testsuite/20_util/function_objects/not_fn/111327.cc
> new file mode 100644
> index 00000000000..93e00ee8057
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/function_objects/not_fn/111327.cc
> @@ -0,0 +1,29 @@
> +// PR libstdc++/111327 - std::bind_front (and std::not_fn) doesn't perfectly
> +// forward according to value category of the call wrapper object
> +// { dg-do compile { target c++17 } }
> +
> +#include <functional>
> +#include <utility>
> +
> +struct F {
> +  void operator()(...) & = delete;
> +  bool operator()(...) const &;
> +};
> +
> +struct G {
> +  void operator()(...) && = delete;
> +  bool operator()(...) const &&;
> +};
> +
> +int main() {
> +  auto f = std::not_fn(F{});
> +  f(); // { dg-error "deleted" }
> +  std::move(f)();
> +  std::as_const(f)();
> +  std::move(std::as_const(f))();
> +
> +  auto g = std::not_fn(G{});
> +  g(); // { dg-error "deleted" }
> +  std::move(g)(); // { dg-error "deleted" }
> +  std::move(std::as_const(g))();
> +}
> --
> 2.42.0.158.g94e83dcf5b
>