https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115059
Bug ID: 115059
Summary: Constraints/Mandates on the comparison operators of
std::optional and std::variant are overly strict
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: de34 at live dot cn
Target Milestone: ---
The following example should compile in C++23/26 (or some other modes where
explicit object parameters are available) due to CWG DR 2813
(https://cplusplus.github.io/CWG/issues/2813.html), given that
[optional.relops] and [variant.relops] only require the result to be
convertible to bool.
```
#include <optional>
#include <variant>
class PinnedBoolean {
private:
bool val_;
PinnedBoolean(const PinnedBoolean&) = delete;
PinnedBoolean& operator=(const PinnedBoolean&) = delete;
public:
constexpr explicit PinnedBoolean(bool b) noexcept : val_{b} {}
constexpr operator bool(this PinnedBoolean x) noexcept { return x.val_; }
};
static_assert(PinnedBoolean{true});
struct X {
friend constexpr PinnedBoolean operator==(X, X) noexcept { return
PinnedBoolean{true}; }
friend constexpr PinnedBoolean operator!=(X, X) noexcept { return
PinnedBoolean{false}; }
};
static_assert(std::optional<X>{} == std::optional<X>{});
static_assert(std::variant<X>{} == std::variant<X>{});
```
However, libc++ is currently applying stricter requirements
(https://godbolt.org/z/Mc3bse5h7).
Since CWG2813, both __boolean_testable and is_convertible_v are stricter than
the plain "convertible to", because the implicit conversion from the result
type to bool can be only well-formed for prvalues but not xvalues.
