Private static interface members forbidden in Java 8

[FroMage]

If you try to generate static interface members you get the backend error; modifier private not allowed here, because Java 8 does not allow that. We should forbid this in the typechecker, unless we generate the static methods in the impl class.

[gavinking]

Can't we just automatically promote them to default access?

[gavinking]

Package access, I mean.

[FroMage]

There's no default access in interfaces: by default they're all public.

[fwgreen]

Private interface methods are supported in Java 9.