In both C# and Java, methods themselves can be generic:

    class ListExtras {
      static <T extends Comparable<T>> T max (List<T> ts) {
        T res = ts.get(0);
        for (T t : ts)
          if (res.compareTo(t) < 0)
            res = t;
        return res;
      }
      
      // ...
    }

C#/Java-style bounded quantification is enough for this particular use case. My complaint is that bounded quantification in these languages is tied to inheritance as shown by <T extends Comparable<T>>. In Haskell and Rust, bounded quantification is tied to type classes, which, as you said, are open - a type class instance for a given type may be made anywhere in the program, not just where the type is defined. So bounded quantification in Haskell and Rust (and its emulation in Scala via implicits) is strictly more powerful than bounded quantification in C# and Java.

As an aside, C# is a little less retarded than Java, so even primitive types like int are objects, and, in particular, int implements IComparable<int>.