When I said,
"Which will obviously not compile."
I was, of course, talking complete rubbish!

Let's just take another look at that expansion, with syntax colouring:

if( A > B ) return A; // A is biggest else  return B; // B is biggest

Clearly this will compile (nested comments permitting), but is not quite what one might have expected from the original macro definition (the 'else' clause is missing - it's "disappeared" into the comment)