On Fri, 5 Dec 2025, Tomasz Kaminski wrote:
>
>
> On Fri, Dec 5, 2025 at 4:26 PM Patrick Palka <[email protected]> wrote:
> On Fri, 5 Dec 2025, Tomasz Kaminski wrote:
>
> > Just one small suggestion, otherwise LGTM.
> >
> > On Fri, Dec 5, 2025 at 3:57 PM Patrick Palka <[email protected]>
> wrote:
> > Tested on x86_64-pc-linu-xgnu, does this look OK for trunk?
> >
> > -- >8 --
> >
> > Implement the cv-qual forwarding required by std::bind using
> deducing
> > this when available, instead of needing 4 operator() overloads.
> Using
> > deducing this here is more complicated here than in other call
> > wrappers because std::bind is not really "perfect forwarding":
> it
> > doesn't forward value category, and along with const-ness it
> also
> > forwards volatile-ness (before C++20).
> >
> > The old implementation suffers from the same problem that other
> > SFINAE-friendly call wrappers have which is solved by using
> deducing
> > this (see p5.5 of the deducing this paper P0847R7).
> >
> > PR libstdc++/80564
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/std/functional (__cv_like): New.
> > (_Bind::_Res_type): Don't define when not needed.
> > (_Bind::__dependent): Likewise.
> > (_Bind::_Res_type_cv): Likewise.
> > (_Bind::operator()) [_GLIBCXX_EXPLICIT_THIS_PARAMETER]:
> > Define as two instead of four overloads using deducing
> > this.
> > * testsuite/20_util/bind/cv_quals_2.cc: Ignore SFINAE
> > diagnostics inside headers.
> > * testsuite/20_util/bind/ref_neg.cc: Likewise.
> > * testsuite/20_util/bind/cv_quals_4.cc: New test.
> > ---
> > libstdc++-v3/include/std/functional | 81
> +++++++++++++++++++
> > .../testsuite/20_util/bind/cv_quals_2.cc | 3 +
> > .../testsuite/20_util/bind/cv_quals_4.cc | 39 +++++++++
> > .../testsuite/20_util/bind/ref_neg.cc | 1 +
> > 4 files changed, 124 insertions(+)
> > create mode 100644
> libstdc++-v3/testsuite/20_util/bind/cv_quals_4.cc
> >
> > diff --git a/libstdc++-v3/include/std/functional
> b/libstdc++-v3/include/std/functional
> > index 1928a27d3fd6..9348356f03ce 100644
> > --- a/libstdc++-v3/include/std/functional
> > +++ b/libstdc++-v3/include/std/functional
> > @@ -495,6 +495,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > # define _GLIBCXX_DEPR_BIND
> > #endif
> >
> > +#if _GLIBCXX_EXPLICIT_THIS_PARAMETER
> > + // Return a _Up that has the same cv-quals as _Tp.
> > + template<typename _Tp, typename _Up>
> > + struct __cv_like
> > + { using type = _Up; };
> > +
> > + template<typename _Tp, typename _Up>
> > + struct __cv_like<const _Tp, _Up>
> > + { using type = const _Up; };
> > +
> > + template<typename _Tp, typename _Up>
> > + struct __cv_like<volatile _Tp, _Up>
> > + { using type = volatile _Up; };
> > +
> > + template<typename _Tp, typename _Up>
> > + struct __cv_like<const volatile _Tp, _Up>
> > + { using type = const volatile _Up; };
> > +
> > + template<typename _Tp, typename _Up>
> > + using __cv_like_t = typename __cv_like<_Tp, _Up>::type;
> > +#endif
> > +
> > /// Type of the function object returned from bind().
