[PATCH] libstdc++: Implement P2328 changes to join_view

[PATCH] libstdc++: Implement P2328 changes to join_view

[Patrick Palka]

This implements the wording changes of "join_view should join all views
of ranges".

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__non_propating_cache): Define
	as per P2328.
	(join_view): Remove constraints on the value and reference types
	of the wrapped iterator type as per P2328.
	(join_view::_Iterator::_M_satisfy): Adjust as per P2328.
	(join_view::_Iterator::operator++): Likewise.
	(join_view::_M_inner): Use __non_propating_cache as per P2328.
	Remove now-redundant use of __maybe_present_t.
	* testsuite/std/ranges/adaptors/join.cc (test10): New test.
---
 libstdc++-v3/include/std/ranges               | 70 ++++++++++++++++---
 .../testsuite/std/ranges/adaptors/join.cc     | 12 ++++
 2 files changed, 71 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 4be643baeaa..b43f9b337a4 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2258,10 +2258,61 @@ namespace views::__adaptor
     inline constexpr _DropWhile drop_while;
   } // namespace views
 
+  namespace __detail
+  {
+    template<typename _Tp>
+      struct __non_propagating_cache
+      {
+	// When _Tp is not an object type (e.g. is a reference type), we make
+	// __non_propagating_cache<_Tp> empty rather than an invalid type so
+	// that users can easily conditionally declare data members with this
+	// type (such as join_view::_M_inner).
+      };
+
+    template<typename _Tp>
+      requires is_object_v<_Tp>
+      struct __non_propagating_cache<_Tp> : private optional<_Tp>
+      {
+	__non_propagating_cache() = default;
+
+	constexpr
+	__non_propagating_cache(const __non_propagating_cache&) noexcept
+	{ }
+
+	constexpr
+	__non_propagating_cache(__non_propagating_cache&& __other) noexcept
+	{ __other.reset(); }
+
+	constexpr __non_propagating_cache&
+	operator=(const __non_propagating_cache& __other) noexcept
+	{
+	  if (std::__addressof(__other) != this)
+	    this->reset();
+	  return *this;
+	}
+
+	constexpr __non_propagating_cache&
+	operator=(__non_propagating_cache&& __other) noexcept
+	{
+	  this->reset();
+	  __other.reset();
+	  return *this;
+	}
+
+	using optional<_Tp>::operator*;
+
+	template<typename _Iter>
+	  _Tp&
+	  _M_emplace_deref(const _Iter& __i)
+	  {
+	    this->reset();
+	    return this->emplace(*__i);
+	  }
+      };
+  }
+
   template<input_range _Vp>
     requires view<_Vp> && input_range<range_reference_t<_Vp>>
-      && (is_reference_v<range_reference_t<_Vp>>
-	  || view<range_value_t<_Vp>>)
     class join_view : public view_interface<join_view<_Vp>>
     {
     private:
@@ -2327,17 +2378,16 @@ namespace views::__adaptor
 	  constexpr void
 	  _M_satisfy()
 	  {
-	    auto __update_inner = [this] (range_reference_t<_Base> __x) -> auto&
-	    {
+	    auto __update_inner = [this] (const iterator_t<_Base>& __x) -> auto&& {
 	      if constexpr (_S_ref_is_glvalue)
-		return __x;
+		return *__x;
 	      else
-		return (_M_parent->_M_inner = views::all(std::move(__x)));
+		return _M_parent->_M_inner._M_emplace_deref(__x);
 	    };
 
 	    for (; _M_outer != ranges::end(_M_parent->_M_base); ++_M_outer)
 	      {
-		auto& __inner = __update_inner(*_M_outer);
+		auto&& __inner = __update_inner(_M_outer);
 		_M_inner = ranges::begin(__inner);
 		if (_M_inner != ranges::end(__inner))
 		  return;
@@ -2413,7 +2463,7 @@ namespace views::__adaptor
 	      if constexpr (_S_ref_is_glvalue)
 		return *_M_outer;
 	      else
-		return _M_parent->_M_inner;
+		return *_M_parent->_M_inner;
 	    }();
 	    if (++_M_inner == ranges::end(__inner_range))
 	      {
@@ -2524,10 +2574,8 @@ namespace views::__adaptor
 	  friend _Sentinel<!_Const>;
 	};
 
