recursive type member cause assertion failure

[xuwei-k]

Compiler version

3.0.2-RC1-bin-20210605-f1252d8-NIGHTLY, 3.0.1-RC1, 3.0.0

Minimized code

trait A {
  type B <: A {
    type B <: A.this.B
  }

  def c: B
}

object Main {
  def x: A = null

  def main(args: Array[String]): Unit = {
    x.c.c.c
  }
}

Output (click arrow to expand)

[info] compiling 1 Scala source to /Users/kenji/recursion-type-member-scala-3/target/scala-3.0.2-RC1-bin-20210605-f1252d8-NIGHTLY/classes ...
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure for type bounds [... (caught cyclic reference) ...] <:< type bounds [ <: A{B <: (A{B <: A#B#B} & A#B)#B} & A#B], frozen = true
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure for A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure for A#B#B <:< A{B <: A{B <: A#B#B}#B}, frozen = true
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure for type bounds [... (caught cyclic reference) ...] <:< type bounds [ <: A{B <: (A{B <: A#B#B} & A#B)#B} & A#B], frozen = true
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since java.lang.AssertionError: assertion failed was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[info] assertion failure [[cannot display since dotty.tools.dotc.core.CyclicReference:  was thrown]]
[error] -- Error: /Users/kenji/recursion-type-member-scala-3/A.scala:13:8 --------------
[error] 13 |    x.c.c.c
[error]    |        ^
[error]    |Recursion limit exceeded.
[error]    |Maybe there is an illegal cyclic reference?
[error]    |If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
[error]    |A recurring operation is (inner to outer):
[error]    |
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  subtype ... (caught cyclic reference) ... <:< ... (caught cyclic reference) ...
[error]    |  ...
[error]    |
[error]    |  subtype A#B#B <:< A{B <: A{B <: A#B#B}#B}
[error]    |  subtype A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}
[error]    |  subtype A#B#B <:< A{B <: A{B <: A#B#B}#B}
[error]    |  subtype A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}
[error]    |  subtype A#B#B <:< A{B <: A{B <: A#B#B}#B}
[error]    |  subtype A{B <: A#B#B} & A#B <:< A{B <: A{B <: A#B#B}#B}
[error]    |  subtype A#B#B <:< A{B <: A{B <: A#B#B}#B}
[error]    |  subtype A{B <: A#B#B} & A#B <:< [cannot display due to assertion failed, raw string = RefinedType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),trait A),B,TypeBounds(TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Nothing),LazyRef(null)))]
[error]    |  subtype A#B#B <:< ... (caught cyclic reference) ...
[error]    |  subtype (A#B#c : => A#B#B) <:< ?{ c: ? }
[error] one error found

[mkurz]

Is this something that needs and can be fixed in a 3.0.x release?

[odersky]

It could be fixed in a patch release. The question is who will fix it. I unfortunately won't have the time. F-bounds is an area where Scala 2 and Scala 3 differ quite a bit. Scala 3 has to do more checks and therefore is more likely to fall into cyclic references.

I have advocated elsewhere that F-bounds should be replaced by intersection types. @japgolly has tried that out with positive results.

[japgolly]

@japgolly has tried that out with positive results.

Yep! (And thanks again to @odersky 's guidance btw). The issue we're talking about is #13133. You can implement f-bounds using intersection types. It was wary as to whether it would work in Scala 2 where with is non-commutative but my little experiment worked out fine! I think as long as no user of your api is explicitly swapping the order around themselves, if you're consistent in the way your order your A with Bs within your own api then the solution works in Scala 2 just as well as it does in Scala 3! (Not to mention that cross-compilation works too).

[smarter]

type B <: A.this.B

If you replace this line by:

type B = A.this.B

Then the example works (and I think this preserves the behavior that was intended?)