[gcc r16-5918] libstdc++: Use deducing this in range adaptors even in C++20 [PR111550]

[Patrick Palka]

https://gcc.gnu.org/g:5a2a527b26c22df4f1764f61b2285ac0c380ff68

commit r16-5918-g5a2a527b26c22df4f1764f61b2285ac0c380ff68
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Dec 5 12:16:30 2025 -0500

    libstdc++: Use deducing this in range adaptors even in C++20 [PR111550]
    
    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.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/std/ranges                    | 10 ++++++++--
 libstdc++-v3/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 b3105c553839..b81ee784ec37 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1259,7 +1259,9 @@ 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
@@ -1268,6 +1270,7 @@ 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&...>
@@ -1336,7 +1339,9 @@ 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
@@ -1346,6 +1351,7 @@ 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();
 }