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[libc++] P3379R1: Constrain std::expected equality operators #117664

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[libc++] P3379R1: Constrain std::expected equality operators #117664

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[libc++] P3379R1: Constrain 'std::expected' equality operators
Nov 26, 2024
408b331
[libc++] P3379R1: Constrain 'std::expected' equality operators
Nov 28, 2024
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[libc++] P3379R1: Constrain 'std::expected' equality operators
Dec 1, 2024
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[libc++] P3379R1: Constrain 'std::expected' equality operators
Dec 1, 2024
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[libc++] P3379R1: Constrain 'std::expected' equality operators
Dec 1, 2024
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[libc++] P3379R1: Constrain 'std::expected' equality operators
Dec 1, 2024
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[libc++] P3379R1: Constrain 'std::expected' equality operators
Dec 1, 2024
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1 change: 1 addition & 0 deletions libcxx/docs/ReleaseNotes/20.rst
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Expand Up @@ -45,6 +45,7 @@ Implemented Papers
- ``std::jthread`` and ``<stop_token>`` are not guarded behind ``-fexperimental-library`` anymore
- P2674R1: A trait for implicit lifetime types (`Github <https://github.com/llvm/llvm-project/issues/105259>`__)
- P0429R9: A Standard ``flat_map`` is partially implemented and ``flat_map`` is provided (`Github <https://github.com/llvm/llvm-project/issues/105190>`__)
- P3379R1: Constrain ``std::expected`` equality operators
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The paper number is R0, please adjust all references in the PR

Suggested change
- P3379R1: Constrain ``std::expected`` equality operators
- P3379R0: Constrain ``std::expected`` equality operators

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@jwakely jwakely Apr 10, 2025 •
edited
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There was a D3379R1 draft but I forgot to publish it, and we voted on R0. The only difference is that R1 said to add __cpp_lib_constrained_equality in <expected>. That addition got done editorially: cplusplus/draft#7449

It looks like this PR neither bumps the macro value nor adds the macro to <expected>.

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I've submitted brevzin/sd6#28 to update SD-6.


Improvements and New Features
-----------------------------
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32 changes: 32 additions & 0 deletions libcxx/include/__expected/expected.h
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Expand Up @@ -10,6 +10,7 @@
#define _LIBCPP___EXPECTED_EXPECTED_H

#include <__assert>
#include <__concepts/convertible_to.h>
#include <__config>
#include <__expected/bad_expected_access.h>
#include <__expected/unexpect.h>
Expand Down Expand Up @@ -1139,7 +1140,15 @@ class expected : private __expected_base<_Tp, _Err> {

// [expected.object.eq], equality operators
template <class _T2, class _E2>
# if _LIBCPP_STD_VER >= 26
requires(!is_void_v<_T2> &&
requires(const _Tp& __tp, const _T2& __t2, const _Err& __err, const _E2& __e2) {
{ __tp == __t2 } -> convertible_to<bool>;
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The convertible_to concept is stricter than the plain "(implicitly) convertible to". The latter can be expressed by __is_core_convertible in <__type_traits/is_core_convertible.h>.

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I guess we can have the following.

Internal concept __core_convertible_to in the <__concepts/core_convertible_to.h> internal header 

#ifndef _LIBCPP___CONCEPTS_CORE_CONVERTIBLE_TO_H
#define _LIBCPP___CONCEPTS_CORE_CONVERTIBLE_TO_H

#include <__config>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER >= 20

// [conv.general]/3 says "E is convertible to T" whenever "T t=E;" is well-formed.
// We can't test for that, but we can test implicit convertibility by passing it
// to a function. Unlike std::convertible_to, __core_convertible_to doesn't test
// static_cast or handle cv void, while accepting move-only types.

template <class _Tp, class _Up>
concept __core_convertible_to = requires {
  // rejects function and array types which are adjusted to pointer types in parameter lists
  static_cast<_Up (*)()>(nullptr)();
  static_cast<void (*)(_Up)>(nullptr)(static_cast<_Tp (*)()>(nullptr)());
};

#endif

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___CONCEPTS_CORE_CONVERTIBLE_TO_H

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Thank you for writing this! I'll take a look.

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Would you like to draft a patch for this header if you have time? I can rebase and use your header. This will ensure that you receive credit for your work! Otherwise, I can include it in this patch myself.

