Method overloading chooses a signature using the static types available at the call site. The receiver m has static type M, whose visible overloads accept X and Y; the Z overload introduced only in N is not a candidate.
For m.f(x), the argument's static type is X, so the chosen signature is f(X). N does not override it, and the inherited M body runs. For m.f(y), the chosen signature is f(Y), and dynamic dispatch on the receiver chooses N's override. For m.f(z), both visible signatures apply, but f(Y) is more specific than f(X). Dynamic dispatch again chooses N.f(Y). Thus
f with X at M
f with Y at N
f with Y at N
Equivalently,
(1)
The runtime type of every argument is Z, but ordinary single-receiver dynamic dispatch does not redo method overloading using that runtime type. This is overload selection followed by dynamic dispatch.

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