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Implement LWG-4243 as_bytes/as_writable_bytes is broken with span<volatile T> #5867

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Merged
merged 1 commit into from
Nov 19, 2025
+19 −5
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Implement LWG-4243 as_bytes/as_writable_bytes is broken with span<volatile T> #5867

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3 changes: 2 additions & 1 deletion stl/inc/span
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Original file line number Diff line number Diff line change
Expand Up @@ -558,13 +558,14 @@ span(_Rng&&) -> span<remove_reference_t<_RANGES range_reference_t<_Rng>>>;
#ifdef __cpp_lib_byte
// [span.objectrep] Views of object representation
_EXPORT_STD template <class _Ty, size_t _Extent>
requires (!is_volatile_v<_Ty>)
_NODISCARD auto as_bytes(span<_Ty, _Extent> _Sp) noexcept {
using _ReturnType = span<const byte, _Extent == dynamic_extent ? dynamic_extent : sizeof(_Ty) * _Extent>;
return _ReturnType{reinterpret_cast<const byte*>(_Sp.data()), _Sp.size_bytes()};
}

_EXPORT_STD template <class _Ty, size_t _Extent>
requires (!is_const_v<_Ty>)
requires (!is_const_v<_Ty> && !is_volatile_v<_Ty>)
_NODISCARD auto as_writable_bytes(span<_Ty, _Extent> _Sp) noexcept {
using _ReturnType = span<byte, _Extent == dynamic_extent ? dynamic_extent : sizeof(_Ty) * _Extent>;
return _ReturnType{reinterpret_cast<byte*>(_Sp.data()), _Sp.size_bytes()};
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21 changes: 17 additions & 4 deletions tests/std/tests/P0122R7_span/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -153,11 +153,11 @@ using BorrowedContiguousSizedRange = BasicRange<int, true>;
template <typename T, size_t Extent = dynamic_extent>
constexpr void FunctionTakingSpan(type_identity_t<span<T, Extent>>) {}

template <typename U, typename = void>
constexpr bool AsWritableBytesCompilesFor = false;
template <typename U>
constexpr bool AsBytesCompilesFor = requires { as_bytes(declval<U>()); };

template <typename U>
constexpr bool AsWritableBytesCompilesFor<U, void_t<decltype(as_writable_bytes(declval<U>()))>> = true;
constexpr bool AsWritableBytesCompilesFor = requires { as_writable_bytes(declval<U>()); };

constexpr bool test() {
{
Expand Down Expand Up @@ -1016,10 +1016,23 @@ void test_non_constexpr() {
static_assert(noexcept(as_writable_bytes(sp_dyn)));
static_assert(noexcept(as_writable_bytes(sp_nine)));

static_assert(AsBytesCompilesFor<span<int>>);
static_assert(AsBytesCompilesFor<span<int, 5>>);
static_assert(AsBytesCompilesFor<span<const int>>);
static_assert(AsBytesCompilesFor<span<const int, 6>>);
static_assert(!AsBytesCompilesFor<span<volatile int>>);
static_assert(!AsBytesCompilesFor<span<volatile int, 7>>);
static_assert(!AsBytesCompilesFor<span<const volatile int>>);
static_assert(!AsBytesCompilesFor<span<const volatile int, 8>>);

static_assert(AsWritableBytesCompilesFor<span<int>>);
static_assert(AsWritableBytesCompilesFor<span<int, 9>>);
static_assert(!AsWritableBytesCompilesFor<span<const int>>);
static_assert(!AsWritableBytesCompilesFor<span<const int, 9>>);
static_assert(!AsWritableBytesCompilesFor<span<const int, 10>>);
static_assert(!AsWritableBytesCompilesFor<span<volatile int>>);
static_assert(!AsWritableBytesCompilesFor<span<volatile int, 11>>);
static_assert(!AsWritableBytesCompilesFor<span<const volatile int>>);
static_assert(!AsWritableBytesCompilesFor<span<const volatile int, 12>>);

auto sp_1 = as_bytes(sp_dyn);
auto sp_2 = as_bytes(sp_nine);
Expand Down