libstdc++: Convert _RangeAdaptorClosure into a CRTP base [PR108827]

Using the CRTP idiom for this base class avoids bloating the size of a
pipeline when adding distinct empty range adaptor closure objects to it,
as detailed in section 4.1 of P2387R3.

But it means we can no longer define its operator| overloads as hidden
friends, since it'd mean each instantiation of _RangeAdaptorClosure
introduces its own distinct set of hidden friends.  So e.g. for the
outer | in

  x | (views::reverse | views::join)

ADL would find 6 distinct hidden operator| friends:

  two from _RangeAdaptorClosure<_Reverse>
  two from _RangeAdaptorClosure<_Join>
  two from _RangeAdaptorClosure<_Pipe<_Reverse, _Join>>

but we really only want to consider the last two.

We avoid this issue by instead defining the operator| overloads at
namespace scope alongside _RangeAdaptorClosure.  This should be fine
because the only types defined in this namespace are _RangeAdaptorClosure,
_RangeAdaptor, _Pipe and _Partial, so we don't have to worry about
unintentional ADL.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

	PR libstdc++/108827

libstdc++-v3/ChangeLog:

	* include/std/ranges (__adaptor::_RangeAdaptorClosure):
	Convert into a CRTP class template.  Move hidden operator|
	friends into namespace scope and adjust their constraints.
	(__closure::__is_range_adaptor_closure_fn): Define.
	(__closure::__is_range_adaptor_closure): Define.
	(__adaptor::_Partial): Adjust use of _RangeAdaptorClosure.
	(__adaptor::_Pipe): Likewise.
	(views::_All): Likewise.
	(views::_Join): Likewise.
	(views::_Common): Likewise.
	(views::_Reverse): Likewise.
	(views::_Elements): Likewise.
	(views::_Adjacent): Likewise.
	(views::_AsRvalue): Likewise.
	(views::_Enumerate): Likewise.
	(views::_AsConst): Likewise.
	* testsuite/std/ranges/adaptors/all.cc: Reinstate assertion
	expecting that adding empty range adaptor closure objects to a
	pipeline doesn't increase the size of a pipeline.

2 files changed, 45 insertions, 42 deletions

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index fb4a6a8..ffa3e97 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -892,25 +892,35 @@ namespace views::__adaptor
   // The derived class should define the optional static data member
   // _S_has_simple_call_op to true if the behavior of this adaptor is
   // independent of the constness/value category of the adaptor object.
-  struct _RangeAdaptorClosure
-  {
-    // range | adaptor is equivalent to adaptor(range).
-    template<typename _Self, typename _Range>
-      requires derived_from<remove_cvref_t<_Self>, _RangeAdaptorClosure>
-	&& __adaptor_invocable<_Self, _Range>
-      friend constexpr auto
-      operator|(_Range&& __r, _Self&& __self)
-      { return std::forward<_Self>(__self)(std::forward<_Range>(__r)); }
-
-    // Compose the adaptors __lhs and __rhs into a pipeline, returning
-    // another range adaptor closure object.
-    template<typename _Lhs, typename _Rhs>
-      requires derived_from<_Lhs, _RangeAdaptorClosure>
-	&& derived_from<_Rhs, _RangeAdaptorClosure>
-      friend constexpr auto
-      operator|(_Lhs __lhs, _Rhs __rhs)
-      { return _Pipe<_Lhs, _Rhs>{std::move(__lhs), std::move(__rhs)}; }
-  };
+  template<typename _Derived>
+    struct _RangeAdaptorClosure
+    { };
+
+  template<typename _Tp, typename _Up>
+    requires (!same_as<_Tp, _RangeAdaptorClosure<_Up>>)
+    void __is_range_adaptor_closure_fn
+      (const _Tp&, const _RangeAdaptorClosure<_Up>&); // not defined
+
+  template<typename _Tp>
+    concept __is_range_adaptor_closure
+      = requires (_Tp __t) { __adaptor::__is_range_adaptor_closure_fn(__t, __t); };
+
+  // range | adaptor is equivalent to adaptor(range).
+  template<typename _Self, typename _Range>
+    requires __is_range_adaptor_closure<_Self>
+      && __adaptor_invocable<_Self, _Range>
+    constexpr auto
+    operator|(_Range&& __r, _Self&& __self)
+    { return std::forward<_Self>(__self)(std::forward<_Range>(__r)); }
+
+  // Compose the adaptors __lhs and __rhs into a pipeline, returning
+  // another range adaptor closure object.
+  template<typename _Lhs, typename _Rhs>
+    requires __is_range_adaptor_closure<_Lhs>
+      && __is_range_adaptor_closure<_Rhs>
+    constexpr auto
+    operator|(_Lhs __lhs, _Rhs __rhs)
+    { return _Pipe<_Lhs, _Rhs>{std::move(__lhs), std::move(__rhs)}; }
 
