libstdc++: Reuse _Bind_back_t functor in ranges::_PartialHEADtrunkmaster

This patch refactors ranges::_Partial to be implemented using _Bind_back_t.
This allows it to benefit from the changes in r16-3398-g250dd5b5604fbc,
specifically making the closure trivially copyable. Since _Bind_back_t
already provides an optimized implementation for a single bound argument,
specializations for _Partial with a single argument are now removed.

We still preserve a specialization of _Partial for trivially copy-constructible
arguments that define only a const overload of operator(). To avoid
re-checking invocability constraints, this specialization calls the now-public,
unconstrained _Binder::_S_call static method instead of the constrained
_Binder::operator().

The primary specialization of _Partial retains its operator(), which
uses a simpler __adaptor_invocable constraint that does not consider
member pointers, as they are not relevant here. This implementation also
calls _Binder::_S_call to avoid re-performing overload resolution and
invocability checks for _Binder::operator().

Finally, the _M_binder member (_Bind_back_t) is now marked
[[no_unique_address]]. This is beneficial as ranges::_Partial is used with
ranges::to, which commonly has zero or empty bound arguments (e.g., stateless
allocators, comparators, or hash functions).

libstdc++-v3/ChangeLog:

	* include/bits/binders.h (_Binder::_S_call): Make public.
	* include/std/ranges (ranges::_Partial<_Adaptor, _Args...>):
	Replace tuple<_Args...> with _Bind_back_t<_Adaptor, _Args...>.
	(ranges::_Partial<_Adaptor, _Arg>): Remove.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

2 files changed, 17 insertions, 97 deletions

diff --git a/libstdc++-v3/include/bits/binders.h b/libstdc++-v3/include/bits/binders.h
index c324bf5..6489edd 100644
--- a/libstdc++-v3/include/bits/binders.h
+++ b/libstdc++-v3/include/bits/binders.h
@@ -186,11 +186,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	void operator()(_CallArgs&&...) const && = delete;
 #endif
 
-    private:
-      using _BoundArgsStorage
-	// _BoundArgs are required to be move-constructible, so this is valid.
-	= decltype(__make_bound_args<_BoundArgs...>(std::declval<_BoundArgs>()...));
-
       template<typename _Tp, typename... _CallArgs>
 	static constexpr
 	decltype(auto)
@@ -214,6 +209,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				 std::forward<_CallArgs>(__call_args)...);
 	}
 
+    private:
+      using _BoundArgsStorage
+	// _BoundArgs are required to be move-constructible, so this is valid.
+	= decltype(__make_bound_args<_BoundArgs...>(std::declval<_BoundArgs>()...));
+
       [[no_unique_address]] _Fd _M_fd;
       [[no_unique_address]] _BoundArgsStorage _M_bound_args;
     };
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 2970b2c..156d12b 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -51,6 +51,7 @@
 #include <utility>
 #include <variant>
 #endif
+#include <bits/binders.h>
 #include <bits/ranges_util.h>
 #include <bits/refwrap.h>
 
@@ -1047,14 +1048,15 @@ namespace views::__adaptor
   template<typename _Adaptor, typename... _Args>
     struct _Partial : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
     {
-      tuple<_Args...> _M_args;
+      using _Binder = _Bind_back_t<_Adaptor, _Args...>;
+      [[no_unique_address]] _Binder _M_binder;
 
       // First parameter is to ensure this constructor is never used
       // instead of the copy/move constructor.
       template<typename... _Ts>
 	constexpr
 	_Partial(int, _Ts&&... __args)
-	  : _M_args(std::forward<_Ts>(__args)...)
+	  : _M_binder(0, _Adaptor(), std::forward<_Ts>(__args)...)
 	{ }
 
       // Invoke _Adaptor with arguments __r, _M_args... according to the
@@ -1065,75 +1067,21 @@ namespace views::__adaptor
 	constexpr auto
 	operator()(this _Self&& __self, _Range&& __r)
 	{
-	  auto __forwarder = [&__r] (auto&&... __args) {
-	    return _Adaptor{}(std::forward<_Range>(__r),
-			      std::forward<decltype(__args)>(__args)...);
-	  };
-	  return std::apply(__forwarder, __like_t<_Self, _Partial>(__self)._M_args);
+	  return _Binder::_S_call(__like_t<_Self, _Partial>(__self)._M_binder,
+				  std::forward<_Range>(__r));
 	}
 #else
       template<typename _Range>
 	requires __adaptor_invocable<_Adaptor, _Range, const _Args&...>
 	constexpr auto
 	operator()(_Range&& __r) const &
-	{
-	  auto __forwarder = [&__r] (const auto&... __args) {
-	    return _Adaptor{}(std::forward<_Range>(__r), __args...);
-	  };
-	  return std::apply(__forwarder, _M_args);
-	}
+	{ return _Binder::_S_call(_M_binder, std::forward<_Range>(__r)); }
 