-      // XXX: _M_inner is "present only when !is_reference_v<_InnerRange>"
       [[no_unique_address]]
-	__detail::__maybe_present_t<!is_reference_v<_InnerRange>,
-				    views::all_t<_InnerRange>> _M_inner;
+	__detail::__non_propagating_cache<remove_cv_t<_InnerRange>> _M_inner;
       _Vp _M_base = _Vp();
 
     public:
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
index fb06a7698af..6890b105eab 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
@@ -160,6 +160,17 @@ test09()
   static_assert(!requires { 0 | join; });
 }
 
+void
+test10()
+{
+// Verify P2328 changes.
+  int r[] = {1, 2, 3};
+  auto v = r
+    | views::transform([] (int n) { return std::vector{{n, -n}}; })
+    | views::join;
+  VERIFY( ranges::equal(v, (int[]){1, -1, 2, -2, 3, -3}) );
+}
+
 int
 main()
 {
@@ -172,4 +183,5 @@ main()
   test07();
   test08();
   test09();
+  test10();
 }
-- 
2.31.1.362.g311531c9de

Re: [PATCH] libstdc++: Implement P2328 changes to join_view

[Jonathan Wakely]

On 30/04/21 10:38 -0400, Patrick Palka via Libstdc++ wrote:
>This implements the wording changes of "join_view should join all views
>of ranges".
>
>Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

OK, thanks.

>libstdc++-v3/ChangeLog:
>
>	* include/std/ranges (__detail::__non_propating_cache): Define
>	as per P2328.
>	(join_view): Remove constraints on the value and reference types
>	of the wrapped iterator type as per P2328.
>	(join_view::_Iterator::_M_satisfy): Adjust as per P2328.
>	(join_view::_Iterator::operator++): Likewise.
>	(join_view::_M_inner): Use __non_propating_cache as per P2328.
>	Remove now-redundant use of __maybe_present_t.
>	* testsuite/std/ranges/adaptors/join.cc (test10): New test.
>---
> libstdc++-v3/include/std/ranges               | 70 ++++++++++++++++---
> .../testsuite/std/ranges/adaptors/join.cc     | 12 ++++
> 2 files changed, 71 insertions(+), 11 deletions(-)
>
>diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
>index 4be643baeaa..b43f9b337a4 100644
>--- a/libstdc++-v3/include/std/ranges
>+++ b/libstdc++-v3/include/std/ranges
>@@ -2258,10 +2258,61 @@ namespace views::__adaptor
>     inline constexpr _DropWhile drop_while;
>   } // namespace views
>
>+  namespace __detail
>+  {
>+    template<typename _Tp>
>+      struct __non_propagating_cache
>+      {
>+	// When _Tp is not an object type (e.g. is a reference type), we make
>+	// __non_propagating_cache<_Tp> empty rather than an invalid type so
>+	// that users can easily conditionally declare data members with this
>+	// type (such as join_view::_M_inner).
>+      };
>+
>+    template<typename _Tp>
>+      requires is_object_v<_Tp>
>+      struct __non_propagating_cache<_Tp> : private optional<_Tp>
>+      {
>+	__non_propagating_cache() = default;
>+
>+	constexpr
>+	__non_propagating_cache(const __non_propagating_cache&) noexcept
>+	{ }
>+
>+	constexpr
>+	__non_propagating_cache(__non_propagating_cache&& __other) noexcept
>+	{ __other.reset(); }
>+
