libstdc++: Conditionalize LWG 3569 changes to join_view

LWG 3569 adjusted join_view's iterator specification to handle non
default-constructible iterators by wrapping the corresponding data member
in std::optional, which 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-forward (and
thus non default-constructible).  We check forwardness instead of
default-constructibility in order to accommodate input-only iterators
that satisfy but do not model default_initializable, e.g. whose default
constructor is underconstrained.

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 forward.  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.

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

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 156d12b31ba1533e2dc46390bb8273bf9fb19e6b..bae50d06ca810ba7d4f0bd7e17ad17d0e85fd1d0 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2889,7 +2889,12 @@ namespace views::__adaptor
              }
 
            if constexpr (_S_ref_is_glvalue)
-             _M_inner.reset();
+             {
+               if constexpr (forward_iterator<_Inner_iter>)
+                 _M_inner = _Inner_iter();
+               else
+                 _M_inner.reset();
+             }
          }
 
          static constexpr auto
@@ -2929,6 +2934,24 @@ namespace views::__adaptor
              return *_M_parent->_M_outer;
          }
 
+         constexpr _Inner_iter&
+         _M_get_inner()
+         {
+           if constexpr (forward_iterator<_Inner_iter>)
+             return _M_inner;
+           else
+             return *_M_inner;
+         }
+
+         constexpr const _Inner_iter&
+         _M_get_inner() const
+         {
+           if constexpr (forward_iterator<_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)
@@ -2942,7 +2965,9 @@ 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<forward_iterator<_Inner_iter>,
+                         _Inner_iter, optional<_Inner_iter>> _M_inner
+           = decltype(_M_inner)();
          _Parent* _M_parent = nullptr;
 
        public:
@@ -2966,7 +2991,7 @@ 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
@@ -2974,7 +2999,7 @@ namespace views::__adaptor
          operator->() const
            requires __detail::__has_arrow<_Inner_iter>
              && copyable<_Inner_iter>
-         { return *_M_inner; }
+         { return _M_get_inner(); }
 
          constexpr _Iterator&
          operator++()
@@ -2985,7 +3010,7 @@ 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();
@@ -3015,9 +3040,9 @@ 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;
          }
 
@@ -3044,14 +3069,14 @@ 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;