[gcc r11-9561] libstdc++: Back out some changes from P2325R3 backport [PR103904]

[gcc r11-9561] libstdc++: Back out some changes from P2325R3 backport [PR103904]

[Patrick Palka]

https://gcc.gnu.org/g:3273e704d04a98bcfccdc93c8d07e88f2b032043

commit r11-9561-g3273e704d04a98bcfccdc93c8d07e88f2b032043
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Feb 11 15:39:06 2022 -0500

    libstdc++: Back out some changes from P2325R3 backport [PR103904]
    
    In the P2325R3 backport r11-9555 the relaxation of the constraints on
    the partial specialization of __box (which is semantically equivalent to
    the primary template, only more space efficient) means some
    specializations of __box will now use the partial specialization instead
    of the primary template, which constitutes an ABI change unsuitable for
    a release branch.  So this patch reverts this constraint change, which
    isn't needed for correctness anyway.
    
    Similarly, the change to use __non_propagating_cache for the data member
    split_view::_M_current (so that it's always default-initializable) also
    constitutes an unsuitable ABI change.  This patch reverts this change
    too, and instead further constrains split_view's default constructor to
    require that we can default-initialize _M_current.
    
            PR libstdc++/103904
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges (__detail::__box): Revert r11-9555 changes
            to the constraints on the partial specialization and the
            now-unnecessary special member functions.
            (__detail::__non_propagating_cache::operator=): Remove
            now-unused overload added by r11-9555.
            (split_view::_OuterIter::__current): Adjust after reverting the
            r11-9555 change to the type of _M_current.
            (split_view::_M_current): Revert r11-9555 change to its type.
            (split_view::split_view): Constrain the default constructor
            further.
            * testsuite/std/ranges/adaptors/detail/copyable_box.cc: Disable
            now-irrelevant test for the r11-9555 changes to the partial
            specialization of __box.

Diff:
---
 libstdc++-v3/include/std/ranges                    | 55 +++-------------------
 .../std/ranges/adaptors/detail/copyable_box.cc     |  4 ++
 2 files changed, 11 insertions(+), 48 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 03c6275801f..4a65a49ed30 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -144,8 +144,7 @@ namespace ranges
     // std::optional.  It provides just the subset of the primary template's
     // API that we currently use.
     template<__boxable _Tp>
-      requires copyable<_Tp> || (is_nothrow_move_constructible_v<_Tp>
-				 && is_nothrow_copy_constructible_v<_Tp>)
+      requires copyable<_Tp>
       struct __box<_Tp>
       {
       private:
@@ -174,38 +173,6 @@ namespace ranges
 	  : _M_value(std::forward<_Args>(__args)...)
 	  { }
 
-	__box(const __box&) = default;
-	__box(__box&&) = default;
-	__box& operator=(const __box&) requires copyable<_Tp> = default;
-	__box& operator=(__box&&) requires copyable<_Tp> = default;
-
-	// When _Tp is nothrow_copy_constructible but not copy_assignable,
-	// copy assignment is implemented via destroy-then-copy-construct.
-	constexpr __box&
-	operator=(const __box& __that) noexcept
-	{
-	  static_assert(is_nothrow_copy_constructible_v<_Tp>);
-	  if (this != std::__addressof(__that))
-	    {
-	      _M_value.~_Tp();
-	      std::construct_at(std::__addressof(_M_value), *__that);
-	    }
-	  return *this;
-	}
-
-	// Likewise for move assignment.
-	constexpr __box&
-	operator=(__box&& __that) noexcept
-	{
-	  static_assert(is_nothrow_move_constructible_v<_Tp>);
-	  if (this != std::__addressof(__that))
-	    {
-	      _M_value.~_Tp();
-	      std::construct_at(std::__addressof(_M_value), std::move(*__that));
-	    }
-	  return *this;
-	}
-
 	constexpr bool
 	has_value() const noexcept
 	{ return true; };
@@ -1180,16 +1147,6 @@ namespace views::__adaptor
 	  return *this;
 	}
 
-	constexpr __non_propagating_cache&
-	operator=(_Tp __val)
-	{
-	  this->_M_reset();
-	  std::construct_at(std::__addressof(this->_M_payload._M_payload),
-			    std::in_place, std::move(__val));
-	  this->_M_payload._M_engaged = true;
-	  return *this;
-	}
-
 	constexpr _Tp&
 	operator*() noexcept
 	{ return this->_M_get(); }
@@ -2886,7 +2843,7 @@ namespace views::__adaptor
 	    if constexpr (forward_range<_Vp>)
 	      return _M_current;
 	    else
-	      return *_M_parent->_M_current;
+	      return _M_parent->_M_current;
 	  }
 
 	  constexpr auto&
@@ -2895,7 +2852,7 @@ namespace views::__adaptor
 	    if constexpr (forward_range<_Vp>)
 	      return _M_current;
 	    else
-	      return *_M_parent->_M_current;
+	      return _M_parent->_M_current;
 	  }
 
 	  _Parent* _M_parent = nullptr;
@@ -3143,13 +3100,15 @@ namespace views::__adaptor
       // XXX: _M_current is "present only if !forward_range<V>"
       [[no_unique_address]]
 	__detail::__maybe_present_t<!forward_range<_Vp>,
-	  __detail::__non_propagating_cache<iterator_t<_Vp>>> _M_current;
+				    iterator_t<_Vp>> _M_current;
       _Vp _M_base = _Vp();
 
 
     public:
       split_view() requires (default_initializable<_Vp>
-			     && default_initializable<_Pattern>)
+			     && default_initializable<_Pattern>
+			     && (forward_range<_Vp>
+				 || default_initializable<iterator_t<_Vp>>))
 	= default;
 
       constexpr
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/detail/copyable_box.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/detail/copyable_box.cc
index fa6d4d56816..2078d442447 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/detail/copyable_box.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/detail/copyable_box.cc
@@ -101,6 +101,9 @@ test02()
   __box<A<false>> x1(std::in_place, 0, 0);
 }
 
+#if 0
+// On the 11 branch, the partial specialization of __box admits only copyable types
+// so this test doesn't apply.
 constexpr bool
 test03()
 {
@@ -142,3 +145,4 @@ test03()
   return true;
 }
 static_assert(test03());
+#endif