>+	constexpr __non_propagating_cache&
>+	operator=(const __non_propagating_cache& __other) noexcept
>+	{
>+	  if (std::__addressof(__other) != this)
>+	    this->reset();
>+	  return *this;
>+	}
>+
>+	constexpr __non_propagating_cache&
>+	operator=(__non_propagating_cache&& __other) noexcept
>+	{
>+	  this->reset();
>+	  __other.reset();
>+	  return *this;
>+	}
>+
>+	using optional<_Tp>::operator*;
>+
>+	template<typename _Iter>
>+	  _Tp&
>+	  _M_emplace_deref(const _Iter& __i)
>+	  {
>+	    this->reset();
>+	    return this->emplace(*__i);
>+	  }
>+      };
>+  }
>+
>   template<input_range _Vp>
>     requires view<_Vp> && input_range<range_reference_t<_Vp>>
>-      && (is_reference_v<range_reference_t<_Vp>>
>-	  || view<range_value_t<_Vp>>)
>     class join_view : public view_interface<join_view<_Vp>>
>     {
>     private:
>@@ -2327,17 +2378,16 @@ namespace views::__adaptor
> 	  constexpr void
> 	  _M_satisfy()
> 	  {
>-	    auto __update_inner = [this] (range_reference_t<_Base> __x) -> auto&
>-	    {
>+	    auto __update_inner = [this] (const iterator_t<_Base>& __x) -> auto&& {
> 	      if constexpr (_S_ref_is_glvalue)
>-		return __x;
>+		return *__x;
> 	      else
>-		return (_M_parent->_M_inner = views::all(std::move(__x)));
>+		return _M_parent->_M_inner._M_emplace_deref(__x);
> 	    };
>
> 	    for (; _M_outer != ranges::end(_M_parent->_M_base); ++_M_outer)
> 	      {
>-		auto& __inner = __update_inner(*_M_outer);
>+		auto&& __inner = __update_inner(_M_outer);
> 		_M_inner = ranges::begin(__inner);
> 		if (_M_inner != ranges::end(__inner))
> 		  return;
>@@ -2413,7 +2463,7 @@ namespace views::__adaptor
> 	      if constexpr (_S_ref_is_glvalue)
> 		return *_M_outer;
> 	      else
>-		return _M_parent->_M_inner;
>+		return *_M_parent->_M_inner;
> 	    }();
> 	    if (++_M_inner == ranges::end(__inner_range))
> 	      {
>@@ -2524,10 +2574,8 @@ namespace views::__adaptor
> 	  friend _Sentinel<!_Const>;
> 	};
>
>-      // XXX: _M_inner is "present only when !is_reference_v<_InnerRange>"
>       [[no_unique_address]]
>-	__detail::__maybe_present_t<!is_reference_v<_InnerRange>,
>-				    views::all_t<_InnerRange>> _M_inner;
>+	__detail::__non_propagating_cache<remove_cv_t<_InnerRange>> _M_inner;
>       _Vp _M_base = _Vp();
>
>     public:
>diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
>index fb06a7698af..6890b105eab 100644
>--- a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
>+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
>@@ -160,6 +160,17 @@ test09()
>   static_assert(!requires { 0 | join; });
> }
>
>+void
>+test10()
>+{
>+// Verify P2328 changes.
>+  int r[] = {1, 2, 3};
>+  auto v = r
>+    | views::transform([] (int n) { return std::vector{{n, -n}}; })
>+    | views::join;
>+  VERIFY( ranges::equal(v, (int[]){1, -1, 2, -2, 3, -3}) );
>+}
>+
> int
> main()
> {
>@@ -172,4 +183,5 @@ main()
>   test07();
>   test08();
>   test09();
>+  test10();
> }
>-- 
>2.31.1.362.g311531c9de
>

