[PATCH 2/2] libstdc++: Conditionalize LWG 3569 changes to join_view

[Patrick Palka]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk only
(since it impacts ABI)?

-- >8 --

LWG 3569 adjusted join_view's iterator to handle adapting
non-default-constructible (input) iterators by wrapping the
corresponding data member with std::optional, and we followed suit in
r13-2649-g7aa80c82ecf3a3.

But this wrapping is unnecessary for iterators that are already
default-constructible.  Rather than unconditionally using std::optional
here, which introduces time/space overhead, this patch conditionalizes
our LWG 3569 changes on the iterator in question being
non-default-constructible.

libstdc++-v3/ChangeLog:

	* include/std/ranges (join_view::_Iterator::_M_satisfy):
	Adjust to handle non-std::optional _M_inner as per before LWG 3569.
	(join_view::_Iterator::_M_get_inner): New.
	(join_view::_Iterator::_M_inner): Don't wrap in std::optional if
	the iterator is already default constructible.  Initialize.
	(join_view::_Iterator::operator*): Use _M_get_inner instead
	of *_M_inner.
	(join_view::_Iterator::operator++): Likewise.
	(join_view::_Iterator::iter_move): Likewise.
	(join_view::_Iterator::iter_swap): Likewise.
---
 libstdc++-v3/include/std/ranges | 49 +++++++++++++++++++++++++--------
 1 file changed, 37 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index efe62969d657..799fa7611ce2 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2971,7 +2971,12 @@ namespace views::__adaptor
 	      }
 
 	    if constexpr (_S_ref_is_glvalue)
-	      _M_inner.reset();
+	      {
+		if constexpr (default_initializable<_Inner_iter>)
+		  _M_inner = _Inner_iter();
+		else
+		  _M_inner.reset();
+	      }
 	  }
 
 	  static constexpr auto
@@ -3011,6 +3016,24 @@ namespace views::__adaptor
 	      return *_M_parent->_M_outer;
 	  }
 
+	  constexpr _Inner_iter&
+	  _M_get_inner()
+	  {
+	    if constexpr (default_initializable<_Inner_iter>)
+	      return _M_inner;
+	    else
+	      return *_M_inner;
+	  }
+
+	  constexpr const _Inner_iter&
+	  _M_get_inner() const
+	  {
+	    if constexpr (default_initializable<_Inner_iter>)
+	      return _M_inner;
+	    else
+	      return *_M_inner;
+	  }
+
 	  constexpr
 	  _Iterator(_Parent* __parent, _Outer_iter __outer) requires forward_range<_Base>
 	    : _M_outer(std::move(__outer)), _M_parent(__parent)
@@ -3024,7 +3047,9 @@ namespace views::__adaptor
 	  [[no_unique_address]]
 	    __detail::__maybe_present_t<forward_range<_Base>, _Outer_iter> _M_outer
 	      = decltype(_M_outer)();
-	  optional<_Inner_iter> _M_inner;
+	  __conditional_t<default_initializable<_Inner_iter>,
+			  _Inner_iter, optional<_Inner_iter>> _M_inner
+	    = decltype(_M_inner)();
 	  _Parent* _M_parent = nullptr;
 
 	public:
@@ -3048,7 +3073,7 @@ namespace views::__adaptor
 
 	  constexpr decltype(auto)
 	  operator*() const
-	  { return **_M_inner; }
+	  { return *_M_get_inner(); }
 
 	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
 	  // 3500. join_view::iterator::operator->() is bogus
@@ -3056,7 +3081,7 @@ namespace views::__adaptor
 	  operator->() const
 	    requires __detail::__has_arrow<_Inner_iter>
 	      && copyable<_Inner_iter>
-	  { return *_M_inner; }
+	  { return _M_get_inner(); }
 
 	  constexpr _Iterator&
 	  operator++()
@@ -3067,7 +3092,7 @@ namespace views::__adaptor
 	      else
 		return *_M_parent->_M_inner;
 	    }();
-	    if (++*_M_inner == ranges::end(__inner_range))
+	    if (++_M_get_inner() == ranges::end(__inner_range))
 	      {
 		++_M_get_outer();
 		_M_satisfy();
@@ -3097,9 +3122,9 @@ namespace views::__adaptor
 	  {
 	    if (_M_outer == ranges::end(_M_parent->_M_base))
 	      _M_inner = ranges::end(__detail::__as_lvalue(*--_M_outer));
-	    while (*_M_inner == ranges::begin(__detail::__as_lvalue(*_M_outer)))
-	      *_M_inner = ranges::end(__detail::__as_lvalue(*--_M_outer));
-	    --*_M_inner;
+	    while (_M_get_inner() == ranges::begin(__detail::__as_lvalue(*_M_outer)))
+	      _M_get_inner() = ranges::end(__detail::__as_lvalue(*--_M_outer));
+	    --_M_get_inner();
 	    return *this;
 	  }
 
@@ -3126,14 +3151,14 @@ namespace views::__adaptor
 
 	  friend constexpr decltype(auto)
 	  iter_move(const _Iterator& __i)
-	  noexcept(noexcept(ranges::iter_move(*__i._M_inner)))
-	  { return ranges::iter_move(*__i._M_inner); }
+	  noexcept(noexcept(ranges::iter_move(__i._M_get_inner())))
+	  { return ranges::iter_move(__i._M_get_inner()); }
 
 	  friend constexpr void
 	  iter_swap(const _Iterator& __x, const _Iterator& __y)
-	    noexcept(noexcept(ranges::iter_swap(*__x._M_inner, *__y._M_inner)))
+	    noexcept(noexcept(ranges::iter_swap(__x._M_get_inner(), __y._M_get_inner())))
 	    requires indirectly_swappable<_Inner_iter>
-	  { return ranges::iter_swap(*__x._M_inner, *__y._M_inner); }
+	  { return ranges::iter_swap(__x._M_get_inner(), __y._M_get_inner()); }
 
 	  friend _Iterator<!_Const>;
 	  template<bool> friend struct _Sentinel;
-- 
2.50.1.271.gd30e120486