[Bug libstdc++/99846] [11 regression] std::variant comparison operator error for recursive type

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99846

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #7 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Before CWG 2369 (i.e. in GCC 10), for the original testcase, overload
resolution for 'left < right' would reject the operator<=> candidate due to
substitution failure into its return type before ever checking its loopy
constraints, and because we also define the (non-constrained) relational
operators for std::variant alongside operator<=>, overload resolution would
just end up selecting the explicitly defined operator<.

After CWG 2369 (i.e. in GCC 11), we check constraints sooner during overload
resolution, so when considering the operator<=> candidate we end up looping
during constraint checking, which we consider to be a fatal error.

Distilled testcase that exhibits the issue:

template <typename L, typename R> concept comparable_with = requires(L l, R r)
{ l < r; };

template <typename b> concept comparable = comparable_with<b, b>;

template <typename> class variant;

template <typename T> void operator<(variant<T>, variant<T>);

template <typename T> requires(comparable<T>)
typename T::fail operator<=>(variant<T>, variant<T>);

template <typename> class variant {};

struct g;
struct g : variant<g> {};
int main()
{
  g left, right;
  left < right;
}