[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