libstdc++: Implement LWG 3564 for ranges::transform_view

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.

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

1 files changed, 19 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
index bcb18a3..ca69534 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();
 }