Under the covers, `x.f(y)` is actually something like `_ZN1X1fEi(&x, y)`, where the mangled name indicates the class of X and the type of y. The implicit 'this' pointer is actually explicit in the implementation. Similarly, a call to `f(&x, y)` would actually be a call to `_Z1fP1Xi(&x, y)`, with similar reasoning.

The author is proposing that writing

  f(x, y)

Which would normally only bind to the second mangled function, would bind to the first mangled function if the second is not available.

The additional proposal, that:

  x.f(y)

would work in the other direction was rejected. That's a bit of a shame, as this kind of duality is powerful in languages that support it (e.g. scala).