   // The base class of every range adaptor non-closure.
   //
@@ -952,7 +962,7 @@ namespace views::__adaptor
   // A range adaptor closure that represents partial application of
   // the range adaptor _Adaptor with arguments _Args.
   template<typename _Adaptor, typename... _Args>
-    struct _Partial : _RangeAdaptorClosure
+    struct _Partial : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
     {
       tuple<_Args...> _M_args;
 
@@ -993,7 +1003,7 @@ namespace views::__adaptor
   // 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
+    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
     {
       _Arg _M_arg;
 
@@ -1026,7 +1036,7 @@ namespace views::__adaptor
   template<typename _Adaptor, typename... _Args>
     requires __adaptor_has_simple_extra_args<_Adaptor, _Args...>
       && (is_trivially_copyable_v<_Args> && ...)
-    struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure
+    struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
     {
       tuple<_Args...> _M_args;
 
@@ -1056,7 +1066,7 @@ namespace views::__adaptor
   template<typename _Adaptor, typename _Arg>
     requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
       && is_trivially_copyable_v<_Arg>
-    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
+    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
     {
       _Arg _M_arg;
 
@@ -1081,7 +1091,7 @@ namespace views::__adaptor
   // A range adaptor closure that represents composition of the range
   // adaptor closures _Lhs and _Rhs.
   template<typename _Lhs, typename _Rhs>
-    struct _Pipe : _RangeAdaptorClosure
+    struct _Pipe : _RangeAdaptorClosure<_Pipe<_Lhs, _Rhs>>
     {
       [[no_unique_address]] _Lhs _M_lhs;
       [[no_unique_address]] _Rhs _M_rhs;
@@ -1117,7 +1127,7 @@ namespace views::__adaptor
   template<typename _Lhs, typename _Rhs>
     requires __closure_has_simple_call_op<_Lhs>
       && __closure_has_simple_call_op<_Rhs>
-    struct _Pipe<_Lhs, _Rhs> : _RangeAdaptorClosure
+    struct _Pipe<_Lhs, _Rhs> : _RangeAdaptorClosure<_Pipe<_Lhs, _Rhs>>
     {
       [[no_unique_address]] _Lhs _M_lhs;
       [[no_unique_address]] _Rhs _M_rhs;
@@ -1141,7 +1151,7 @@ namespace views::__adaptor
   template<typename _Derived>
     requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
     class range_adaptor_closure
-    : public views::__adaptor::_RangeAdaptorClosure
+    : public views::__adaptor::_RangeAdaptorClosure<_Derived>
     { };
 #endif
 
@@ -1287,7 +1297,7 @@ namespace views::__adaptor
 	concept __can_owning_view = requires { owning_view{std::declval<_Range>()}; };
     } // namespace __detail
 
-    struct _All : __adaptor::_RangeAdaptorClosure
+    struct _All : __adaptor::_RangeAdaptorClosure<_All>
     {
       template<typename _Range>
 	static constexpr bool
@@ -3071,7 +3081,7 @@ namespace views::__adaptor
 	  = requires { join_view<all_t<_Range>>{std::declval<_Range>()}; };
     } // namespace __detail
 
