Separate compilation and subtype checking of HK types

[allanrenucci]

In a file collections.scala:

package collection

trait Seq[+A] extends SeqOps[A, Seq, Seq[A]]
trait SeqOps[+A, +CC[_], +C] extends Any

package immutable {
  trait Seq[+A] extends collection.Seq[A] with SeqOps[A, Seq, Seq[A]]
  trait SeqOps[+A, +CC[_], +C] extends collection.SeqOps[A, CC, C]
}

class StringOps extends collection.SeqOps[Char, immutable.Seq, String]

In a file Test.scala:

package collection

object Test {
  type AnyConstr[X] = Any
  val test: SeqOps[Char, AnyConstr, _] = null.asInstanceOf[StringOps]
}

$ dotc -d out collections.scala
$ dotc -d out -classpath out Test.scala -explain-types
-- [E007] Type Mismatch Error: tests/allan/Test.scala:5:58 ---------------------
5 |  val test: SeqOps[Char, AnyConstr, _] = null.asInstanceOf[StringOps]
  |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |found:    collection.StringOps
  |required: collection.SeqOps[Char, collection.Test.AnyConstr, _]
  |
  |
  |Constraint(
  | uninstVars = ;
  | constrained types = 
  | bounds = 
  | ordering = 
  |)
  |Subtype trace:
  |  ==> collection.StringOps <:< collection.SeqOps[Char, collection.Test.AnyConstr, _]  
  |    ==> collection.SeqOps[Char, collection.immutable.Seq, String] <:< collection.SeqOps[Char, collection.Test.AnyConstr, _] LoApprox 
  |      ==> collection.type <:< collection.type(collection)  
  |      <== collection.type <:< collection.type(collection)   = true
  |      ==> Char <:< Char  
  |        ==> scala.type <:< scala.type  
  |        <== scala.type <:< scala.type   = true
  |      <== Char <:< Char   = true
  |      ==> collection.immutable.Seq <:< collection.Test.AnyConstr  
  |      <== collection.immutable.Seq <:< collection.Test.AnyConstr   = false
  |    <== collection.SeqOps[Char, collection.immutable.Seq, String] <:< collection.SeqOps[Char, collection.Test.AnyConstr, _] LoApprox  = false
  |  <== collection.StringOps <:< collection.SeqOps[Char, collection.Test.AnyConstr, _]   = false
one error found