StackOverflowError in scalac (reg. NoArgsTypeRef.typeParams)

[noresttherein]

Reproduction steps

Scala version: 2.13.10

	trait Base[L, R] {
		type U
		val swap :Base[R, L] { type U = Base.this.U }
	}
	trait Sub[L, R] extends Base[L, R] {
		type U
		override val swap :Sub[R, L] { type U = Sub.this. U }
	}

Problem

	at scala.reflect.internal.Types$NoArgsTypeRef.typeParams(Types.scala:2160)
	at scala.reflect.internal.Symbols$Symbol.typeParams(Symbols.scala:1809)
	at scala.reflect.internal.Types$NoArgsTypeRef.typeParams(Types.scala:2160)
	at scala.reflect.internal.Symbols$Symbol.typeParams(Symbols.scala:1809)
	at scala.reflect.internal.Types$NoArgsTypeRef.typeParams(Types.scala:2160)
	at scala.reflect.internal.Symbols$Symbol.typeParams(Symbols.scala:1809)
        ...

[SethTisue]

more minimal:

trait Base {
  type U
  def swap: Base
}
trait Sub extends Base {
  override val swap: Sub { type U = Sub.this.U }
}

note that the override must be a val, though

[som-snytt]

@noresttherein never rests. Nor does @SethTisue when minimizing.

I observe that no one begins by saying, "I don't wish to appear to be minimizing your problem, but..."

[noresttherein]

Sorry for not minimizing it. I mean, the type on the stack trace is an unapplied higher type, and I was so surprised by it being already so simple, because I was so sure I did it already countless times, and I kind of assumed it's already minimized.

[noresttherein]

Also, in case someone stumbles upon it, this works:

	trait Base {
		type U
		type BaseSwap = Base { type U = Base.this.U }
		val swap :BaseSwap
	}
	trait Sub extends Base {
		type SubSwap = Sub { type U = Sub.this.U }
		val swap :SubSwap
	}

Which is probably the reason for my astonishment - I must have always been doing the above.

[joroKr21]

The most confusing part is that in the minimal version there are no typeParams involved 🤔