Re: [PATCH] libstdc++: Implement P2328 changes to join_view

[Tim Song]

On Fri, Apr 30, 2021 at 12:11 PM Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> +       template<typename _Iter>
> +         _Tp&
> +         _M_emplace_deref(const _Iter& __i)
> +         {
> +           this->reset();
> +           return this->emplace(*__i);
> +         }

This incurs a move, avoiding of which is the sole reason for
emplace-deref's existence.

> +      };
> +  }
> +

Re: [PATCH] libstdc++: Implement P2328 changes to join_view

[Patrick Palka]

On Fri, 30 Apr 2021, Tim Song wrote:

> On Fri, Apr 30, 2021 at 12:11 PM Patrick Palka via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > +       template<typename _Iter>
> > +         _Tp&
> > +         _M_emplace_deref(const _Iter& __i)
> > +         {
> > +           this->reset();
> > +           return this->emplace(*__i);
> > +         }
> 
> This incurs a move, avoiding of which is the sole reason for
> emplace-deref's existence.

Ah thanks, I had missed that...  IIUC, if we instead derive from
optional's internal base class _Optional_base, we can easily get at the
underlying storage for the object and directly initialize it with '*__i'
and avoid the move. How does the the following adjustment to the patch
look?

-- >8 --

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges

index 971898f5492..f1db4a238ef 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2254,7 +2254,7 @@ namespace views::__adaptor
 
     template<typename _Tp>
       requires is_object_v<_Tp>
-      struct __non_propagating_cache<_Tp> : private optional<_Tp>
+      struct __non_propagating_cache<_Tp> : private _Optional_base<_Tp>
       {
 	__non_propagating_cache() = default;
 
@@ -2264,32 +2264,36 @@ namespace views::__adaptor
 
 	constexpr
 	__non_propagating_cache(__non_propagating_cache&& __other) noexcept
-	{ __other.reset(); }
+	{ __other._M_reset(); }
 
 	constexpr __non_propagating_cache&
 	operator=(const __non_propagating_cache& __other) noexcept
 	{
 	  if (std::__addressof(__other) != this)
-	    this->reset();
+	    this->_M_reset();
 	  return *this;
 	}
 
 	constexpr __non_propagating_cache&
 	operator=(__non_propagating_cache&& __other) noexcept
 	{
-	  this->reset();
-	  __other.reset();
+	  this->_M_reset();
+	  __other._M_reset();
 	  return *this;
 	}
 
-	using optional<_Tp>::operator*;
+	constexpr _Tp&
+	operator*() noexcept
+	{ return this->_M_get(); }
 
 	template<typename _Iter>
 	  _Tp&
 	  _M_emplace_deref(const _Iter& __i)
 	  {
-	    this->reset();
-	    return this->emplace(*__i);
+	    this->_M_reset();
+	    ::new ((void *) std::__addressof(this->_M_payload._M_payload)) _Tp(*__i);
+	    this->_M_payload._M_engaged = true;
+	    return this->_M_get();
 	  }
       };
   }

Re: [PATCH] libstdc++: Implement P2328 changes to join_view

[Jonathan Wakely]

On 30/04/21 17:34 -0400, Patrick Palka via Libstdc++ wrote:
>On Fri, 30 Apr 2021, Tim Song wrote:
>
>> On Fri, Apr 30, 2021 at 12:11 PM Patrick Palka via Libstdc++
>> <libstdc++@gcc.gnu.org> wrote:
>> >
>> > +       template<typename _Iter>
>> > +         _Tp&
>> > +         _M_emplace_deref(const _Iter& __i)
>> > +         {
>> > +           this->reset();
>> > +           return this->emplace(*__i);
>> > +         }
>>
>> This incurs a move, avoiding of which is the sole reason for
>> emplace-deref's existence.
>
>Ah thanks, I had missed that...  IIUC, if we instead derive from
>optional's internal base class _Optional_base, we can easily get at the
>underlying storage for the object and directly initialize it with '*__i'
>and avoid the move. How does the the following adjustment to the patch
>look?

Looks good to me, thanks.

>-- >8 --
>
>diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
>
>index 971898f5492..f1db4a238ef 100644
>--- a/libstdc++-v3/include/std/ranges
>+++ b/libstdc++-v3/include/std/ranges
>@@ -2254,7 +2254,7 @@ namespace views::__adaptor
>
>     template<typename _Tp>
>       requires is_object_v<_Tp>
>-      struct __non_propagating_cache<_Tp> : private optional<_Tp>
>+      struct __non_propagating_cache<_Tp> : private _Optional_base<_Tp>
>       {
> 	__non_propagating_cache() = default;
>
>@@ -2264,32 +2264,36 @@ namespace views::__adaptor
>
> 	constexpr
> 	__non_propagating_cache(__non_propagating_cache&& __other) noexcept
>-	{ __other.reset(); }
>+	{ __other._M_reset(); }
>
> 	constexpr __non_propagating_cache&
> 	operator=(const __non_propagating_cache& __other) noexcept
> 	{
> 	  if (std::__addressof(__other) != this)
>-	    this->reset();
>+	    this->_M_reset();
> 	  return *this;
> 	}
>
> 	constexpr __non_propagating_cache&
> 	operator=(__non_propagating_cache&& __other) noexcept
> 	{
>-	  this->reset();
>-	  __other.reset();
>+	  this->_M_reset();
>+	  __other._M_reset();
> 	  return *this;
> 	}
>
>-	using optional<_Tp>::operator*;
>+	constexpr _Tp&
>+	operator*() noexcept
>+	{ return this->_M_get(); }
>
> 	template<typename _Iter>
> 	  _Tp&
> 	  _M_emplace_deref(const _Iter& __i)
> 	  {
>-	    this->reset();
>-	    return this->emplace(*__i);
>+	    this->_M_reset();
>+	    ::new ((void *) std::__addressof(this->_M_payload._M_payload)) _Tp(*__i);
>+	    this->_M_payload._M_engaged = true;
>+	    return this->_M_get();
> 	  }
>       };
>   }
>