       template<typename _Range>
 	requires __adaptor_invocable<_Adaptor, _Range, _Args...>
 	constexpr auto
 	operator()(_Range&& __r) &&
-	{
-	  auto __forwarder = [&__r] (auto&... __args) {
-	    return _Adaptor{}(std::forward<_Range>(__r), std::move(__args)...);
-	  };
-	  return std::apply(__forwarder, _M_args);
-	}
-
-      template<typename _Range>
-	constexpr auto
-	operator()(_Range&& __r) const && = delete;
-#endif
-    };
-
-  // A lightweight specialization of the above primary template for
-  // the common case where _Adaptor accepts a single extra argument.
-  template<typename _Adaptor, typename _Arg>
-    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
-    {
-      _Arg _M_arg;
-
-      template<typename _Tp>
-	constexpr
-	_Partial(int, _Tp&& __arg)
-	  : _M_arg(std::forward<_Tp>(__arg))
-	{ }
-
-#if __cpp_explicit_this_parameter
-      template<typename _Self, typename _Range>
-	requires __adaptor_invocable<_Adaptor, _Range, __like_t<_Self, _Arg>>
-	constexpr auto
-	operator()(this _Self&& __self, _Range&& __r)
-	{
-	  return _Adaptor{}(std::forward<_Range>(__r),
-			    __like_t<_Self, _Partial>(__self)._M_arg);
-	}
-#else
-      template<typename _Range>
-	requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
-	constexpr auto
-	operator()(_Range&& __r) const &
-	{ return _Adaptor{}(std::forward<_Range>(__r), _M_arg); }
-
-      template<typename _Range>
-	requires __adaptor_invocable<_Adaptor, _Range, _Arg>
-	constexpr auto
-	operator()(_Range&& __r) &&
-	{ return _Adaptor{}(std::forward<_Range>(__r), std::move(_M_arg)); }
+	{ return _Binder::_S_call(std::move(_M_binder), std::forward<_Range>(__r)); }
 
       template<typename _Range>
 	constexpr auto
@@ -1150,12 +1098,13 @@ namespace views::__adaptor
       && (is_trivially_copy_constructible_v<_Args> && ...)
     struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
     {
-      tuple<_Args...> _M_args;
+      using _Binder = _Bind_back_t<_Adaptor, _Args...>;
+      [[no_unique_address]] _Binder _M_binder;
 
       template<typename... _Ts>
 	constexpr
 	_Partial(int, _Ts&&... __args)
-	  : _M_args(std::forward<_Ts>(__args)...)
+	  : _M_binder(0, _Adaptor(), std::forward<_Ts>(__args)...)
 	{ }
 
       // Invoke _Adaptor with arguments __r, const _M_args&... regardless
@@ -1164,36 +1113,7 @@ namespace views::__adaptor
 	requires __adaptor_invocable<_Adaptor, _Range, const _Args&...>
 	constexpr auto
 	operator()(_Range&& __r) const
-	{
-	  auto __forwarder = [&__r] (const auto&... __args) {
-	    return _Adaptor{}(std::forward<_Range>(__r), __args...);
-	  };
-	  return std::apply(__forwarder, _M_args);
-	}
-
-      static constexpr bool _S_has_simple_call_op = true;
-    };
-
-  // A lightweight specialization of the above template for the common case
-  // where _Adaptor accepts a single extra argument.
-  template<typename _Adaptor, typename _Arg>
-    requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
-      && is_trivially_copy_constructible_v<_Arg>
-    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
-    {
-      _Arg _M_arg;
-
-      template<typename _Tp>
-	constexpr
-	_Partial(int, _Tp&& __arg)
-	  : _M_arg(std::forward<_Tp>(__arg))
-	{ }
-
-      template<typename _Range>
-	requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
-	constexpr auto
-	operator()(_Range&& __r) const
-	{ return _Adaptor{}(std::forward<_Range>(__r), _M_arg); }
+	{ return _Binder::_S_call(_M_binder, std::forward<_Range>(__r)); }
 
       static constexpr bool _S_has_simple_call_op = true;
     };