-    struct _Join : __adaptor::_RangeAdaptorClosure
+    struct _Join : __adaptor::_RangeAdaptorClosure<_Join>
     {
       template<viewable_range _Range>
 	requires __detail::__can_join_view<_Range>
@@ -3865,7 +3875,7 @@ namespace views::__adaptor
 	  = requires { common_view{std::declval<_Range>()}; };
     } // namespace __detail
 
-    struct _Common : __adaptor::_RangeAdaptorClosure
+    struct _Common : __adaptor::_RangeAdaptorClosure<_Common>
     {
       template<viewable_range _Range>
 	requires __detail::__already_common<_Range>
@@ -3986,7 +3996,7 @@ namespace views::__adaptor
 	  = requires { reverse_view{std::declval<_Range>()}; };
     } // namespace __detail
 
-    struct _Reverse : __adaptor::_RangeAdaptorClosure
+    struct _Reverse : __adaptor::_RangeAdaptorClosure<_Reverse>
     {
       template<viewable_range _Range>
 	requires __detail::__is_reverse_view<remove_cvref_t<_Range>>
@@ -4386,7 +4396,7 @@ namespace views::__adaptor
     } // namespace __detail
 
     template<size_t _Nm>
-      struct _Elements : __adaptor::_RangeAdaptorClosure
+      struct _Elements : __adaptor::_RangeAdaptorClosure<_Elements<_Nm>>
       {
 	template<viewable_range _Range>
 	  requires __detail::__can_elements_view<_Nm, _Range>
@@ -5504,7 +5514,7 @@ namespace views::__adaptor
     }
 
     template<size_t _Nm>
-      struct _Adjacent : __adaptor::_RangeAdaptorClosure
+      struct _Adjacent : __adaptor::_RangeAdaptorClosure<_Adjacent<_Nm>>
       {
 	template<viewable_range _Range>
 	  requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
@@ -8629,7 +8639,7 @@ namespace views::__adaptor
 	concept __can_as_rvalue_view = requires { as_rvalue_view(std::declval<_Tp>()); };
     }
 
-    struct _AsRvalue : __adaptor::_RangeAdaptorClosure
+    struct _AsRvalue : __adaptor::_RangeAdaptorClosure<_AsRvalue>
     {
       template<viewable_range _Range>
 	requires __detail::__can_as_rvalue_view<_Range>
@@ -8938,7 +8948,7 @@ namespace views::__adaptor
 	  = requires { enumerate_view<all_t<_Tp>>(std::declval<_Tp>()); };
     }
 
-    struct _Enumerate : __adaptor::_RangeAdaptorClosure
+    struct _Enumerate : __adaptor::_RangeAdaptorClosure<_Enumerate>
     {
       template<viewable_range _Range>
 	requires __detail::__can_enumerate_view<_Range>
@@ -9024,7 +9034,7 @@ namespace views::__adaptor
 	concept __can_as_const_view = requires { as_const_view(std::declval<_Range>()); };
     }
 
-    struct _AsConst : __adaptor::_RangeAdaptorClosure
+    struct _AsConst : __adaptor::_RangeAdaptorClosure<_AsConst>
     {
       template<viewable_range _Range>
       constexpr auto
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
index 57cb542..6bbaf73 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
@@ -105,12 +105,6 @@ static_assert(std::is_empty_v<decltype(views::common
 				       | views::common
 				       | views::keys
 				       | views::reverse)>);
-#if 0
-// Adding empty range adaptor closure objects to a pipeline used to not
-// increase the size of the pipeline, but now that our range adaptor closure
-// objects derive from a common empty base class, [[no_unique_address]] can no
-// longer make two empty adjacent range adaptor closure objects occupy the same
-// data member address.
 static_assert(sizeof(decltype(views::take(5) | views::drop(5)))
 	      == sizeof(decltype(views::take(5)
 				 | views::join
@@ -119,7 +113,6 @@ static_assert(sizeof(decltype(views::take(5) | views::drop(5)))
 				 | views::keys
 				 | views::drop(5)
 				 | views::reverse)));
-#endif
 
 template<auto all = views::all>
 void