[PATCH 3/3] libstdc++: Implement remaining piece of LWG 3448

[Patrick Palka via Gcc-patches]

Almost all of the proposed resolution for LWG 3448 is already
implemented; the only part left is to adjust the return type of
transform_view::sentinel::operator-.

libstdc++-v3/ChangeLog:

        LWG 3448
        PR libstdc++/95322
        * include/std/ranges (transform_view::__distance_from): Give
        this a deduced return type.
        (transform_view::sentinel::operator-): Adjust the return type so
        that it's based on the constness of the iterator rather than
        that of the sentinel.
        * testsuite/std/ranges/adaptors/95322.cc: Refer to LWG 3488.
---
 libstdc++-v3/include/std/ranges                | 18 +++++++++---------
 .../testsuite/std/ranges/adaptors/95322.cc     |  2 +-
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 42028354b81..2d0017f1750 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1865,7 +1865,7 @@ namespace views
          using _Base = __detail::__maybe_const_t<_Const, _Vp>;
 
          template<bool _Const2>
-           constexpr range_difference_t<_Base>
+           constexpr auto
            __distance_from(const _Iterator<_Const2>& __i) const
            { return _M_end - __i._M_current; }
 
@@ -1902,17 +1902,17 @@ namespace views
            operator==(const _Iterator<_Const2>& __x, const _Sentinel& __y)
            { return __y.__equal(__x); }
 
-         template<bool _Const2>
-           requires sized_sentinel_for<sentinel_t<_Base>,
-                      iterator_t<__detail::__maybe_const_t<_Const2, _Vp>>>
-           friend constexpr range_difference_t<_Base>
+         template<bool _Const2,
+                  typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
+           requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
+           friend constexpr range_difference_t<_Base2>
            operator-(const _Iterator<_Const2>& __x, const _Sentinel& __y)
            { return -__y.__distance_from(__x); }
 
-         template<bool _Const2>
-           requires sized_sentinel_for<sentinel_t<_Base>,
-                      iterator_t<__detail::__maybe_const_t<_Const2, _Vp>>>
-           friend constexpr range_difference_t<_Base>
+         template<bool _Const2,
+                  typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
+           requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
+           friend constexpr range_difference_t<_Base2>
            operator-(const _Sentinel& __y, const _Iterator<_Const2>& __x)
            { return __y.__distance_from(__x); }
 
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/95322.cc 
b/libstdc++-v3/testsuite/std/ranges/adaptors/95322.cc
index 9279d5520a8..67bc7d33917 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/95322.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/95322.cc
@@ -26,7 +26,7 @@ using __gnu_test::test_forward_range;
 void
 test01()
 {
-  // PR libstdc++/95322
+  // PR libstdc++/95322 and LWG 3488
   int a[2]{1, 2};
   test_forward_range<int> v{a};
   auto view1 = v | std::views::take(2);
-- 
2.28.0.337.ge9b77c84a0