> > template<typename _Signature>
> > class _Bind;
> > @@ -564,6 +586,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > using _Res_type_impl
> > = __invoke_result_t<_Fn&, _Mu_type<_BArgs,
> _CallArgs>&&...>;
> >
> > +#if !_GLIBCXX_EXPLICIT_THIS_PARAMETER
> > template<typename _CallArgs>
> > using _Res_type = _Res_type_impl<_Functor, _CallArgs,
> _Bound_args...>;
> >
> > @@ -576,6 +599,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > typename __cv_quals<__dependent<_CallArgs>>::type,
> > _CallArgs,
> > typename __cv_quals<_Bound_args>::type...>;
> > +#endif
> >
> > public:
> > template<typename... _Args>
> > @@ -593,6 +617,62 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > _Bind(const _Bind&) = default;
> > _Bind(_Bind&&) = default;
> >
> > +#if _GLIBCXX_EXPLICIT_THIS_PARAMETER
> > +# pragma GCC diagnostic push
> > +# pragma GCC diagnostic ignored "-Wc++17-extensions" // if
> constexpr
> > +# pragma GCC diagnostic ignored "-Wc++23-extensions" //
> deducing this
> > + // Call unqualified
> > + template<typename... _Args,
> > + typename _Self,
> > + typename _Self_nonref = typename
> remove_reference<_Self>::type,
> > + __enable_if_t<!is_volatile<_Self_nonref>::value,
> int> = 0,
> > + typename _Result
> > + = _Res_type_impl<__cv_like_t<_Self_nonref,
> _Functor>,
> > + tuple<_Args...>,
> > + __cv_like_t<_Self_nonref,
> _Bound_args>...>>
> > + _GLIBCXX20_CONSTEXPR
> > + _Result
> > + operator()(this _Self&& __self, _Args&&... __args)
> > + {
> > + if constexpr (is_const<_Self_nonref>::value)
> > + return static_cast<const _Bind&>(__self)
> >
> > Once you have __cv_like_t, is there any reason to not use something
> like here:
> > using _Bind_ref_t = __cv_like_t<_Self_nonref, _Bind>&;
> > _Bind_ref_t(__self).
> > I think we also need to use c-cast, if someone privately inherited
> from bind specialization,
> > for some reason. I thinking of version of overload:
> > template<typename... Funcs>
> > class Overload : Funcs....
> > {
> > public:
> > Overload(Funcs... func) : Funcs(funcs)... {}
> > using Funcs::operator()....;
> > };
> >
> > And then used with bind.
>
> Good catch, I keep forgetting we need a C-style cast with deducing
> this.
>
> Changes in v2:
> - use a C-style cast instead of static_cast when casting explicit
> this parameter to the curret instantiation, and add test for that
>
> Testing on x86_64-pc-ilnux-gnu in progress, OK for trunk once
> passes?
>
> LGTM.
Thanks!
>
> -- >8 --
>
> Subject: [PATCH] libstdc++: Use deducing this in std::bind when
> available
> [PR80564]
>
> Implement the cv-qual forwarding required by std::bind using deducing
> this when available, instead of needing 4 operator() overloads. Using
> deducing this here is more complicated here than in other call
> wrappers because std::bind is not really "perfect forwarding": it
> doesn't forward value category, and along with const-ness it also
> forwards volatile-ness (before C++20).
>
> The old implementation suffers from the same problem that other
> SFINAE-friendly call wrappers have which is solved by using deducing
> this (see p5.5 of the deducing this paper P0847R7).
>
> PR libstdc++/80564
>
> libstdc++-v3/ChangeLog:
>
> * include/std/functional (__cv_like): New.
> (_Bind::_Res_type): Don't define when not needed.
> (_Bind::__dependent): Likewise.
> (_Bind::_Res_type_cv): Likewise.
> (_Bind::operator()) [_GLIBCXX_EXPLICIT_THIS_PARAMETER]:
> Define as two instead of four overloads using deducing
> this.
> * testsuite/20_util/bind/cv_quals_2.cc: Ignore SFINAE
> diagnostics inside headers.
> * testsuite/20_util/bind/ref_neg.cc: Likewise.
> * testsuite/20_util/bind/cv_quals_4.cc: New test.
> ---
> libstdc++-v3/include/std/functional | 82 +++++++++++++++++++
> .../testsuite/20_util/bind/cv_quals_2.cc | 3 +
> .../testsuite/20_util/bind/cv_quals_4.cc | 50 +++++++++++
> .../testsuite/20_util/bind/ref_neg.cc | 1 +
> 4 files changed, 136 insertions(+)
> create mode 100644 libstdc++-v3/testsuite/20_util/bind/cv_quals_4.cc
>
> diff --git a/libstdc++-v3/include/std/functional
> b/libstdc++-v3/include/std/functional
> index 1928a27d3fd6..06ff32752d4a 100644
> --- a/libstdc++-v3/include/std/functional
> +++ b/libstdc++-v3/include/std/functional
> @@ -495,6 +495,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> # define _GLIBCXX_DEPR_BIND
> #endif
>
> +#if _GLIBCXX_EXPLICIT_THIS_PARAMETER
> + // Return a _Up that has the same cv-quals as _Tp.