Re: [PATCH] libstdc++: Implement P2328 changes to join_view

[Jonathan Wakely]

On 30/04/21 12:43 -0500, Tim Song via Libstdc++ wrote:
>On Fri, Apr 30, 2021 at 12:11 PM Patrick Palka via Libstdc++
><libstdc++@gcc.gnu.org> wrote:
>>
>> +       template<typename _Iter>
>> +         _Tp&
>> +         _M_emplace_deref(const _Iter& __i)
>> +         {
>> +           this->reset();
>> +           return this->emplace(*__i);
>> +         }
>
>This incurs a move, avoiding of which is the sole reason for
>emplace-deref's existence.

I completely forgot about that, even though we only discussed it last
week. Thanks, Tim.

Re: [PATCH] libstdc++: Implement P2328 changes to join_view

[Patrick Palka]

On Fri, 30 Apr 2021, Jonathan Wakely wrote:

> On 30/04/21 17:34 -0400, Patrick Palka via Libstdc++ wrote:
> > On Fri, 30 Apr 2021, Tim Song wrote:
> > 
> > > On Fri, Apr 30, 2021 at 12:11 PM Patrick Palka via Libstdc++
> > > <libstdc++@gcc.gnu.org> wrote:
> > > >
> > > > +       template<typename _Iter>
> > > > +         _Tp&
> > > > +         _M_emplace_deref(const _Iter& __i)
> > > > +         {
> > > > +           this->reset();
> > > > +           return this->emplace(*__i);
> > > > +         }
> > > 
> > > This incurs a move, avoiding of which is the sole reason for
> > > emplace-deref's existence.
> > 
> > Ah thanks, I had missed that...  IIUC, if we instead derive from
> > optional's internal base class _Optional_base, we can easily get at the
> > underlying storage for the object and directly initialize it with '*__i'
> > and avoid the move. How does the the following adjustment to the patch
> > look?
> 
> Looks good to me, thanks.

Here's what I ended up committing, complete with a comment to
__non_propagating_cache::_M_emplace_deref and a test to verify we're
avoiding the extra move:

-- >8 --

Subject: [PATCH] libstdc++: Implement P2328 changes to join_view

This implements the wording changes of P2328R0 "join_view should join
all views of ranges".

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__non_propating_cache): Define
	as per P2328.
	(join_view): Remove constraints on the value and reference types
	of the wrapped iterator type as per P2328.
	(join_view::_Iterator::_M_satisfy): Adjust as per P2328.
	(join_view::_Iterator::operator++): Likewise.
	(join_view::_M_inner): Use __non_propating_cache as per P2328.
	Remove now-redundant use of __maybe_present_t.
	* testsuite/std/ranges/adaptors/join.cc: Include <array>.
	(test10): New test.
---
 libstdc++-v3/include/std/ranges               | 77 ++++++++++++++++---
 .../testsuite/std/ranges/adaptors/join.cc     | 24 ++++++
 2 files changed, 90 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 4975d5c630b..4a7ca49b45d 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2241,10 +2241,68 @@ namespace views::__adaptor
     inline constexpr _DropWhile drop_while;
   } // namespace views
 
