Divergent Implicit Type Error for SeqProperty

[KGoodale13]

The following code puts the compiler into an infinite loop possibly because of a DivergentImplicitTypeError (Not completely sure it is infinite but I gave it a solid 20 minutes before I killed it):

private case class Chunk[T](
    var requested: Boolean = false,
    data: SeqProperty[T] = SeqProperty.blank
)

If I change SeqProperty.blank to SeqProperty.blank[T] it will throw an error saying it cannot find a implicit ProperyCreator[T] as it should.

[ddworak]

Thanks for the report!

The underlying issue is that you actually cannot do it like this in Scala, e.g. https://scalafiddle.io/sf/OVcJbld/0

case class Property[T]()
object Property {
  def create[T]: Property[T] = Property()
}

case class Dummy[T](
  p: Property[T] = Property.create  
)

val x = Dummy[Int]()

won't compile due to:

error: type mismatch;
 found   : ScalaFiddle.this.Property[scala.this.Nothing]
 required: ScalaFiddle.this.Property[scala.this.Int]
Note: scala.this.Nothing <: scala.this.Int, but class Property is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
Error occurred in an application involving default arguments.
  val x = Dummy[Int]()
               ^

This unexpected behaviour is actually a part of the spec:
The default argument expression e is type-checked with an expected type T' obtained by replacing all occurrences of the function's type parameters in T by the undefined type
and in case of these implicits leads to huge mess in implicit resolution. More details: scala/scala-dev#366 (comment)

Anyway, I've managed to reduce the number of implicit defs and macro usage to the point where detecting the issue is acceptably fast and covered by our test suite. #276