// Sub<X, Y>.getSupertype(Base.class) => Base<X, Y>
    // => X=String, Y=Integer
    // => Sub<X, Y>=Sub<String, Integer>
    TypeToken<?> genericSubtype = toGenericType(subclass);
    @SuppressWarnings({"rawtypes", "unchecked"}) // subclass isn't <? extends T>
    Type supertypeWithArgsFromSubtype =
        genericSubtype.getSupertype((Class) getRawType()).runtimeType;