[cfe-dev] Spaceship operator oddity with optional

[Nuno Lopes via cfe-dev]

Hi,

I just bumped into this mysterious behavior with the spaceship operator over
optional:

struct expr {
  std::strong_ordering operator<=>(const expr &rhs) const;
  //bool operator==(const expr &rhs) const;
};

int f() {
  return std::is_eq(std::optional<expr>() <=> std::optional<expr>());
  //return std::optional<expr>() == std::optional<expr>();
}

Function f() returns 0 both on gcc & clang trunk
(https://gcc.godbolt.org/z/fv85eP).
However, if you uncomment the operator==() line, the function starts
returning 1.

The documentation of
https://en.cppreference.com/w/cpp/utility/optional/operator_cmp just says:
    7) If bool(lhs) && bool(rhs) is true returns *x <=> *y
      Otherwise, returns bool(lhs) <=> bool(rhs)

So no word about operator=='s existence.
Is this a bug in both clang & gcc or just some unobvious 3-way comparison
rule I'm not aware?

Thanks,
Nuno