Skip to content

Lwg4304 #146

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
steve-downey merged 2 commits into from
Nov 24, 2025
Merged

Lwg4304 #146

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
758705a
Address LWG4304 value_or ill-formed
steve-downey Nov 22, 2025
ed89d27
Formatting pass
steve-downey Nov 22, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
16 changes: 14 additions & 2 deletions include/beman/optional/optional.hpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1791,6 +1791,16 @@ class optional<T&> {
*/
constexpr T& value() const;

// LWG4304. std::optional<NonReturnable&> is ill-formed due to value_o
// Resolution:
// -?- Constraints: T is a non-array object type.
// -?- Remarks: The return type is unspecified if T is an array type or a
// non-object type. [Note ?: This is to avoid the declaration being
// ill-formed. — end note]
//
// Implementer Note: Using decay_t as a detail as it is remove_cv_t for
// non-array objects, and produces a valid type for arrays and functions,
// which are otherwise `required` out.
/**
* @brief Returns the contained value if there is one, otherwise returns `u`.
*
Expand All @@ -1799,7 +1809,8 @@ class optional<T&> {
* @return std::remove_cv_t<T>
*/
template <class U = std::remove_cv_t<T>>
constexpr std::remove_cv_t<T> value_or(U&& u) const;
requires(std::is_object_v<T> && !std::is_array_v<T>)
constexpr std::decay_t<T> value_or(U&& u) const;

// \ref{optionalref.monadic}, monadic operations
/**
Expand Down Expand Up @@ -1991,7 +2002,8 @@ constexpr T& optional<T&>::value() const {

template <class T>
template <class U>
constexpr std::remove_cv_t<T> optional<T&>::value_or(U&& u) const {
requires(std::is_object_v<T> && !std::is_array_v<T>)
constexpr std::decay_t<T> optional<T&>::value_or(U&& u) const {
static_assert(std::is_constructible_v<std::remove_cv_t<T>, T&>, "T must be constructible from a T&");
static_assert(std::is_convertible_v<U, std::remove_cv_t<T>>, "Must be able to convert u to T");
return has_value() ? *value_ : static_cast<std::remove_cv_t<T>>(std::forward<U>(u));
Expand Down
23 changes: 23 additions & 0 deletions tests/beman/optional/optional_ref.test.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -864,6 +864,29 @@ TEST(OptionalRefTest, OverloadResolutionChecksDangling) {
// static_assert(std::is_same_v<decltype(check_dangling("abc")), void>);
}

namespace {
int int_func(void) { return 7; }
} // namespace

TEST(OptionalRefTest, NonReturnableRef) {
using IntArray5 = int[5];
beman::optional::optional<IntArray5&> o1;
IntArray5 array;
beman::optional::optional<IntArray5&> o2{array};
EXPECT_FALSE(o1.has_value());
EXPECT_TRUE(o2.has_value());
// value_or removed for array types
// IntArray5 array2;
// auto t1 = o1.value_or(array2);
// auto t2 = o2.value_or(array2);

using IntFunc = int(void);
beman::optional::optional<IntFunc&> o3;
beman::optional::optional<IntFunc&> o4{int_func};
EXPECT_FALSE(o3.has_value());
EXPECT_TRUE(o4.has_value());
}

// beman::optional::optional<int const&> foo() {
// beman::optional::optional<int> o(10);
// return o; // Thanks to a simpler implicit move.
Expand Down