libstdc++: P1983R0 Wording for GB301, US296, US292, US291, and US283

Among other changes, P1983R0 resolves LWG 3278 in a different way, so this patch
also reverts the already-applied wording of LWG 3278.

The wording for US291 (the join_view::begin hunk) also required adding the
friend _Iterator<!_Const> to join_view::_Iterator.  This friend is needed so
that _Iterator's converting constructor can access the private members of an
_Iterator of the opposite constness.

The wording for US283 has already been applied it seems.

libstdc++-v3/ChangeLog:

	P1983R0 Wording for GB301, US296, US292, US291, and US283
	* include/std/ranges (filter_view::pred): New member function.
	(join_view::_Iterator::_Iterator): Remove now-redundant comment since
	P1983R0 fixes the highlighted issue in the same way.
	(join_view::_Iterator<_Const>): Add friend
	join_view::_Iterator<!_Const>.
	(join_view::_M_inner): Remove mutable specifier, effectively reverting
	the proposed wording changes of P3278.
	(join_view::begin): Refine the condition for when to return a const
	iterator.
	(split_view::_OuterIter::_OuterIter): Adjust constraints.
	* testsuite/std/ranges/adaptors/filter.cc: Test that filter_view::pred
	exists and works.

3 files changed, 29 insertions, 7 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 458c955..b60f5c3 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,19 @@
+2020-02-18  Patrick Palka  <ppalka@redhat.com>
+
+	P1983R0 Wording for GB301, US296, US292, US291, and US283
+	* include/std/ranges (filter_view::pred): New member function.
+	(join_view::_Iterator::_Iterator): Remove now-redundant comment since
+	P1983R0 fixes the highlighted issue in the same way.
+	(join_view::_Iterator<_Const>): Add friend
+	join_view::_Iterator<!_Const>.
+	(join_view::_M_inner): Remove mutable specifier, effectively reverting
+	the proposed wording changes of P3278.
+	(join_view::begin): Refine the condition for when to return a const
+	iterator.
+	(split_view::_OuterIter::_OuterIter): Adjust constraints.
+	* testsuite/std/ranges/adaptors/filter.cc: Test that filter_view::pred
+	exists and works.
+
 2020-02-18  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/93818
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index e6bdac3..e0eb847 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1488,6 +1488,10 @@ namespace views
       base() &&
       { return std::move(_M_base); }
 
+      constexpr const _Pred&
+      pred() const
+      { return *_M_pred; }
+
       constexpr _Iterator
       begin()
       {
@@ -2336,8 +2340,6 @@ namespace views
 
 	  _Iterator() = default;
 
-	  // XXX: had to change the type of __outer from iterator_t<_Vp> to
-	  // iterator_t<_Base> here, a possible defect in the spec?
 	  constexpr
 	  _Iterator(_Parent& __parent, _Outer_iter __outer)
 	    : _M_outer(std::move(__outer)),
@@ -2437,6 +2439,7 @@ namespace views
 	    noexcept(noexcept(ranges::iter_swap(__x._M_inner, __y._M_inner)))
 	  { return ranges::iter_swap(__x._M_inner, __y._M_inner); }
 
+	  friend _Iterator<!_Const>;
 	  friend _Sentinel<_Const>;
 	};
 
@@ -2475,8 +2478,7 @@ namespace views
       _Vp _M_base = _Vp();
 
       // XXX: _M_inner is "present only when !is_reference_v<_InnerRange>"
-      // Applied P3278 and made this field mutable.
-      [[no_unique_address]] mutable
+      [[no_unique_address]]
 	conditional_t<!is_reference_v<_InnerRange>,
 		      all_view<_InnerRange>, __detail::_Empty> _M_inner;
 
@@ -2509,8 +2511,10 @@ namespace views
       constexpr auto
       begin()
       {
-	return _Iterator<__detail::__simple_view<_Vp>>{*this,
-						       ranges::begin(_M_base)};
+	constexpr bool __use_const
+	  = (__detail::__simple_view<_Vp>
+	     && is_reference_v<range_reference_t<_Vp>>);
+	return _Iterator<__use_const>{*this, ranges::begin(_M_base)};
       }
 
       constexpr auto
@@ -2676,7 +2680,7 @@ namespace views
 	  constexpr
 	  _OuterIter(_OuterIter<!_Const> __i)
 	    requires _Const
-	      && convertible_to<iterator_t<_Vp>, iterator_t<const _Vp>>
+	      && convertible_to<iterator_t<_Vp>, iterator_t<_Base>>
 	    : _M_parent(__i._M_parent), _M_current(std::move(__i._M_current))
 	  { }
 
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/filter.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/filter.cc
index 83d5296..4edbf0b 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/filter.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/filter.cc
@@ -46,6 +46,8 @@ test01()
   static_assert(ranges::range<ranges::all_view<R>>);
   VERIFY( ranges::equal(v, (int[]){1,3,5}) );
   VERIFY( ranges::equal(v | views::reverse, (int[]){5,3,1}) );
+  VERIFY( v.pred()(3) == true );
+  VERIFY( v.pred()(4) == false );
 }
 
 void