[PATCH 4/5] libstdc++: Implement resolution of LWG 3478 as per P2210

[Patrick Palka <ppalka@redhat.com>]

This implements the part of P2210R2 "Superior String Splitting" that
resolves LWG 3478 for split_view (now named lazy_split_view).

libstdc++-v3/ChangeLog:

	* include/std/ranges (lazy_split_view::_OuterIter::__at_end):
	Check _M_trailing_empty.
	(lazy_split_view::_OuterIter::_M_trailing_empty): Define this
	data member.
	(lazy_split_view::_OuterIter::operator++): Set _M_trailing_empty
	appropriately.
	(lazy_split_view::_OuterIter::operator==): Compare
	_M_trailing_empty.
	* testsuite/std/ranges/adaptors/100479.cc (test03): Expect two
	split parts instead of one.
	* testsuite/std/ranges/adaptors/lazy_split.cc (test11): New test.
---
 libstdc++-v3/include/std/ranges               | 21 +++++++++++++++----
 .../testsuite/std/ranges/adaptors/100479.cc   |  6 +++---
 .../std/ranges/adaptors/lazy_split.cc         | 15 +++++++++++++
 3 files changed, 35 insertions(+), 7 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index cc1ef112ff1..78562924bee 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2879,7 +2879,7 @@ namespace views::__adaptor
 
 	  constexpr bool
 	  __at_end() const
-	  { return __current() == ranges::end(_M_parent->_M_base); }
+	  { return __current() == ranges::end(_M_parent->_M_base) && !_M_trailing_empty; }
 
 	  // [range.lazy.split.outer] p1
 	  //  Many of the following specifications refer to the notional member
@@ -2909,6 +2909,7 @@ namespace views::__adaptor
 	  [[no_unique_address]]
 	    __detail::__maybe_present_t<forward_range<_Vp>,
 					iterator_t<_Base>> _M_current;
+	  bool _M_trailing_empty = false;
 
 	public:
 	  using iterator_concept = conditional_t<forward_range<_Base>,
@@ -2977,7 +2978,10 @@ namespace views::__adaptor
 	    // 3505. lazy_split_view::outer-iterator::operator++ misspecified
 	    const auto __end = ranges::end(_M_parent->_M_base);
 	    if (__current() == __end)
-	      return *this;
+	      {
+		_M_trailing_empty = false;
+		return *this;
+	      }
 	    const auto [__pbegin, __pend] = subrange{_M_parent->_M_pattern};
 	    if (__pbegin == __pend)
 	      ++__current();
@@ -2986,7 +2990,11 @@ namespace views::__adaptor
 		__current() = ranges::find(std::move(__current()), __end,
 					   *__pbegin);
 		if (__current() != __end)
-		  ++__current();
+		  {
+		    ++__current();
+		    if (__current() == __end)
+		      _M_trailing_empty = true;
+		  }
 	      }
 	    else
 	      do
@@ -2996,6 +3004,8 @@ namespace views::__adaptor
 		  if (__p == __pend)
 		    {
 		      __current() = __b;
+		      if (__current() == __end)
+			_M_trailing_empty = true;
 		      break;
 		    }
 		} while (++__current() != __end);
@@ -3018,7 +3028,10 @@ namespace views::__adaptor
 	  friend constexpr bool
 	  operator==(const _OuterIter& __x, const _OuterIter& __y)
 	    requires forward_range<_Base>
-	  { return __x._M_current == __y._M_current; }
+	  {
+	    return __x._M_current == __y._M_current
+	      && __x._M_trailing_empty == __y._M_trailing_empty;
+	  }
 
 	  friend constexpr bool
 	  operator==(const _OuterIter& __x, default_sentinel_t)
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/100479.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/100479.cc
index 9899ff92c0b..b8c1e6f4f57 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/100479.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/100479.cc
@@ -95,11 +95,11 @@ test03()
     | views::drop_while([](auto) { return false; })
     | views::filter([](auto) { return true; });
   static_assert(ranges::forward_range<decltype(v)>);
-  VERIFY( ranges::next(v.begin()) == v.end() );
+  VERIFY( ranges::distance(v) == 2 );
   auto w = v;
-  VERIFY( ranges::next(w.begin()) == w.end() );
+  VERIFY( ranges::distance(v) == 2 );
   auto z = std::move(w);
-  VERIFY( ranges::next(z.begin()) == z.end() );
+  VERIFY( ranges::distance(v) == 2 );
   return true;
 }
 
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc
index 12844525d86..133e9a7025b 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/lazy_split.cc
@@ -193,6 +193,20 @@ test10()
   VERIFY( ranges::equal(v, (std::string_view[]){"x", "x"}) );
 }
 
+void
+test11()
+{
+  // LWG 3478
+  static_assert(ranges::distance(views::lazy_split("text"sv, "text"sv)) == 2);
+  static_assert(ranges::distance(views::lazy_split(" text "sv, ' ')) == 3);
+  static_assert(ranges::distance(views::lazy_split(" t e x t "sv, ' ')) == 6);
+  static_assert(ranges::distance(views::lazy_split("  text  "sv, "  "sv)) == 3);
+  static_assert(ranges::distance(views::lazy_split("  text    "sv, "  "sv)) == 4);
+  static_assert(ranges::distance(views::lazy_split("  text     "sv, "  "sv)) == 4);
+  static_assert(ranges::distance(views::lazy_split("t"sv, 't')) == 2);
+  static_assert(ranges::distance(views::lazy_split("text"sv, ""sv)) == 4);
+}
+
 int
 main()
 {
@@ -206,4 +220,5 @@ main()
   test08();
   test09();
   test10();
+  test11();
 }
-- 
2.32.0.93.g670b81a890