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It seems we used __boolean_testable to implement the constraints for operator== in std::reference_wrapper. (https://github.com/llvm/llvm-project/blob/main/libcxx/include/__functional/reference_wrapper.h#L75-L81)
Should we consider making them consistent?

{ __err == __e2 } -> convertible_to<bool>;
})
# else
requires(!is_void_v<_T2>)
# endif
_LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
if (__x.__has_val() != __y.__has_val()) {
return false;
Expand All @@ -1153,11 +1162,22 @@ class expected : private __expected_base<_Tp, _Err> {
}

template <class _T2>
# if _LIBCPP_STD_VER >= 26
requires(!__is_std_expected<_T2>::value &&
requires(const _Tp& __tp, const _T2& __t2) {
{ __tp == __t2 } -> convertible_to<bool>;
})
# endif
_LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const _T2& __v) {
return __x.__has_val() && static_cast<bool>(__x.__val() == __v);
}

template <class _E2>
# if _LIBCPP_STD_VER >= 26
requires requires(const _Err& __err, const _E2& __e2) {
{ __err == __e2 } -> convertible_to<bool>;
}
# endif
_LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __e) {
return !__x.__has_val() && static_cast<bool>(__x.__unex() == __e.error());
}
Expand Down Expand Up @@ -1850,7 +1870,14 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {

// [expected.void.eq], equality operators
template <class _T2, class _E2>
# if _LIBCPP_STD_VER >= 26
requires(is_void_v<_T2> &&
requires(const _Err& __err, const _E2& __e2) {
{ __err == __e2 } -> convertible_to<bool>;
})
# else
requires is_void_v<_T2>
# endif
_LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
if (__x.__has_val() != __y.__has_val()) {
return false;
Expand All @@ -1860,6 +1887,11 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
}

template <class _E2>
# if _LIBCPP_STD_VER >= 26
requires requires(const _Err& __err, const _E2& __e2) {
{ __err == __e2 } -> convertible_to<bool>;
}
# endif
_LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __y) {
return !__x.__has_val() && static_cast<bool>(__x.__unex() == __y.error());
}
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12 changes: 8 additions & 4 deletions ...x/test/std/utilities/expected/expected.expected/equality/equality.other_expected.pass.cpp
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Expand Up @@ -23,14 +23,18 @@
template <class T1, class T2>
concept CanCompare = requires(T1 t1, T2 t2) { t1 == t2; };

struct Foo{};
static_assert(!CanCompare<Foo, Foo>);
struct NonComparable {};
static_assert(!CanCompare<NonComparable, NonComparable>);

static_assert(CanCompare<std::expected<int, int>, std::expected<int, int>>);
static_assert(CanCompare<std::expected<int, int>, std::expected<short, short>>);

// Note this is true because other overloads are unconstrained
static_assert(CanCompare<std::expected<int, int>, std::expected<void, int>>);
#if _LIBCPP_STD_VER >= 26
static_assert(!CanCompare<std::expected<int, int>, std::expected<void, int>>);
static_assert(!CanCompare<std::expected<NonComparable, int>, std::expected<NonComparable, int>>);
static_assert(!CanCompare<std::expected<int, NonComparable>, std::expected<int, NonComparable>>);
static_assert(!CanCompare<std::expected<NonComparable, NonComparable>, std::expected<NonComparable, NonComparable>>);
#endif

constexpr bool test() {
// x.has_value() && y.has_value()
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10 changes: 6 additions & 4 deletions libcxx/test/std/utilities/expected/expected.void/equality/equality.other_expected.pass.cpp
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Expand Up @@ -23,14 +23,16 @@
template <class T1, class T2>
concept CanCompare = requires(T1 t1, T2 t2) { t1 == t2; };

struct Foo{};
static_assert(!CanCompare<Foo, Foo>);
struct NonComparable {};
static_assert(!CanCompare<NonComparable, NonComparable>);

static_assert(CanCompare<std::expected<void, int>, std::expected<void, int>>);
static_assert(CanCompare<std::expected<void, int>, std::expected<void, short>>);

// Note this is true because other overloads in expected<non-void> are unconstrained
static_assert(CanCompare<std::expected<void, int>, std::expected<int, int>>);
#if _LIBCPP_STD_VER >= 26
static_assert(!CanCompare<std::expected<void, int>, std::expected<int, int>>);
static_assert(!CanCompare<std::expected<void, NonComparable>, std::expected<void, NonComparable>>);
#endif

constexpr bool test() {
// x.has_value() && y.has_value()
Expand Down
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