A lint rule for M[_] <: List[_]

[dwijnand]

See https://github.com/lampepfl/dotty/issues/1809, it would be lovely to have a lint rule for this.

[Jacoby6000]

Until this is an official rule, you can this pattern:

(\w+)\[_\]\s*<:\s*(\w+)\[_\]

and after #907 gets merged, you can use

{
  "id" = "hktSubtypeOfExistential"
  "pattern" = "(\\w+)\\[_\\]\\s*<:\\s*(\\w+)\\[_\\]"
  "message" = "{$0} is attempting to express a higher-kinded type as a subtype of an existential type.  You probably meant {$1}[X] <: {$2}[X]"
}

To get a nicely detailed error message

[olafurpg]

Thank you for reporting, this is a good idea! I've hit on it myself. I think it's best that this linter gets implemented as a custom rule so that it can evolve separately from the Scalafix core APIs.