[PATCH] libstdc++: Implement LWG 3564 for ranges::transform_view

[Jonathan Wakely]

Tested x86_64-linux.

-- >8 --

The _Iterator<true> type returned by begin() const uses const F& to
transform the elements, so it should use const F& to determine the
iterator's value_type and iterator_category as well.

This was accepted into the WP in July 2022.

libstdc++-v3/ChangeLog:

        * include/std/ranges (transform_view:_Iterator): Use const F&
        to determine value_type and iterator_category of
        _Iterator<true>, as per LWG 3564.
        * testsuite/std/ranges/adaptors/transform.cc: Check value_type
        and iterator_category.
---
 libstdc++-v3/include/std/ranges               |  9 +++++++--
 .../std/ranges/adaptors/transform.cc          | 19 +++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 6e6e3b97d82..f0d81cbea0c 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1886,8 +1886,12 @@ namespace views::__adaptor
          static auto
          _S_iter_cat()
          {
+           // _GLIBCXX_RESOLVE_LIB_DEFECTS
+           // 3564. transform_view::iterator<true>::value_type and
+           // iterator_category should use const F&
+           using _Fpc = __detail::__maybe_const_t<_Const, _Fp>;
            using _Base = transform_view::_Base<_Const>;
-           using _Res = invoke_result_t<_Fp&, range_reference_t<_Base>>;
+           using _Res = invoke_result_t<_Fpc&, range_reference_t<_Base>>;
            if constexpr (is_lvalue_reference_v<_Res>)
              {
                using _Cat
@@ -1927,6 +1931,7 @@ namespace views::__adaptor
              return input_iterator_tag{};
          }
 
+         using _Fpc = __detail::__maybe_const_t<_Const, _Fp>;
          using _Base_iter = iterator_t<_Base>;
 
          _Base_iter _M_current = _Base_iter();
@@ -1936,7 +1941,7 @@ namespace views::__adaptor
          using iterator_concept = decltype(_S_iter_concept());
          // iterator_category defined in __transform_view_iter_cat
          using value_type
-           = remove_cvref_t<invoke_result_t<_Fp&, range_reference_t<_Base>>>;
+           = remove_cvref_t<invoke_result_t<_Fpc&, range_reference_t<_Base>>>;
          using difference_type = range_difference_t<_Base>;
 
          _Iterator() requires default_initializable<_Base_iter> = default;
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc 
b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
index bcb18a3fc6c..ca695349650 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
@@ -196,6 +196,24 @@ test09()
 #endif
 }
 
+void
+test10()
+{
+  struct F {
+    short operator()(int) { return 0; }
+    const int& operator()(const int& i) const { return i; }
+  };
+
+  int x[] {2, 4};
+  const auto xform = x | views::transform(F{});
+  using const_iterator = decltype(xform.begin());
+  // LWG 3564. transform_view::iterator<true>::value_type and iterator_category
+  // should use const F&
+  static_assert(std::same_as<std::iter_value_t<const_iterator>, int>);
+  using cat = std::iterator_traits<const_iterator>::iterator_category;
+  static_assert(std::same_as<cat, std::random_access_iterator_tag>);
+}
+
 int
 main()
 {
@@ -208,4 +226,5 @@ main()
   test07();
   test08();
   test09();
+  test10();
 }
-- 
2.46.2