Can not compare std::unorded_set with custom KeyEqual

Question

The following program does not compile. But If I do not comment out operator==, it compiles. Why operator== is still needed when I already provide FooEqual

#include <cstddef>
#include <unordered_set>

struct Foo {
};

struct FooHasher {
  size_t operator()(const Foo&) const {
    return 1;
  }
};

struct FooEqual {
  bool operator()(const Foo& lhs, const Foo& rhs) const {
    return true;
  }
};

// bool operator==(const Foo& lhs, const Foo& rhs) {
//   return true;
// }

int main() {
  std::unordered_set<Foo, FooHasher, FooEqual> s1;
  std::unordered_set<Foo, FooHasher, FooEqual> s2;
  (void)(s1 == s2);
  return 0;
}

Answer 1

"23.2.5 Unordered associative containers" states:

Two unordered containers a and b compare equal if a.size() == b.size() and, for every equivalent=key group [Ea1,Ea2) obtained from a.equal_range(Ea1), there exists an equivalent-key group [Eb1,Eb2) obtained from b.equal_range(Ea1), such that distance(Ea1, Ea2) == distance(Eb1, Eb2) and is_permutation(Ea1, Ea2, Eb1) returns true.

Stripping this down, it all comes down to the equality of unordered containers being defined in terms of std::is_permutation().

The important part is that this references the three argument form of std::is_permutation(), and not the four argument form!

In other words, the whole house of cards ends up being reduced to the default operator==, for the contents of the unordered container, rather than the container's official comparison function.

That's my read on this.