+  namespace __detail
+  {
+    template<typename _Tp>
+      struct __non_propagating_cache
+      {
+	// When _Tp is not an object type (e.g. is a reference type), we make
+	// __non_propagating_cache<_Tp> empty rather than ill-formed so that
+	// users can easily conditionally declare data members with this type
+	// (such as join_view::_M_inner).
+      };
+
+    template<typename _Tp>
+      requires is_object_v<_Tp>
+      struct __non_propagating_cache<_Tp> : private _Optional_base<_Tp>
+      {
+	__non_propagating_cache() = default;
+
+	constexpr
+	__non_propagating_cache(const __non_propagating_cache&) noexcept
+	{ }
+
+	constexpr
+	__non_propagating_cache(__non_propagating_cache&& __other) noexcept
+	{ __other._M_reset(); }
+
+	constexpr __non_propagating_cache&
+	operator=(const __non_propagating_cache& __other) noexcept
+	{
+	  if (std::__addressof(__other) != this)
+	    this->_M_reset();
+	  return *this;
+	}
+
+	constexpr __non_propagating_cache&
+	operator=(__non_propagating_cache&& __other) noexcept
+	{
+	  this->_M_reset();
+	  __other._M_reset();
+	  return *this;
+	}
+
+	constexpr _Tp&
+	operator*() noexcept
+	{ return this->_M_get(); }
+
+	template<typename _Iter>
+	  _Tp&
+	  _M_emplace_deref(const _Iter& __i)
+	  {
+	    this->_M_reset();
+	    // Using _Optional_base::_M_construct to initialize from '*__i'
+	    // would incur an extra move due to the indirection, so we instead
+	    // use placement new directly.
+	    ::new ((void *) std::__addressof(this->_M_payload._M_payload)) _Tp(*__i);
+	    this->_M_payload._M_engaged = true;
+	    return this->_M_get();
+	  }
+      };
+  }
+
   template<input_range _Vp>
     requires view<_Vp> && input_range<range_reference_t<_Vp>>
-      && (is_reference_v<range_reference_t<_Vp>>
-	  || view<range_value_t<_Vp>>)
     class join_view : public view_interface<join_view<_Vp>>
     {
     private:
@@ -2310,17 +2368,16 @@ namespace views::__adaptor
 	  constexpr void
 	  _M_satisfy()
 	  {
-	    auto __update_inner = [this] (range_reference_t<_Base> __x) -> auto&
-	    {
+	    auto __update_inner = [this] (const iterator_t<_Base>& __x) -> auto&& {
 	      if constexpr (_S_ref_is_glvalue)
-		return __x;
+		return *__x;
 	      else
-		return (_M_parent->_M_inner = views::all(std::move(__x)));
+		return _M_parent->_M_inner._M_emplace_deref(__x);
 	    };
 
 	    for (; _M_outer != ranges::end(_M_parent->_M_base); ++_M_outer)
 	      {
-		auto& __inner = __update_inner(*_M_outer);
+		auto&& __inner = __update_inner(_M_outer);
 		_M_inner = ranges::begin(__inner);
 		if (_M_inner != ranges::end(__inner))
 		  return;
@@ -2396,7 +2453,7 @@ namespace views::__adaptor
 	      if constexpr (_S_ref_is_glvalue)
 		return *_M_outer;
 	      else
-		return _M_parent->_M_inner;
+		return *_M_parent->_M_inner;
 	    }();
 	    if (++_M_inner == ranges::end(__inner_range))
 	      {
@@ -2507,10 +2564,8 @@ namespace views::__adaptor
 	  friend _Sentinel<!_Const>;
 	};
 
-      // XXX: _M_inner is "present only when !is_reference_v<_InnerRange>"
       [[no_unique_address]]
-	__detail::__maybe_present_t<!is_reference_v<_InnerRange>,
-				    views::all_t<_InnerRange>> _M_inner;
+	__detail::__non_propagating_cache<remove_cv_t<_InnerRange>> _M_inner;
       _Vp _M_base = _Vp();
 
     public:
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
index e6c71d771de..d774e8d9385 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
@@ -19,6 +19,7 @@
 // { dg-do run { target c++2a } }
 
 #include <algorithm>
+#include <array>
 #include <ranges>
 #include <string>
 #include <string_view>
@@ -170,6 +171,28 @@ test10()
   VERIFY( ranges::next(v.begin()) == v.end() );
 }
 
+void
+test11()
+{
+  // Verify P2328 changes.
+  int r[] = {1, 2, 3};
+  auto v = r
+    | views::transform([] (int n) { return std::vector{{n, -n}}; })
+    | views::join;
+  VERIFY( ranges::equal(v, (int[]){1, -1, 2, -2, 3, -3}) );
+
+  struct S {
+    S() = default;
+    S(const S&) = delete;
+    S(S&&) = delete;
+  };
+  auto w = r
+    | views::transform([] (int) { return std::array<S, 2>{}; })
+    | views::join;
+  for (auto& i : w)
+    ;
+}
+
 int
 main()
 {
@@ -183,4 +206,5 @@ main()
   test08();
   test09();
   test10();
+  test11();
 }
-- 
2.31.1.442.g7e39198978