Using this.type, an expression can compile if broken into assignments and not if given inline.

[scabug]

This is the narrowest test case I could arrange. If necessary, I can provide a larger code sample where the bug actually harms clarity:

object Bug {
  abstract class FChain {
    type T

    def chain(constant:String) =
      new FConstant[this.type](constant, this) //removing [this.type], everything compiles
  }

  case class FConstant[E <: FChain](constant:String, tail:E) extends FChain {
    type T = tail.T
  }
  
  object FNil extends FChain {
    type T = Unit
  }

}

import Bug._
println("Compiles:")
val a1 = FNil.chain("a").chain("a")
val a2 = a1.chain("a")

println("\nDoesn't compile:")
val a = FNil.chain("a").chain("a").chain("a")

This code is the same as the attached file.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-2435?orig=1
Reporter: Rafael de F. Ferreira (rafaeldff)
Attachments:

• Bug.scala (created on Oct 3, 2009 4:59:42 PM UTC, 513 bytes)

[scabug]

Rafael de F. Ferreira (rafaeldff) said:
Same code as provided in the description, can be run as a script or loaded in the REPL

[scabug]

@adriaanm said:
ok, and the error message is:

 found   : Bug.FConstant[_2.type(in value a)] where type _2.type(in value a) <: Bug.FConstant[_1.type] with Singleton
 required: Bug.FConstant[_2.type(in value a)] forSome { type _2.type(in value a) <: Bug.FConstant[_1.type] with Singleton; val _1: Bug.FConstant[Bug.FNil.type] }
  val a = FNil.chain("a").chain("a").chain("a")
      ^

seems like some existential type got stripped or something.. I'll take a look ASAP.

[scabug]

@retronym said:
Test case in:

scala/scala#528

[scabug]

@retronym said:
scala/scala@e40f3c8