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

[Jonathan Wakely]

On 27/04/21 10:55 -0400, Patrick Palka via Libstdc++ wrote:
>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?

Yes for all three, thanks.

>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
>