[gcc r14-11512] libstdc++: Fix std::ranges::iter_move for function references [PR119469]

[Jonathan Wakely]

https://gcc.gnu.org/g:88f0c1fe2825056f6d5fefa7809d878b109e7019

commit r14-11512-g88f0c1fe2825056f6d5fefa7809d878b109e7019
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 26 11:21:32 2025 +0000

    libstdc++: Fix std::ranges::iter_move for function references [PR119469]
    
    The result of std::move (or a cast to an rvalue reference) on a function
    reference is always an lvalue. Because std::ranges::iter_move was using
    the type std::remove_reference_t<X>&& as the result of std::move, it was
    giving the wrong type for function references. Use a decltype-specifier
    with declval<remove_reference_t<X>>() instead of just using the
    remove_reference_t<X>&& type directly. This gives the right result,
    while still avoiding the cost of doing overload resolution for
    std::move.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/119469
            * include/bits/iterator_concepts.h (_IterMove::__result): Use
            decltype-specifier instead of an explicit type.
            * testsuite/24_iterators/customization_points/iter_move.cc:
            Check results for function references.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    (cherry picked from commit 3e52eb28c537aaa03afb78ef9dff8325c5f41f78)

Diff:
---
 libstdc++-v3/include/bits/iterator_concepts.h                 | 11 +++++++++--
 .../testsuite/24_iterators/customization_points/iter_move.cc  | 11 +++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index acd8e9d5ae11..f70a5d4e0609 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -128,12 +128,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  struct __result<_Tp>
 	  { using type = decltype(iter_move(std::declval<_Tp>())); };
 
-	// Otherwise, if *E if an lvalue, use std::move(*E).
+	// Otherwise, if *E is an lvalue, use std::move(*E).
 	template<typename _Tp>
 	  requires (!__adl_imove<_Tp>)
 	    && is_lvalue_reference_v<__iter_ref_t<_Tp>>
 	  struct __result<_Tp>
-	  { using type = remove_reference_t<__iter_ref_t<_Tp>>&&; };
+	  {
+	    // Instead of decltype(std::move(*E)) we define the type as the
+	    // return type of std::move, i.e. remove_reference_t<iter_ref>&&.
+	    // N.B. the use of decltype(declval<X>()) instead of just X&& is
+	    // needed for function reference types, see PR libstdc++/119469.
+	    using type
+	      = decltype(std::declval<remove_reference_t<__iter_ref_t<_Tp>>>());
+	  };
 
 	template<typename _Tp>
 	  static constexpr bool
diff --git a/libstdc++-v3/testsuite/24_iterators/customization_points/iter_move.cc b/libstdc++-v3/testsuite/24_iterators/customization_points/iter_move.cc
index fbd7a481296b..f352a7ad05e4 100644
--- a/libstdc++-v3/testsuite/24_iterators/customization_points/iter_move.cc
+++ b/libstdc++-v3/testsuite/24_iterators/customization_points/iter_move.cc
@@ -157,9 +157,20 @@ test_pr106612()
   static_assert( std::same_as<decltype(std::ranges::iter_move(I3{})), F> );
 }
 
+void
+test_pr119469()
+{
+  // rvalue references to function types are weird.
+  using F = int();
+  static_assert( std::same_as<std::iter_rvalue_reference_t<F>, F&> );
+  static_assert( std::same_as<std::iter_rvalue_reference_t<F&>, F&> );
+  static_assert( std::same_as<std::iter_rvalue_reference_t<F&&>, F&> );
+}
+
 int
 main()
 {
   test01();
   test_adl();
+  test_pr119469();
 }