 Solution

Apply overload selection followed by dynamic dispatch: the receiver's static type determines visible signatures, and argument static types choose the overload. The receiver's runtime type then selects its override. A super call starts at the inherited implementation instead of redispatching to the override just entered.

For c.f1(y), the Y overload is visible on C, so its body in C runs. For a.f2(x), the X signature is selected and its override in C calls C.f1(X). For a.f2(y), the Y signature is selected; its body in C calls super.f1(y). The superclass B has only f1(X), so that call uses B.f1(X).

For b.f3(y), the static type B exposes only f3(X). Its override in C calls super.f2(x), entering B.f2(X). The unqualified f1(x) inside that body still uses dynamic dispatch on the original C instance, so it calls C.f1(X).

Finally c.f4(x) invokes the companion object C, which is a separate receiver extending B. Its f3(X) calls inherited B.f2(X), whose f1 call dispatches to ObjC.f1(X).^[ref]

The exact five output lines, including the final comma and space on each line, are: