libstdc++: Implement LWG 3798 for range adaptors [PR106676]

LWG 3798 modified the iterator_category of the iterator types for
transform_view, join_with_view, zip_transform_view and
adjacent_transform_view, to allow the iterator's reference type to be an
rvalue reference.

libstdc++-v3/ChangeLog:

	PR libstdc++/106676
	* include/bits/iterator_concepts.h (__cpp17_fwd_iterator): Use
	is_reference instead of is_value_reference.
	rvalue references.
	* include/std/ranges (transform_view:__iter_cat::_S_iter_cat):
	Likewise.
	(zip_transform_view::__iter_cat::_S_iter_cat): Likewise.
	(adjacent_transform_view::__iter_cat::_S_iter_cat): Likewise.
	(join_with_view::__iter_cat::_S_iter_cat): Likewise.
	* testsuite/std/ranges/adaptors/transform.cc: Check
	iterator_category when the transformation function returns an
	rvalue reference type.

Reviewed-by: Patrick Palka <ppalka@redhat.com>

3 files changed, 32 insertions, 6 deletions

diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index 490a362..669d3dd 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -333,10 +333,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    typename incrementable_traits<_Iter>::difference_type>;
 	};
 
+    // _GLIBCXX_RESOLVE_LIB_DEFECTS
+    // 3798. Rvalue reference and iterator_category
     template<typename _Iter>
       concept __cpp17_fwd_iterator = __cpp17_input_iterator<_Iter>
 	&& constructible_from<_Iter>
-	&& is_lvalue_reference_v<iter_reference_t<_Iter>>
+	&& is_reference_v<iter_reference_t<_Iter>>
 	&& same_as<remove_cvref_t<iter_reference_t<_Iter>>,
 		   typename indirectly_readable_traits<_Iter>::value_type>
 	&& requires(_Iter __it)
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 98442dc..5b45588 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1892,7 +1892,9 @@ namespace views::__adaptor
 	    using _Base = transform_view::_Base<_Const>;
 	    using _Res = invoke_result_t<__maybe_const_t<_Const, _Fp>&,
 					 range_reference_t<_Base>>;
-	    if constexpr (is_lvalue_reference_v<_Res>)
+	    // _GLIBCXX_RESOLVE_LIB_DEFECTS
+	    // 3798. Rvalue reference and iterator_category
+	    if constexpr (is_reference_v<_Res>)
 	      {
 		using _Cat
 		  = typename iterator_traits<iterator_t<_Base>>::iterator_category;
@@ -5047,7 +5049,9 @@ namespace views::__adaptor
 	  using __detail::__range_iter_cat;
 	  using _Res = invoke_result_t<__maybe_const_t<_Const, _Fp>&,
 				       range_reference_t<__maybe_const_t<_Const, _Vs>>...>;
-	  if constexpr (!is_lvalue_reference_v<_Res>)
+	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+	  // 3798. Rvalue reference and iterator_category
+	  if constexpr (!is_reference_v<_Res>)
 	    return input_iterator_tag{};
 	  else if constexpr ((derived_from<__range_iter_cat<_Vs, _Const>,
 					   random_access_iterator_tag> && ...))
@@ -5820,7 +5824,9 @@ namespace views::__adaptor
       using _Res = invoke_result_t<__unarize<__maybe_const_t<_Const, _Fp>&, _Nm>,
 				   range_reference_t<_Base>>;
       using _Cat = typename iterator_traits<iterator_t<_Base>>::iterator_category;
-      if constexpr (!is_lvalue_reference_v<_Res>)
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3798. Rvalue reference and iterator_category
+      if constexpr (!is_reference_v<_Res>)
 	return input_iterator_tag{};
       else if constexpr (derived_from<_Cat, random_access_iterator_tag>)
 	return random_access_iterator_tag{};
@@ -7228,8 +7234,10 @@ namespace views::__adaptor
 	  using _OuterCat = typename iterator_traits<_OuterIter>::iterator_category;
 	  using _InnerCat = typename iterator_traits<_InnerIter>::iterator_category;
 	  using _PatternCat = typename iterator_traits<_PatternIter>::iterator_category;
-	  if constexpr (!is_lvalue_reference_v<common_reference_t<iter_reference_t<_InnerIter>,
-								  iter_reference_t<_PatternIter>>>)
+	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+	  // 3798. Rvalue reference and iterator_category
+	  if constexpr (!is_reference_v<common_reference_t<iter_reference_t<_InnerIter>,
+							   iter_reference_t<_PatternIter>>>)
 	    return input_iterator_tag{};
 	  else if constexpr (derived_from<_OuterCat, bidirectional_iterator_tag>
 			     && derived_from<_InnerCat, bidirectional_iterator_tag>
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
index ca69534..e1b7435 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
@@ -214,6 +214,21 @@ test10()
   static_assert(std::same_as<cat, std::random_access_iterator_tag>);
 }
 
+void
+test11()
+{
+  struct MoveIt {
+    int&& operator()(int& i) const { return std::move(i); }
+  };
+
+  int x[] {2, 4};
+  auto xform = x | views::transform(MoveIt{});
+  using iterator = decltype(xform.begin());
+  // LWG 3798. Rvalue reference and iterator_category
+  using cat = std::iterator_traits<iterator>::iterator_category;
+  static_assert(std::same_as<cat, std::random_access_iterator_tag>);
+}
+
 int
 main()
 {
@@ -227,4 +242,5 @@ main()
   test08();
   test09();
   test10();
+  test11();
 }