[PATCH] libstdc++: Use deducing this in std::bind when available [PR80564]
Tomasz Kaminski
tkaminsk@redhat.com
Fri Dec 5 15:07:47 GMT 2025
On Fri, Dec 5, 2025 at 4:06 PM Tomasz Kaminski <tkaminsk@redhat.com> wrote:
> Just one small suggestion, otherwise LGTM.
>
> On Fri, Dec 5, 2025 at 3:57 PM Patrick Palka <ppalka@redhat.com> 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.
>
Could you add a test like this.
> + .template __call_c<_Result>(std::forward_as_tuple
>> +
>> (std::forward<_Args>(__args)...),
>> + _Bound_indexes());
>> + else
>> + return static_cast<_Bind&>(__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>...>>
>> + _GLIBCXX_DEPR_BIND
>> + _Result
>> + operator()(this _Self&& __self, _Args&&... __args)
>> + {
>> + if constexpr (is_const<_Self_nonref>::value)
>> + return static_cast<const volatile _Bind&>(__self)
>> + .template __call_c_v<_Result>(std::forward_as_tuple
>> +
>> (std::forward<_Args>(__args)...),
>> + _Bound_indexes());
>> + else
>> + return static_cast<volatile _Bind&>(__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 +722,7 @@ _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..365a6ff4e00f
>> --- /dev/null
>> +++ b/libstdc++-v3/testsuite/20_util/bind/cv_quals_4.cc
>> @@ -0,0 +1,39 @@
>> +// PR libstdc++/80564 - bind on SFINAE unfriendly generic lambda
>> +// { dg-do compile { target c++14 } }
>> +
>> +#include <functional>
>> +
>> +struct A
>> +{
>> + template<class T>
>> + auto operator()(T&)
>> + { }
>> +
>> + template<class 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)();
>> +}
>> +
>> +int
>> +main()
>> +{
>> + test01();
>> + test02();
>> +}
>> 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
>>
>>
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