libstdc++: Fix up lambda in join_view::_Iterator::operator++ [PR100290]

Currently, the return type of this lambda is decltype(auto), so the
lambda ends up returning a copy of _M_parent->_M_inner rather than a
reference to it when _S_ref_glvalue is false.  This means _M_inner and
ranges::end(__inner_range) are respectively an iterator and sentinel for
different ranges, so comparing them is undefined.

libstdc++-v3/ChangeLog:

	PR libstdc++/100290
	* include/std/ranges (join_view::_Iterator::operator++): Correct
	the return type of the lambda to avoid returning a copy of
	_M_parent->_M_inner.
	* testsuite/std/ranges/adaptors/join.cc (test10): New test.

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 74075a2..09115e9 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2389,7 +2389,7 @@ namespace views::__adaptor
          constexpr _Iterator&
          operator++()
          {
-           auto&& __inner_range = [this] () -> decltype(auto) {
+           auto&& __inner_range = [this] () -> auto&& {
              if constexpr (_S_ref_is_glvalue)
                return *_M_outer;
              else

diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
index fb06a76..e6c71d7 100644 (file)
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
@@ -160,6 +160,16 @@ test09()
   static_assert(!requires { 0 | join; });
 }
 
+void
+test10()
+{
+  // PR libstdc++/100290
+  auto v = views::single(0)
+    | views::transform([](const auto& s) { return views::single(s); })
+    | views::join;
+  VERIFY( ranges::next(v.begin()) == v.end() );
+}
+
 int
 main()
 {
@@ -172,4 +182,5 @@ main()
   test07();
   test08();
   test09();
+  test10();
 }