The following works:
def error(c: Context { type PrefixType = Logger })(message: c.Expr[String]): c.Expr[Unit] = { ...

But the following not:
type LoggerContext = Context { type PrefixType = Logger }
def error(c: LoggerContext)(message: c.Expr[String]): c.Expr[Unit] = { ...

The compiler will complain:
[error] required: (c: scala.reflect.makro.Context)(message: c.Expr[String]): c.Expr[Unit]
[error] found : (c: name.heikoseeberger.scalalogging.LoggerMacros.LoggerContext)(message: c.Expr[String]): c.Exp