[PATCH 4/4] libstdc++: Use deducing this in range adaptors even in C++20 [PR111550]

[Patrick Palka]

Use deducing this to implement perfect forwarding even in C++20 mode
by using the _GLIBCXX_EXPLICIT_THIS_PARAMETER internal FTM instead of
the standard __cpp_explicit_this_parameter.  This fixes the original
testcase from this PR even in C++20 mode.

        PR libstdc++/111550

libstdc++-v3/ChangeLog:

        * include/std/ranges (views::__adaptor::_Partial::operator())
        [_GLIBCXX_EXPLICIT_THIS_PARAMETER]: Also use deducing this
        in C++20 mode when possible.
        (views::__adaptor::_Pipe::Operator())
        [_GLIBCXX_EXPLICIT_THIS_PARAMETER]: Likewise.
        * testsuite/std/ranges/adaptors/take.cc (test07): New test.
---
 libstdc++-v3/include/std/ranges                   | 10 ++++++++--
 .../testsuite/std/ranges/adaptors/take.cc         | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index fd290ea362cc..9b8aa8d734e6 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1061,7 +1061,9 @@ namespace views::__adaptor
 
       // Invoke _Adaptor with arguments __r, _M_args... according to the
       // value category of this _Partial object.
-#if __cpp_explicit_this_parameter
+#if _GLIBCXX_EXPLICIT_THIS_PARAMETER
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wc++23-extensions" // deducing this
       template<typename _Self, typename _Range>
        requires __adaptor_invocable<_Adaptor, _Range, __like_t<_Self, 
_Args>...>
        constexpr auto
@@ -1070,6 +1072,7 @@ namespace views::__adaptor
          return _Binder::_S_call(__like_t<_Self, _Partial>(__self)._M_binder,
                                  std::forward<_Range>(__r));
        }
+# pragma GCC diagnostic pop
 #else
       template<typename _Range>
        requires __adaptor_invocable<_Adaptor, _Range, const _Args&...>
@@ -1138,7 +1141,9 @@ namespace views::__adaptor
 
       // Invoke _M_rhs(_M_lhs(__r)) according to the value category of this
       // range adaptor closure object.
-#if __cpp_explicit_this_parameter
+#if _GLIBCXX_EXPLICIT_THIS_PARAMETER
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wc++23-extensions" // deducing this
       template<typename _Self, typename _Range>
        requires __pipe_invocable<__like_t<_Self, _Lhs>, __like_t<_Self, _Rhs>, 
_Range>
        constexpr auto
@@ -1148,6 +1153,7 @@ namespace views::__adaptor
                  (__like_t<_Self, _Pipe>(__self)._M_lhs
                   (std::forward<_Range>(__r))));
        }
+# pragma GCC diagnostic pop
 #else
       template<typename _Range>
        requires __pipe_invocable<const _Lhs&, const _Rhs&, _Range>
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/take.cc 
b/libstdc++-v3/testsuite/std/ranges/adaptors/take.cc
index 167377264cbf..9584c576b965 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/take.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/take.cc
@@ -118,6 +118,20 @@ test06()
   static_assert(!requires { views::all | take; });
 }
 
+void
+test07()
+{
+  // PR libstdc++/111550
+  struct Five {
+    operator int() & { return 5; }
+    operator int() && = delete;
+  };
+  auto take_five = views::take(Five{});
+  auto r = take_five(views::iota(0));
+  auto take_five_piped = views::take(Five{}) | 
views::transform(std::identity{});
+  auto s = take_five_piped(views::iota(0));
+}
+
 int
 main()
 {
@@ -127,4 +141,5 @@ main()
   test04();
   test05();
   test06();
+  test07();
 }
-- 
2.51.0.491.g4b71b29477