Speculatively implement LWG-3664 (#2522)

to fix a regression when calling `ranges::distance` with array arguments which _should_ decay to pointers. While we're here, let's merge `distance(i, s)` overloads and use `if constexpr` dispatch instead of concept overloading.

Co-authored-by: Stephan T. Lavavej <[email protected]>

Author: CaseyCarter

stl/inc/xutility

@@ -2868,19 +2868,16 @@ namespace ranges {
     public:
         using _Not_quite_object::_Not_quite_object;
 
-        // clang-format off
-        template <input_or_output_iterator _It, sentinel_for<_It> _Se>
-            requires (!sized_sentinel_for<_Se, _It>)
-        _NODISCARD constexpr iter_difference_t<_It> operator()(_It _First, _Se _Last) const {
-            // clang-format on
-            _Adl_verify_range(_First, _Last);
-            return _Distance_unchecked(_Get_unwrapped(_STD move(_First)), _Get_unwrapped(_STD move(_Last)));
-        }
-
-        template <input_or_output_iterator _It, sized_sentinel_for<_It> _Se>
-        _NODISCARD constexpr iter_difference_t<_It> operator()(const _It& _First, const _Se& _Last) const
-            noexcept(noexcept(_Last - _First)) /* strengthened */ {
-            return _Last - _First;
+        template <class _It, sentinel_for<decay_t<_It>> _Se>
+        _NODISCARD constexpr iter_difference_t<decay_t<_It>> operator()(_It&& _Raw_first, _Se _Last) const
+            noexcept(_Nothrow_dist<_It, _Se>) /* strengthened */ {
+            if constexpr (sized_sentinel_for<_Se, decay_t<_It>>) {
+                return _Last - static_cast<const decay_t<_It>&>(_Raw_first); // Per LWG-3664
+            } else {
+                auto _First = _STD forward<_It>(_Raw_first);
+                _Adl_verify_range(_First, _Last);
+                return _Distance_unchecked(_Get_unwrapped(_STD move(_First)), _Get_unwrapped(_STD move(_Last)));
+            }
         }
 
         template <range _Rng>
@@ -2894,6 +2891,13 @@ namespace ranges {
         }
 
     private:
+        template <class _It, class _Se>
+        static constexpr bool _Nothrow_dist = false;
+
+        template <class _It, sized_sentinel_for<decay_t<_It>> _Se>
+        static constexpr bool _Nothrow_dist<_It, _Se> = noexcept(
+            _STD declval<_Se&>() - _STD declval<const decay_t<_It>&>());
+
         template <class _It, class _Se>
         _NODISCARD static constexpr iter_difference_t<_It> _Distance_unchecked(_It _First, const _Se _Last) noexcept(
             noexcept(++_First != _Last)) {

tests/std/tests/P0896R4_ranges_iterator_machinery/test.cpp

@@ -2861,6 +2861,32 @@ namespace iter_ops {
         }
     };
 
+    template <class Element>
+    struct pointer_sentinel {
+        Element* ptr = nullptr;
+
+        pointer_sentinel() = default;
+        constexpr explicit pointer_sentinel(Element* const p) noexcept : ptr{p} {}
+
+        template <class T>
+        [[nodiscard]] constexpr bool operator==(T* that) const noexcept {
+            static_assert(std::same_as<T, Element>);
+            return ptr == that;
+        }
+
+        template <class T>
+        [[nodiscard]] friend constexpr std::ptrdiff_t operator-(T* x, const pointer_sentinel& y) noexcept {
+            static_assert(std::same_as<T, Element>);
+            return x - y.ptr;
+        }
+
+        template <class T>
+        [[nodiscard]] friend constexpr std::ptrdiff_t operator-(const pointer_sentinel& y, T* x) noexcept {
+            static_assert(std::same_as<T, Element>);
+            return y.ptr - x;
+        }
+    };
+
     constexpr bool test_distance() {
         using ranges::distance, ranges::size;
         using std::iter_difference_t, std::same_as;
@@ -2974,6 +3000,26 @@ namespace iter_ops {
             assert(r.t == expected);
         }
 
+        {
+            // Call distance(i, s) with arrays which must be decayed to pointers.
+            // (This behavior was regressed by LWG-3392.)
+            int some_ints[] = {1, 2, 3};
+            assert(distance(some_ints, pointer_sentinel{some_ints + 1}) == 1);
+            STATIC_ASSERT(noexcept(distance(some_ints, pointer_sentinel{some_ints + 1})));
+            assert(distance(some_ints + 1, some_ints) == -1);
+            STATIC_ASSERT(noexcept(distance(some_ints + 1, some_ints)));
+            assert(distance(some_ints, some_ints) == 0);
+            STATIC_ASSERT(noexcept(distance(some_ints, some_ints)));
+
+            const auto& const_ints = some_ints;
+            assert(distance(const_ints, pointer_sentinel{const_ints + 1}) == 1);
+            STATIC_ASSERT(noexcept(distance(const_ints, pointer_sentinel{const_ints + 1})));
+            assert(distance(const_ints + 1, const_ints) == -1);
+            STATIC_ASSERT(noexcept(distance(const_ints + 1, const_ints)));
+            assert(distance(const_ints, const_ints) == 0);
+            STATIC_ASSERT(noexcept(distance(const_ints, const_ints)));
+        }
+
         return true;
     }
     STATIC_ASSERT(test_distance());