[gcc r10-9789] libstdc++: Fix up lambda in join_view::_Iterator::operator++ [PR100290]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:b81834eaf85d5c4e0df8e4fc2307cbdd083dea6c

commit r10-9789-gb81834eaf85d5c4e0df8e4fc2307cbdd083dea6c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Apr 27 14:07:46 2021 -0400

    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.
    
    (cherry picked from commit 85ef4b8d4eb3313a48b79c7e752891f9646bb246)

Diff:
---
 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 446585f0003..cb7c6c8b5b7 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2528,7 +2528,7 @@ namespace views
 	  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 8bbea9a6b25..9c0231b704b 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
@@ -149,6 +149,16 @@ test08()
   VERIFY( i->a == 5 );
 }
 
+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()
 {
@@ -160,4 +170,5 @@ main()
   test06();
   test07();
   test08();
+  test10();
 }