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?
-- >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>...>>
+ _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
