[gcc r12-173] libstdc++: Fix up lambda in join_view::_Iterator::operator++ [PR100290]

[Patrick Palka]

https://gcc.gnu.org/g:85ef4b8d4eb3313a48b79c7e752891f9646bb246

commit r12-173-g85ef4b8d4eb3313a48b79c7e752891f9646bb246
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.

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 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();
 }