immutable method not callable using argument types () - doesn't make sense

[bearophile]

Steven Schveighoffer:

> So what the compiler is saying is that you can't call dup with 
> arguments (), you must call it with arguments '() immutable', 
> meaning you must call it on an immutable B, not a mutable B.

Any space for compiler error message improvements here?

Bye,
bearophile