In [expr.spaceship]/7, => strong_ordering::unequal

If the composite pointer type is a function pointer type, a pointer-to-member type, or std​::​nullptr_­t, the result is of type std​::​strong_­equality; the result is std​::​strong_­equality​::​equal if the (possibly converted) operands compare equal ([expr.eq]) and std​::​strong_­equality​::unequal if they compare unequal, otherwise the result of the operator is unspecified.

In [cmp.strongeq] => strong_ordering::nonequal

public:
  // valid values
  static const strong_equality equal;
  static const strong_equality nonequal;