 Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2016/imperative-programming-resit/5/a/solution

The output is BADDCD, without a newline. Method overloading chooses a signature using static types, including the instantiated class type parameter; numeric widening can make a Double parameter applicable to an Int argument.

a.m(o1) chooses m(Int), which is more specific than the polymorphic method: B.
a.m(o2) uses m[Double](Double): A.
b.m(o1) compares m(Any) and m(Double). Int can widen to Double, and Double is more specific than Any: D.
b.m(o2) chooses m(Double) over m(Any): D.
c.m(o1) compares the instantiated m(Int) with m(Double) and chooses m(Int): C.
c.m(o2) can use only m(Double): D.

The third call is the subtle one: the existence of m(Any) does not prevent the more specific numeric overload from winning. These are Scala 2 method overloading and numeric widening rules. ^[ref]

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