struct X {
  /* something */ operator<=>(const X&);   // member, compares X
};
X x;

struct Y : X { };
Y y;

x == x (obviously) works.
x == y gets us (x <=> y) == 0 which works (conversion on second param)

y == x: We check whether 0 == (x <=> y) works (yes, it does), so we add the
synthesized candidate

  op<=>(const X&, const X& implicit_obj_param)

to the candidate set for ==. The total candidate set for y==x is

• op<=>(const X& implicit_obj_param, const X&);
• op<=>(const X&, const X& implicit_obj_param); // #2

Then, we match y==x against this and we convert y to X (first param of #2).

What bothers me a bit is that the property "implicit object param" (which prevents conversions on that operand) is not explicitly mentioned as being swapped in 16.3.1.2p6.2.