Interpreters shouldn't have default parameters

[dcsobral]

Default parameters are implemented as forwarding methods. If they were not present in the algebra but present in the interpreter, they would not be accessible. Since they are present in the algebra, having them on the interpreter will simply override the trait with the same thing (hopefully!).

I actually posed a question about this on twitter, to confirm my intuition. While twitter crowd isn't a particularly reliable source, Odersky himself replied in less than ten minutes stating that default parameters should be final.

[BenFradet]

created #462