> + template<typename _Tp, typename _Up> // _Up should be cv-unqualified
> + struct __cv_like
> + { using type = _Up; };
> +
> + template<typename _Tp, typename _Up>
> + struct __cv_like<const _Tp, _Up>
> + { using type = const _Up; };
> +
> + template<typename _Tp, typename _Up>
> + struct __cv_like<volatile _Tp, _Up>
> + { using type = volatile _Up; };
> +
> + template<typename _Tp, typename _Up>
> + struct __cv_like<const volatile _Tp, _Up>
> + { using type = const volatile _Up; };
> +
> + template<typename _Tp, typename _Up>
> + using __cv_like_t = typename __cv_like<_Tp, _Up>::type;
> +#endif
> +
> /// Type of the function object returned from bind().
> template<typename _Signature>
> class _Bind;
> @@ -564,6 +586,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> using _Res_type_impl
> = __invoke_result_t<_Fn&, _Mu_type<_BArgs, _CallArgs>&&...>;
>
> +#if !_GLIBCXX_EXPLICIT_THIS_PARAMETER
> template<typename _CallArgs>
> using _Res_type = _Res_type_impl<_Functor, _CallArgs,
> _Bound_args...>;
>
> @@ -576,6 +599,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> typename __cv_quals<__dependent<_CallArgs>>::type,
> _CallArgs,
> typename __cv_quals<_Bound_args>::type...>;
> +#endif
>
> public:
> template<typename... _Args>
> @@ -593,6 +617,63 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> _Bind(const _Bind&) = default;
> _Bind(_Bind&&) = default;
>
> +#if _GLIBCXX_EXPLICIT_THIS_PARAMETER
> +# pragma GCC diagnostic push
> +# pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
> +# pragma GCC diagnostic ignored "-Wc++23-extensions" // deducing this
> + template<typename... _Args,
> + typename _Self,
> + typename _Self_nonref = typename
> remove_reference<_Self>::type,
> + __enable_if_t<!is_volatile<_Self_nonref>::value, int> =
> 0,
> + typename _Result
> + = _Res_type_impl<__cv_like_t<_Self_nonref, _Functor>,
> + tuple<_Args...>,
> + __cv_like_t<_Self_nonref,
> _Bound_args>...>>
> + _GLIBCXX20_CONSTEXPR
> + _Result
> + operator()(this _Self&& __self, _Args&&... __args)
> + {
> + using _Bind_ref = __cv_like_t<_Self_nonref, _Bind>&;
> + if constexpr (is_const<_Self_nonref>::value)
> + return _Bind_ref(__self)
> + .template __call_c<_Result>(std::forward_as_tuple
> +
> (std::forward<_Args>(__args)...),
> + _Bound_indexes());
> + else
> + return _Bind_ref(__self)
> + .template __call<_Result>(std::forward_as_tuple
> +
> (std::forward<_Args>(__args)...),
> + _Bound_indexes());
> + }
> +
> +# if defined(_GLIBCXX_VOLATILE_BIND)
> + template<typename... _Args,
> + typename _Self,
> + typename _Self_nonref = typename
> remove_reference<_Self>::type,
> + __enable_if_t<is_volatile<_Self_nonref>::value, int> = 0,
> + typename _Result
> + = _Res_type_impl<__cv_like_t<_Self_nonref, _Functor>,
> + tuple<_Args...>,
> + __cv_like_t<_Self_nonref,
> _Bound_args>...>>
By the way the reason we need this separate overload for the volatile
case is solely to emit a deprecation diagnostic in C++17 mode, otherwise
we could get away with just one overload in all modes. (In C++20
the volatile overloads got removed.) So in theory we could have just one
overload before C++17, since we don't need a deprecation message in that
case, but that'd result in more complexity and code duplication as the
main overload would need to forward to __call_c_v and __call_v as well
and doesn't seem worth it.
