[Bug libstdc++/115059] New: Constraints/Mandates on the comparison operators of std::optional and std::variant are overly strict

[Bug libstdc++/115059] New: Constraints/Mandates on the comparison operators of std::optional and std::variant are overly strict

[de34 at live dot cn]

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.

[Bug libstdc++/115059] Constraints/Mandates on the comparison operators of std::optional and std::variant are overly strict

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115059

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't want to change anything in libstdc++ here. Either std::is_convertible
should be sufficient to check "convertible to" constraints, or "convertible to"
should exclude these kind of games.

[Bug libstdc++/115059] Constraints/Mandates on the comparison operators of std::optional and std::variant are overly strict

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115059

--- Comment #2 from Jiang An <de34 at live dot cn> ---
(In reply to Jonathan Wakely from comment #1)
> I don't want to change anything in libstdc++ here. Either
> std::is_convertible should be sufficient to check "convertible to"
> constraints, or "convertible to" should exclude these kind of games.

Yeah. This is perphaps in the scope of LWG484 and LWG3105, but guaranteed copy
elision doesn't seem discussed yet.