Existentially quantified variables that are not used
The compiler currently allows types like :: T = E.a: T Int & toString a without warning. This type cannot be instantiated as far as I'm aware. :: T = E.a: T Int can be instantiated, but the type variable is meaningless. It would help people to keep their code clean if the compiler would issue a warning or even an error in such cases.
I'd like to try to implement a warning/error but thought I'd first synchronize what the preferred way to deal with this would be: warning or error, and at what stage (parsing?)?