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

[Patrick Palka via Gcc-patches]

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

Tested on x86_64-pc-linux-gnu, does this look OK for trunk/11/10?

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.
---
 libstdc++-v3/include/std/ranges                    |  2 +-
 libstdc++-v3/testsuite/std/ranges/adaptors/join.cc | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 74075a2d6d3..09115e9b45f 100644
--- 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 fb06a7698af..e6c71d771de 100644
--- 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();
 }
-- 
2.31.1.362.g311531c9de