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Implement LWG-3891 LWG-3870 breaks std::expected<cv T, E> #6233

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Implement LWG-3891 LWG-3870 breaks std::expected<cv T, E> #6233

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4 changes: 2 additions & 2 deletions stl/inc/expected
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Original file line number Diff line number Diff line change
Expand Up @@ -578,7 +578,7 @@ public:
}
_STD construct_at(_STD addressof(_Unexpected), _STD move(_Tmp));
} else {
_Ty _Tmp(_STD move(_Value));
remove_cv_t<_Ty> _Tmp(_STD move(_Value));
if constexpr (!is_trivially_destructible_v<_Ty>) {
_Value.~_Ty();
}
Expand Down Expand Up @@ -1206,7 +1206,7 @@ private:
}

union {
_Ty _Value;
remove_cv_t<_Ty> _Value;
_Err _Unexpected;
};
bool _Has_value;
Expand Down
116 changes: 110 additions & 6 deletions tests/std/tests/P0323R12_expected/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2393,7 +2393,6 @@ constexpr bool test_lwg_3886() {
expected<const QualDistinction, char> ex{unexpect};
assert(ex.value_or({}).qual_ == Qualification::None);
}
#if 0 // TRANSITION, LWG-3891
assert(
(expected<const QualDistinction, char>{unexpect} = {QualDistinction{}}).value().qual_ == Qualification::None);
{
Expand All @@ -2402,7 +2401,6 @@ constexpr bool test_lwg_3886() {
ex = unexpected<char>{'*'};
assert((ex = {QualDistinction{}}).value().qual_ == Qualification::None);
}
#endif // ^^^ no workaround ^^^

return true;
}
Expand All @@ -2413,7 +2411,6 @@ void test_lwg_3886_volatile() {
expected<volatile QualDistinction, char> ex{unexpect};
assert(ex.value_or({}).qual_ == Qualification::None);
}
#if 0 // TRANSITION, LWG-3891
assert((expected<volatile QualDistinction, char>{unexpect} = {QualDistinction{}}).value().qual_
== Qualification::None);
{
Expand All @@ -2422,14 +2419,12 @@ void test_lwg_3886_volatile() {
ex = unexpected<char>{'*'};
assert((ex = {QualDistinction{}}).value().qual_ == Qualification::None);
}
#endif // ^^^ no workaround ^^^

assert((expected<const volatile QualDistinction, char>{unexpect}.value_or({}).qual_ == Qualification::None));
{
expected<const volatile QualDistinction, char> ex{unexpect};
assert(ex.value_or({}).qual_ == Qualification::None);
}
#if 0 // TRANSITION, LWG-3891
assert((expected<const volatile QualDistinction, char>{unexpect} = {QualDistinction{}}).value().qual_
== Qualification::None);
{
Expand All @@ -2438,7 +2433,114 @@ void test_lwg_3886_volatile() {
ex = unexpected<char>{'*'};
assert((ex = {QualDistinction{}}).value().qual_ == Qualification::None);
}
#endif // ^^^ no workaround ^^^
}

// Test LWG-3891: "LWG 3870 breaks std::expected<cv T, E>"

struct FakeCvAssignable {
#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, DevCom-11070772
char c_{};
#endif // ^^^ workaround ^^^

FakeCvAssignable() = default;
FakeCvAssignable(const FakeCvAssignable&) = default;
FakeCvAssignable(FakeCvAssignable&&) = default;
FakeCvAssignable& operator=(const FakeCvAssignable&) = default;
FakeCvAssignable& operator=(FakeCvAssignable&&) = default;

template <class T = FakeCvAssignable>
constexpr FakeCvAssignable(const volatile type_identity_t<T>&) noexcept {}
template <class T = FakeCvAssignable>
constexpr FakeCvAssignable(const volatile type_identity_t<T>&&) noexcept {}

// Per LWG-3891, the following functions shouldn't be called within expected<FakeCvAssignable, E>.

template <class T = FakeCvAssignable>
FakeCvAssignable& operator=(const volatile type_identity_t<T>&) noexcept {
assert(false);
}
template <class T = FakeCvAssignable>
FakeCvAssignable& operator=(const volatile type_identity_t<T>&&) noexcept {
assert(false);
}
template <class T = FakeCvAssignable>
const volatile FakeCvAssignable& operator=(const volatile type_identity_t<T>&) const volatile noexcept {
assert(false);
}
template <class T = FakeCvAssignable>
const volatile FakeCvAssignable& operator=(const volatile type_identity_t<T>&&) const volatile noexcept {
assert(false);
}
};

constexpr bool test_lwg_3891() {
{
expected<const int, char> oc{unexpect};
assert(!oc.has_value());
oc.emplace(0);
assert(oc.value() == 0);
static_assert(!is_copy_assignable_v<decltype(oc)>);
static_assert(!is_move_assignable_v<decltype(oc)>);
static_assert(!is_swappable_v<decltype(oc)>);

expected<volatile int, char> ov{unexpect}, ov2{unexpect};
assert(!ov.has_value());
ov.emplace(0);
assert(ov.has_value());
assert(ov.value_or(-1) == 0);
swap(ov, ov);
assert(ov.has_value());
assert(ov.value_or(-1) == 0);
ov = ov2;
assert(!ov.has_value());
ov = move(ov2);
assert(!ov.has_value());

expected<const volatile int, char> ocv{unexpect};
assert(!ocv.has_value());
ocv.emplace(0);
assert(ocv.has_value());
assert(ocv.value_or(-1) == 0);
static_assert(!is_copy_assignable_v<decltype(ocv)>);
static_assert(!is_move_assignable_v<decltype(ocv)>);
static_assert(!is_swappable_v<decltype(ocv)>);
}
{
expected<const FakeCvAssignable, char> oc{unexpect}, oc2{unexpect};
assert(!oc.has_value());
oc.emplace(FakeCvAssignable{});
assert(oc.has_value());
swap(oc, oc);
assert(oc.has_value());
oc = oc2;
assert(!oc.has_value());
oc = move(oc2);
assert(!oc.has_value());

expected<volatile FakeCvAssignable, char> ov{unexpect}, ov2{unexpect};
assert(!ov.has_value());
ov.emplace(FakeCvAssignable{});
assert(ov.has_value());
swap(ov, ov);
assert(ov.has_value());
ov = ov2;
assert(!ov.has_value());
ov = move(ov2);
assert(!ov.has_value());

expected<const volatile FakeCvAssignable, char> ocv{unexpect}, ocv2{unexpect};
assert(!ocv.has_value());
ocv.emplace(FakeCvAssignable{});
assert(ocv.has_value());
swap(ocv, ocv);
assert(ocv.has_value());
ocv = ocv2;
assert(!ocv.has_value());
ocv = move(ocv2);
assert(!ocv.has_value());
}

return true;
}

// Test GH-4011: these predicates triggered constraint recursion.
Expand Down Expand Up @@ -2508,6 +2610,7 @@ static_assert(!is_assignable_v<expected<void, move_only>&, ambiguating_expected_
#endif // ^^^ no workaround ^^^

static_assert(test_lwg_3886());
static_assert(test_lwg_3891());

// Test LWG-4222 "expected constructor from a single value missing a constraint"

Expand Down Expand Up @@ -2608,6 +2711,7 @@ int main() {
test_lwg_3843();
test_lwg_3886();
test_lwg_3886_volatile();
test_lwg_3891();
test_inherited_constructors();

test_lwg_4366::test();
Expand Down