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Implement LWG-4510 Ambiguity of std::ranges::advance and std::ranges::next when the difference type is also a sentinel type #6224

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Merged
StephanTLavavej merged 6 commits into from
Apr 17, 2026
+30 −1
Merged

Implement LWG-4510 Ambiguity of std::ranges::advance and std::ranges::next when the difference type is also a sentinel type #6224

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6824949
Implement LWG-4510 Ambiguity of `std::ranges::advance` and `std::rang…
mirion-dev Apr 5, 2026
1318f97
Extract to `namespace lwg4510`.
StephanTLavavej Apr 13, 2026
3f3a39b
Avoid quasi-shadowing: `iterator` => `IterType`, `any` => `AnyType`.
StephanTLavavej Apr 13, 2026
3c316da
Avoid abbreviated function templates.
StephanTLavavej Apr 13, 2026
c5c410d
Postincrement should return by value.
StephanTLavavej Apr 13, 2026
064d419
Add a positive test for `sentinel_for`.
StephanTLavavej Apr 13, 2026
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3 changes: 2 additions & 1 deletion stl/inc/__msvc_iter_core.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -406,7 +406,8 @@ concept input_or_output_iterator = requires(_It __i) {
} && weakly_incrementable<_It>;

_EXPORT_STD template <class _Se, class _It>
concept sentinel_for = semiregular<_Se> && input_or_output_iterator<_It> && _Weakly_equality_comparable_with<_Se, _It>;
concept sentinel_for = semiregular<_Se> && !_Integer_like<_Se> && input_or_output_iterator<_It>
&& _Weakly_equality_comparable_with<_Se, _It>;

_EXPORT_STD template <class _Se, class _It> // specializations allowed by N5014 [iterator.concept.sizedsentinel]/3
constexpr bool disable_sized_sentinel_for = false;
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28 changes: 28 additions & 0 deletions tests/std/tests/P0896R4_ranges_iterator_machinery/test.cpp
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
// Copyright (c) Microsoft Corporation.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include <__msvc_int128.hpp>
#include <cassert>
#include <compare>
#include <concepts>
Expand Down Expand Up @@ -3677,6 +3678,33 @@ namespace lwg3420 {
static_assert(!has_member_value_type<std::iterator_traits<X>>);
} // namespace lwg3420

namespace lwg4510 {
// Test LWG-4510
// "Ambiguity of std::ranges::advance and std::ranges::next when the difference type is also a sentinel type"
template <class T>
struct IterType {
using difference_type = int;

IterType& operator++();
IterType operator++(int);
IterType& operator*() const;

friend bool operator==(const IterType&, const IterType&);
};

struct AnyType {
template <class T>
AnyType(const T&);

friend bool operator==(const AnyType&, const AnyType&);
};

static_assert(std::input_or_output_iterator<IterType<AnyType>>);
static_assert(std::sentinel_for<IterType<AnyType>, IterType<AnyType>>);
static_assert(!std::sentinel_for<int, IterType<AnyType>>);
static_assert(!std::sentinel_for<std::_Unsigned128, IterType<AnyType>>);
} // namespace lwg4510

namespace vso1121031 {
// Validate that indirectly_readable_traits accepts type arguments with both value_type and element_type nested
// types if they are consistent.
Expand Down