> + _GLIBCXX_DEPR_BIND
> + _Result
> + operator()(this _Self&& __self, _Args&&... __args)
> + {
> + using _Bind_ref = __cv_like_t<_Self_nonref, _Bind>&;
> + if constexpr (is_const<_Self_nonref>::value)
> + return _Bind_ref(__self)
> + .template __call_c_v<_Result>(std::forward_as_tuple
> +
> (std::forward<_Args>(__args)...),
> + _Bound_indexes());
> + else
> + return _Bind_ref(__self)
> + .template __call_v<_Result>(std::forward_as_tuple
> +
> (std::forward<_Args>(__args)...),
> + _Bound_indexes());
> + }
> +# endif
> +# pragma GCC diagnostic pop
> +#else
> // Call unqualified
> template<typename... _Args,
> typename _Result = _Res_type<tuple<_Args...>>>
> @@ -642,6 +723,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> _Bound_indexes());
> }
> #endif // volatile
> +#endif
> };
>
> /// Type of the function object returned from bind<R>().
> diff --git a/libstdc++-v3/testsuite/20_util/bind/cv_quals_2.cc
> b/libstdc++-v3/testsuite/20_util/bind/cv_quals_2.cc
> index e4c348f7a3ca..6d37cc43fd3a 100644
> --- a/libstdc++-v3/testsuite/20_util/bind/cv_quals_2.cc
> +++ b/libstdc++-v3/testsuite/20_util/bind/cv_quals_2.cc
> @@ -44,6 +44,9 @@ void test01()
> // { dg-error "no match" "" { target c++20 } 43 }
> }
>
> +// Ignore the reasons for deduction/substitution failure in the
> headers.
> +// { dg-prune-output "no type named 'type' in 'struct
> std::enable_if<false" }
> +
> int main()
> {
> test01();
> diff --git a/libstdc++-v3/testsuite/20_util/bind/cv_quals_4.cc
> b/libstdc++-v3/testsuite/20_util/bind/cv_quals_4.cc
> new file mode 100644
> index 000000000000..f6e1a1ec66da
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/bind/cv_quals_4.cc
> @@ -0,0 +1,50 @@
> +// PR libstdc++/80564 - bind on SFINAE unfriendly generic lambda
> +// { dg-do compile { target c++14 } }
> +
> +#include <functional>
> +
> +struct A
> +{
> + template<typename T>
> + auto operator()(T&)
> + { }
> +
> + template<typename T>
> + auto operator()(T&) const
> + { T::fail; }
> +};
> +
> +void
> +test01()
> +{
> + A a;
> + std::bind(a, 0)(); // doesn't consider the const overload
> + std::bind<void>(a, 0)();
> +}
> +
> +void
> +test02()
> +{
> + auto f = [] (auto& x) { x = 1; };
> + int i;
> + std::bind(f, i)(); // doesn't try const-invoking the lambda
> + std::bind<void>(f, i)();
> +}
> +
> +#if __cpp_variadic_using
> +template<typename... Ts>
> +struct overloaded : private Ts...
> +{
> + overloaded(Ts... ts) : Ts(ts)... { }
> + using Ts::operator()...;
> +};
> +
> +void
> +test03()
> +{
> + A a;
> + auto f = std::bind(a, 0);
> + overloaded<decltype(f)> g(f);
> + g();
> +}
> +#endif
> diff --git a/libstdc++-v3/testsuite/20_util/bind/ref_neg.cc
> b/libstdc++-v3/testsuite/20_util/bind/ref_neg.cc
> index dd47c437d426..46cc4bb330e2 100644
> --- a/libstdc++-v3/testsuite/20_util/bind/ref_neg.cc
> +++ b/libstdc++-v3/testsuite/20_util/bind/ref_neg.cc
> @@ -51,6 +51,7 @@ void test02()
> // Ignore the reasons for deduction/substitution failure in the
> headers.
> // Arrange for the match to work on installed trees as well as build
> trees.
> // { dg-prune-output "no type named 'type' in 'struct
> std::__invoke_result" }
> +// { dg-prune-output "no type named 'type' in 'struct
> std::enable_if<false" }
>
> int main()
> {
> --
> 2.52.0.154.gf0ef5b6d9b
>
>
>
